Talk:Overhead valve engine

Why was the pushrod page moved and why wasn't the move discussed first? Overhead valve can also be used to describe an overhead camshaft engine, so this page should be moved back or it should be moved to I-head. IJB TA 15:04, 16 April 2007 (UTC)Reply

I agree, it's better to have a short page here that says "'overhead valve' from a technical standpoint encompasses several engine types, including overhead cam engines and pushrod engines, though now is most commonly used to refer to pushrod engines", and have the larger articles elsewhere. --Interiot 20:04, 17 April 2007 (UTC)Reply

So I guess the question becomes whether to move this page back to pushrod engine or to a new page called I-head. IJB TA 15:41, 18 April 2007 (UTC)Reply

That's something that should be discussed at Talk:Pushrod engine... can we split them back out now? --Interiot 22:26, 3 May 2007 (UTC)Reply

I would, but I'm not really sure what to do with the redirects. IJB TA 21:29, 5 May 2007 (UTC)Reply

I think it should be pushed back to "Pushrod" LOLHI IM AHANIX 05:28, 5 December 2007 (UTC)Reply

• i assure that they should be merged not be split since they co-relate,cause push rod still has a mention on the valves' activity whichever the type of the engine head..whether F-head or T-head...there still lives a mention.

Is nels ornate onmandela-am an architect and still kill this mechanical phenomena and shit. — Preceding unsigned comment added by Nels ornate onmandela (talk • contribs) 18:13, 6 October 2019 (UTC)Reply

Should we add the Renesis rotary engine from the RX-8 to the compairison on the bottom of the page? Xboxstrwrs55

Sure, it's an engine from a sports car and it's naturally aspirated. I'll change the name of the section so it can be added. IJB TA 22:20, 3 May 2007 (UTC)Reply

Ok, I added it, I hope all of the numbers are right. Xboxstrwrs55

Cool, I sorted the chart by power to weight ratio and made the links a little more specific. I had no idea those engines were so light, pretty amazing. IJB TA 21:27, 5 May 2007 (UTC)Reply

I keep looking, but that is the weight I have gotten from 2 places, so I will go with that, thank you. Xboxstrwrs55

Without a proper source, I will delete that spec. Read here: http://www.rx8club.com/showthread.php?t=797 , there is a guy there that weighted the Renesis, and the engine block alone (plus the oil plan) had 220lb. I have seen this engine and there is no way this engine weights only 80kg. I have seen much smaller motorcycle engines weighting 100kg. —Preceding unsigned comment added by MagisterLudi (talk • contribs) 17:25, 22 July 2008 (UTC)Reply

I have added some little understood engine technology to this article. Contrary to popular belief, the pushrod engine is more complex than the later OHC style, with more moving parts. It suffers clearance changes due to head-gasket settlement and thermal effects (hence "adjusting the tappets" - though hydraulic lifters avoid this chore in bigger engines). The big advantage of OHV is that it can safely be drip or splash fed with oil (sometimes pushed through hollow pushrods without assistance from the oil pump). The alternative OHC engines have spinning bearings, and are much more sensitive to lubrication. When there is one oil pump feeding both a crankshaft (requiring perhaps 90% of the oil) and an OHC (requiring 10% of the oil, but which must overcome gravity and reach the cylinder head), correct balance between these feeds is vital. Any contamination in the oil feed, or wear in the system, leads to more expensive damage, sooner, in an OHC engine. TomRawlinson 14:39, 5 May 2007 (UTC)Reply

It's not like engineers would deliberately design OHC engines to have a poor oil supply to the cam(s). In fact consumer reports show that vehicles with pushrod engines are generally less reliable than vehicles with OHC engines, although very few (if any) of those reliability problems are related to the valvetrain. Also the head in this picture:

 

has over 260K miles on it and still has like new cam bearing clearances. IJB TA 21:23, 5 May 2007 (UTC)Reply

Also oil is fed under pressure to the various parts of the engine so gravity wouldn't cause any problems for an OHC engine. IJB TA 21:37, 5 May 2007 (UTC)Reply

I can only report what I know - it is pretty unusual for cam/valve related failures to write off an OHV engine, whereas that is fairly common in OHC engines.

Actually, the reliability problems of OHC engines go further than what I mentioned. Not only do camshaft bearing failures lead to writing off the head, but complete camshaft drive failures in OHC engines are much more likely than in OHV engines, regularily destroying the valves and/or head and damaging the pistons.

Thankyou for not disputing that the later OHC engines are "simpler" than the earlier OHV engines. TomRawlinson 21:33, 6 May 2007 (UTC)Reply

Valvetrain failures are very rare and unusual on any engine type in my experience. I can't find evidence of any valvetrain related reliability problems with OHC engines that don't also apply to pushrod engines. The problems I think you're seeing are a result of timing belt failures where the piston will impact the valves once the cam(s) have stopped turning, this is a result of neglect by the owner not a flaw in the OHC design. Also that type of damage can only happen on interference type engines, most engines are not interference types. Some newer OHC engines have maintenance free timing chains to solve that issue. Last, I have found evidence of cam failure in both pushrod and OHC engines, most failures seem to be a result of improper installation of the cam or a lack of maintenance. Anyway, there isn't nearly enough information here to say one engine type is more reliable than another.

OHC engines of similar configurations have just about always been less complex than pushrod engines. IJB TA 01:37, 7 May 2007 (UTC)Reply

Valve train failures on OHC engines are not all that common, but they're another fault that virtually unknown in OHV engines. Tensioners wear out (the rubber gives way) and occasionally fail. Chains thresh and fail. Belts are pretty reliable, but they wear out (weeping head gaskets ruin them in short order, no warning, just a bang). Belts require replacement at fixed intervals, the work is not cheap and maintenance records get lost. I don't know what you mean by "most engines are not interference types", valve train failures generally (other than a few at tickover) result in virtual destruction of the engine. It's all very well to say "Gee, there was a maintenance failure", but few engines are maintained properly after a few years.

Cam lobe failure seems to have been equally prevalent in both types (though I've not seen any for a while), but cam bearing failure is much more common on OHC engines. Again, you can blame lack of maintenance, what I see is a less reliable engine.

All in all, OHC engines are fine if you're blasting around the country, getting the oil-change done at the reccomended intervals and have someone else paying for the maintenance. They're much more problematical if your old nail and trailer puller is fitted with one. TomRawlinson 20:58, 7 May 2007 (UTC)Reply

Interference engines are those with valves that will occupy the same space as the top of the piston at some point during the operation of the engine. Obviously the valves wont occupy the same space as the piston at the same time during normal operation. They're usually high performance engines with high compression ratios. I've always maintained all my own vehicles and more recently the vehicles owned by my immediate family, all of them have been OHC vehicles, all of them have been more than 10 or 20 years old and none of them have ever had any kind of valvetrain related problems. I have had a timing belt break once, it's annoying (towing the car and all), but it didn't cause any problems and it's wasn't expensive or very difficult to replace on that vehicle. Yes problems can happen with the OHC system, but they can also happen with pushrod systems, as far as what is more prone to failure, that would probably be pretty difficult to prove. IJB TA 14:49, 10 May 2007 (UTC)Reply

Learn something new every day, I didn't think there could be any non-interference engines left, the web suggests there are many, including some modern ones (though neither of ourse). Still, the long drive chain/belt of an OHC is going to wear out faster than the short chain/gears of the OHV system (despite the latter having more parts). And that means either expensive/neglectable maintenance or (in the case of belts) the risk of sudden failure with no warning. TomRawlinson 17:51, 20 June 2007 (UTC)Reply

Maybe, but remember that those chains and belts for OHC engines are designed to handle the load of running that specific system. Engineers don't just grab some random chain or belt off of a shelf and pray that it works for their engine. Obviously belts require maintenance but they offer plenty of benefits when compared with chains. Also the drive systems are always improving, better chains, better belts, better tensioners, etc. One example is Honda's newer 4 cylinder engines (K series, R series), they use a hyvo chain that should never stretch or wear significantly enough to require replacement within an expected engine life (200,000 miles at least). I've seen pushrod engines that needed to have the chain replaced at less than or not much more than 100,000 miles. Newer timing belts generally only require replacement every 100,000 miles, or even more in some cases, that's at least 4 years of use for many people! Anyway, if OHC systems were so unreliable, so expensive and so difficult to maintain there would not be so many 15-20+ year old OHC engines with close to or over 200,000 miles on the road showing no sign of any problems. IJB TA 16:32, 24 June 2007 (UTC)Reply

Also something interesting I came across on a Noble forum, read post #8 "By the end of Day One there were 4 or 5 C6s having brakes rebuilt or timing chains replaced." Seems they may not be all that bullet proof after all. IJB TA 16:36, 24 June 2007 (UTC)Reply

Most modern production engines are "interference" type engines. http://autorepair.about.com/od/glossary/ss/timin-belt-inf2_8.htm Making an engine "non-interference" isn't usually a good thing. Wouldn't these engines need very low valve lift or low compression? Both of these things make for poor efficiency and power. Plus OHV and OHC are both dependable engines, but how can you say that a chain (moreso a belt) is more dependable than a chain that is 1/3 of the length. It is a trade off, either you put the cam next to the crank inside the block for smaller engine size and short distance for timing at the expense of more valvetrain weight or you put the cams above the heads for less valvetrain weight but longer distance to time the cams across and larger overall engine packaging. And what do brakes have to do with an engine being OHV or OHC? You seem to have personal bias against OHV engines. LrngCrv (talk) 09:21, 28 December 2007 (UTC)Reply

Non-interference engines can still be built with reasonably high compression ratios. These engines usually have recesses (eyebrows) cut into the top of the piston. Longer chains are made stronger so reliability is not compromised. I quoted a Noble owner and he just happened to mention something about brakes, it obviously has nothing to do with the conversation here. IJB TA (talk) 01:05, 17 January 2008 (UTC)Reply

Reading your replies leads me to believe that you have very little knowledge on this subject outside of what you can google. Engines are given valve reliefs (or recesses/eyebrows as the term you used...) on pistons to make them non-interference engines. The piston will be nowhere near the top of the stroke when the valve is all the way open. Valve reliefs are for interference type engines with high lifts. You have a very biased and uninformed view on the subject from what I can read and you edit this page as if you are an expert on the subject. Maybe you should restrain a little on your editting.LrngCrv (talk) 04:01, 25 September 2008 (UTC)Reply

Many engines have valve relieves cut into the pistons, and no it does not indicate if an engine is an interference engine or not. But a non-interference engine will require valve relieves in order to achieve a certain compression ratio. I never said that the piston would be near the top of its stroke when the valves were fully open, where did you read that? I said: Interference engines are those with valves that will occupy the same space as the top of the piston at some point during the operation of the engine. Obviously the valves wont occupy the same space as the piston at the same time during normal operation. As in, the valves are not at full lift when the piston is near TDC. There are plenty of non-interference engines still in production. Subaru still builds non-interference engines and I'm sure (I have not checked recently) that plenty of other modern engines are non-interference as well. I don't see what the problem is here. IJB TA (talk) 01:56, 27 September 2008 (UTC)Reply

All of this is side-show to what I said, that OHV engines (though more complex) are intrinsically more reliable than OHC engines. OHV can safely be drip or splash fed with oil (sometimes pushed through hollow pushrods without assistance from the oil pump). Whereas OHC engines have spinning bearings, so they're much more dependent on lubrication. When there is one oil pump feeding both a crankshaft (requiring perhaps 90% of the oil) and an OHC (requiring 10% of the oil, but which must overcome gravity and reach the cylinder head), correct balance between these feeds is vital. Any contamination in the oil feed, or wear in the system, leads to more expensive damage, sooner, in an OHC engine. TomRawlinson (talk) 18:19, 20 October 2008 (UTC)Reply

This was already discussed. Where's the proof of this? IJB TA (talk) 22:59, 22 October 2008 (UTC)Reply

"Comparing engines is not an exact science. This table shows the comparison of some of the most important features when looking at an engine"

I think it's somewhat misleading to say comparing engines is not an exact science. I would argue that it is an exact science, however I understand what the author was trying to communicate, which is that its not possible to accurately compare engines using only a few figures such as peak power/torque figures and rev limits etc. I would put forward a more accurate phrase such as "There are numerous factors to consider when comparing engines. Some of the key features are described in the following table" —The preceding unsigned comment was added by 60.242.154.34 (talk) 12:59, 9 May 2007 (UTC).Reply

Go for it. IJB TA 14:49, 10 May 2007 (UTC)Reply

Removed:

• Less expensive to produce - Having half the number of valvetrain component, plus half the cylinder head machining operations makes it much less expensive to produce. With less complexity, less weight, cast iron cylinder cases, a typical V6 costs the same as an all aluminum dohc 4 cylinder.

I would like to see proof of this. Pushrod engines have quite a few more parts than an equivalent OHC engine. For example, a 1983 Toyota Camry has a single OHC engine with 8 valves and hydraulic tappets, it has a total of 9 components acting on the valves (1 cam, 8 tappets), not including the drive system. A 1981 Datsun 210 has an 8 valve pushrod engine and has 25 components acting on the valves (1 cam, 8 tappets, 8 pushrods, 8 rocker arms), not including the drive system. Quite a difference wouldn't you say? There are fewer head machining operations but more block machining operations for a pushrod engine, they don't just put cams and tappets in raw block castings. Yes iron engines are less expensive to produce, that does not mean pushrod engines are less expensive to produce. IJB TA 01:06, 29 June 2007 (UTC)Reply

Yeah, I came here to make note of the fact engine RPM is somewhat limited by the MASS of the valve train, not the number of parts. EVERY link in an OHC timing chain(s) is a moving part. Plus many more parts involved in tension and alignment, etc. OHC engines have many more moving parts than OHV, and are far more expensive to produce, not to mention tool up TO produce, than an OHV.

The only real advantage an OHC has is RPM, hence why they are and have always been the go-to for small displacement engines needing horsepower made at the top, rather than torque made on the bottom. They are seen so often today where they do not belong, in trucks, etc, because of marketing: "It's more newer and modernerer-er" which, considering both came into existence at the same relative time, is absurd. But nobody accused the buying public of discernment. 70.239.88.118 (talk) 03:20, 28 August 2023 (UTC)Reply

"EVERY link in an OHC timing chain(s) is a moving part."

But a timing chain is not a reciprocating part, so it doesn't affect the maximum allowable rpm of the valvetrain (generally). You're really grasping at straws, there, aren't you?

"OHC engines have many more moving parts than OHV..."

Ok, now I know you're trolling. A timing belt is ONE moving part, factor that into your nonsensical ramblings.

"The only real advantage an OHC has is RPM"

And volumetric efficiency, reduced pumping losses, reduced mechanical losses (especially when comparing engines with the same number of valves), greater freedom of valve placement, greater freedom of port design, greater freedom with regard to VVT systems (a BIG one)... the list goes on. If you want to make everything about money, you're going to doom us all to a very, VERY bleak world indeed. Being stuck with an Iron Duke for all of eternity is just an absolutely nightmarish thought.

Let's see a pushrod engine match the thermal efficiency of the current Formula One engines, or, let's see a production pushrod engine match the thermal efficiency of the Toyota Prius engine, or Mazda's SkyActiv engines. Let's see a pushrod engine match the power to weight ratio of any of the top performing engines on the engine charts below.

Sorry, buddy, but pushrod engines are very MID. They're great for people on a tight budget, but to play with the big boys, you need an OHC engine. IJB TA (talk) 12:15, 27 November 2023 (UTC)Reply

There was a Car and Driver article recently where a GM engine rep was interviewed. He stated that an I4 costs about the same as a pushrod V6. If this article can be cited I would say the point should be added back into the article.

I think I remember reading this article. Then again that isn't proof. —Preceding unsigned comment added by Springee (talk • contribs) 04:37, 1 April 2008 (UTC)Reply

I would suggest removing the article point citing service intervals as an advantage. This point is not intutively obvious and does not cite a source. —Preceding unsigned comment added by 129.59.246.28 (talk) 17:28, 28 March 2008 (UTC)Reply

There are no all aluminum pushrod V6 engines in production as far as I can tell, so an iron V6 would probably cost about as much as an all aluminum I4. IJB TA (talk) 06:26, 31 March 2008 (UTC)Reply

Removed:

• Less expensive to modify - Again, half the number of valves, less machining time, one camshaft (instead of two to four). Incidentally, cast iron while being heavy is also more durable than aluminum. The Buick Series III V6 can withstand 600+ hp with the stock bottom end. GM's LSX V8 cast iron block is reputed to withstand 2000 hp, while the aluminum version is reliable to 800 hp. Standard fare for Honda L4 is to machine away the cylinder bores, install new cast iron cylinder liners, and O-ring the block and heads to achieve 800 hp.

Again this requires proof. Try getting 800 hp from a good old Jeep/AMC 4.0L pushrod engine, then try getting 800 hp from a 3.0 liter Toyota 2JZ-GTE. You can imagine which might be more expensive to achieve that power number with (the Toyota engine will handle that power level with stock internals). There are certain engines that have a large amount of aftermarket support and that does make it less expensive to modify them, but they are not cheap to modify because they are pushrod engines. Also comparing an 800 hp ~1 liter engine to a 800 hp ~6 liter engine is ridiculous, it will be more expensive to produce 800 hp with the ~1 liter engine than the ~6 liter engine no matter what type of valvetrain it may have. IJB TA 01:06, 29 June 2007 (UTC)Reply

And that is the rambling of someone who does not build engines. Your hypothetical 4.0 inline is harder to make HP with due to issues with head design, flow, etc. NOT because of pushrods. Your 'yota makes power because of better flow, not a cam on top. OTOH, which engine will make massive torque down low easier? The inline 4.0. That the 'yota *might* handle your exuberant power guess again has nothing to do with the style of valvetrain, but rather the quality of the bottom-end parts.

Pushrod engines are far cheaper to make power from than OHC not merely because of parts advantages in the type (not any specific engine) but also due to labor. It is simply generally possible to build a pushrod engine in half the time, ie, half the labor cost, vs an OHC. If any of the claims you made were legit the LS/LT engines would not utterly, totally, dominate the performance engine world. That's just shop reality and ,as someone who consulted, co-authored or outright wrote a couple articles in the references cited by the main article, I'll leave it at that. Have a great day. 70.239.88.118 (talk) 03:33, 28 August 2023 (UTC)Reply

"And that is the rambling of someone who does not build engines. Your hypothetical 4.0 inline is harder to make HP with due to issues with head design, flow, etc. NOT because of pushrods"

A) That wasn't what my reply was meant to say. The claim was that ALL pushrod engines are less expensive to modify, but that's clearly not true. B) Yes, in this case, it is more difficult to make power with the pushrod engine, simply because it's a pushrod engine, because it has a hugely complex and inefficient valvetrain which must be extensively modified in order for the engine to produce the same amount of power as the OHC engine. Also, because pushrod engine heads (generally) flow so poorly relative to OHC heads, it's generally more difficult to make power with pushrod heads. Better flow comes with OHC heads, almost universally.

"...which engine will make massive torque down low easier?"

That's still OHC engines, it just depends on how they're tuned. Why do you think the big diesels in semi trucks are all going to 4-valve, OHC heads? Heck, go look at the engine chart below, look at how the 6.2 liter M159 compares to the 7 liter LS7 in torque output, then look at the absolutely dominating torque output of the (still smaller) Aventador engine.

"That the 'yota *might* handle your exuberant power guess again has nothing to do with the style of valvetrain, but rather the quality of the bottom-end parts."

That's somewhat true, but an engine that has a better flowing head is going to have an easier time making more power, so the bottom end components aren't going to be as highly stressed.

"Pushrod engines are far cheaper to make power from than OHC..."

Sure, I'll agree that pushrod V8s offer the most 'bang for the buck' out of any other engine configuration out there, but pushrod engines aren't the king of the mountain, bud.

"If any of the claims you made were legit the LS/LT engines would not utterly, totally, dominate the performance engine world."

But they don't, not in outright performance, anyway. In the production car world, the quickest accelerating cars are all DOHC, the cars with the highest top speeds are all DOHC, the cars with the best track performance are all DOHC, etc. In the racing world, the highest performance engines, BY FAR, are all OHC engines. Formula One, Le Mans prototype cars, Pikes Peak, Rally, etc. Even before Top Fuel banned OHC engines, one started to dominate the series. Heck, the OHV cars in America's top racing series are absolutely DESTROYED by the Ferrari FXX-K, a car with a DOHC V12. You seriously have to be trolling.

Sorry buddy your posturing here doesn't negate the FACTS. Keep losing, America, it's great entertainment. IJB TA (talk) 11:55, 27 November 2023 (UTC)Reply

I think the Less expensive to produce/modify parts still needs to be there just rewrote and more specifically refering to V and flat engines. Mainly because they only require one camshaft to drive all the valves and the cost of the extra valvetrain parts(the pushrods and rocker arms(although alot of OHC engines have rockers as well)) is usually not more than the cost of another extra long timing chain/belt, tensioners, and gears. Let alone factoring in the cost of an extra camshaft and the requirement to make a stronger head to house the camshafts and the extra machining to do so. Pushrod engines also usually only require one head(further driving down costs due to mass production) to be made as well which can fit on either side of the engine, unlike most OHC engines. The same reasoning could be applied as well when modifying said engines.Planingspeed (talk) 21:31, 27 July 2008 (UTC)Reply

You're forgetting some things. Timing belts are much less expensive than chains and can be used to run the water pump, reducing complexity. The head for an OHC engine may be more complex and require more machining operations but the block of an OHC engine is less complex and requires fewer machining operations. All in all it's just speculation that pushrod engines are less expensive to produce, and it's certainly not true of inline engines of a similar configuration (SOHC, two valve/cylinder) which have greatly reduced complexity and part count. IJB TA (talk) 01:45, 14 September 2008 (UTC)Reply

Sorry im new to wikipedia but accusing me of speculation and then doing it yourself is unfair.

OHV engines can use timing belts as well (they do exist for racing applications to allow for easy cam degreeing) but usually do not, because the lower price for a belt that needs scheduled replacement is easily offset by the durability and strength of a generally permanent SHORT chain set up that costs about the same(FOR A OHV ENGINE). OHV engines also can use just gears and no belt/chain at all Ford Straight-6.

OHV engines camshaft drive systems can be used to drive the water pump as wellGM LT engine.

You seem to be thinking more parts/"complexity" equals large amounts of money. With out having done any research on the price of things I can easily see how you could have done this, because hollow (sometimes solid) rods are not expensive. The only things that make a OHV engine different from a OHC engine is the position of the cam and pushrods(though you "could" make a OHC engine with pushrods).

I was mainly refering to V and flat OHV engines being less expensive. The price difference between an inline(straight) OHC and OHV engine is probably debatable. When pricing an INEXPENSIVELY made inline OHV engine valvetrain(like a Gm iron duke or a Jeep straight 4) it cost alot less than a than a similar INEXPENSIVELY made SOHC 2 valve engine(like a K-car engine or possibly a Mitsubishi Saturn engine). When priced out the valvetrain for a early 1980s Chevrolet 350 it came out around half the price of any other engine I priced, even though it has significantly more parts than a 2 valve 4 cylinder SOHC engine.

Such a large price difference now wouldnt be as easy because consumers today want a more powerful and more refined(less NVH) engine. And pushrods are one of the problems of OHV powerplants, deflection and inertia of the pushrods are impediments to NVH control and higher RPM capability. So throwing money at a OHV inline engine to aleviate these problems would bring the cost of it more around the price of a OHC inline engine which can be more refined much easier and with added benefts. Thus making one of the main points of having an oHV inline pointless in modern times. This is probably the reason why there are no more pushrod inlines for automobiles being developed today(that i know of).

OHV V style engines on the other hand can be made refined enough while still meeting the demands for cost and are typically large enough to meet power requirements. There is a reason why GM makes the OHV GM High Value V6 and the OHC GM High Feature V6. The High Value being the base engine in a auto and the High Feature being the extra cost optional engine. Since they could have just as easily made a SOHC engine why did they make a push rod engine? Packaging wouldnt be an answer either when they are both installed in the same car(s).

"But money is a big factor in G.M.'s back-to-the-future powertrains. Brett Smith, director of product and technology forecasting for the Center for Automotive Research in Ann Arbor, Mich., said the company saved an estimated $800 a vehicle by sticking with pushrod motors, which cost less to make largely because they contain fewer parts." http://www.nytimes.com/2005/12/04/automobiles/04CHEVY.html

So what im saying is if cost is the primary concern an OHV engine would be cheaper to produce than a OHC engine. Technically a flathead engine would be the least expensive to make, but several flaws with its design make it unfeasible to produce anymore. I will be adding this section back (reworded though) unless there is some obvious proof or reasoning other than the assumption that more parts/"complexity" equals alot of money.Planingspeed (talk) 17:11, 20 September 2008 (UTC)Reply

My point still stands. OHC engines have considerably lower part count when compared to OHV engines in similar configurations. This quote: "which cost less to make largely because they contain fewer parts." is likely referring to a comparison of 2 valve/cylinder pushrod engines and 4 valve/cylinder OHC engines, this always leads to the common misconception that OHV engines are less complex. Just do the math, OHC engines use fewer parts to achieve the same thing. Also GM has been producing OHV engines continuously for decades, that is likely the reason for the lower cost of their OHV engines. Also consider the performance gained with 4 valve OHC engines:

http://media.gm.com/us/powertrain/en/product_services/2009/09car.htm

The GM 3.9 OHV V6 w/VVT available in the G6: OHV 3.9 output

The GM 3.6 DOHC V6 w/VVT also available in the G6: DOHC 3.6 output

The 3.6 in the G8 performs even better: DOHC 3.6 output

Compare these cars: http://www.fueleconomy.gov/

OHC engine = better torque and power output, better fuel economy. You can see a comparison of the 3.9 with a manual transmission (which should have given it an advantage over the 3.6) here: http://en.wikipedia.org/wiki/Talk:Pushrod

Also concerning what you said here: "The only things that make an OHV engine different from an OHC engine is the position of the cam and pushrods..." There is also the addition of either a rocker or a tappet (depending on the OHC system). In the simplest OHC designs there is only a simple tappet between the cam and the valve, in a pushrod engine there is a tappet, a pushrod AND a rocker arm. Also because a pushrod valvetrain has to compensate for the expansion of the cylinder head AND the block as well as the large number of components between the cam and the valve, pushrod engines require a hydraulic tappet to quiet valve noise. Hydraulic tappets are fairly complex and require as many as eight individual precision components whereas a simple "bucket" tappet in an OHC engine requires only two. IJB TA (talk) 19:19, 4 October 2008 (UTC)Reply

The main advantage of an OHC over an OHV engine is that you can run an OHC engine at higher RPM than an equivalent OHV engine, and hence get more power from the same size of engine. The pushrod system limits engine speed, due to the additional mass of the reciprocating pushrods, as well as being limited by the rigidity of the rods themselves, which can distort and hence minutely vary valve timing. If you are going for power in a big engine running at low speeds pushrods are fine, but if the engine size is limited then going to OHC (or better, DOHC) will enable greater power to be obtained due to being able to run the engine at higher RPM. In addition, the pushrod system limits the angles of the valves that can be used in the head, as the rocker arm is effectively a lever and so provides best advantage over a limited angle re: the pushrod axis. So this limits the valve positioning and hence breathing that can be obtained from a given head.

Otherwise, if a big engine is used, running at not very high RPM, and if power-to-weight ratio is not too important, then OHV is fine. But it places too many restrictions on the designer if he wants performance from a given engine size and weight. If you use DOHC then you could get 600BHP @ 12,000rpm from 1.5 litres (90.8 cu in) as long ago as 1949. — Preceding unsigned comment added by 80.7.147.13 (talk) 17:06, 5 December 2013 (UTC)Reply

This part under the limitations needs sources. It is inaccurate and doesn't describe a limitation of OHV engines. There is no flaw in the OHV engine design that prevents more than two valves per cylinder in the head or prevents variable valve duration. LrngCrv (talk) 13:18, 18 December 2007 (UTC)Reply

• Limited cylinder head design flexibility - The biggest benefit that an OHC design has is the ease of using multiple intake and exhaust valves and variable valve timing. Most modern pushrod engines have two valves per cylinder, while many OHC engines can have three, four or even five valves per cylinder to achieve greater efficiency and power. Recently, however, GM has begun offering a pushrod V6 with VVT, and Cummins' ISB is a 4-valve pushrod straight-6. The GM 3900 was the first mass-produced pushrod engine to feature variable valve timing. The system adjusts both intake and exhaust timing between only two settings, it can not vary the intake and exhaust timing independently. Presently there is even a company called Arao Engineering, formerly Dominion Performance, that has developed, patented, and sold a 4-valve per cylinder aluminum cylinder head for various pushrod engines like the small/big block Chevrolet engines, Ford small/big block engines and others.

I removed this part. The location of the camshaft in the block instead of in the cylinder heads doesn't prevent variable duration or prevent a certain number of valves. There are plenty of examples of OHV engines that have both. Whether companies want to add these to OHV engines or feel that the benifit isn't there for the operation range of most OHV engines is another story but it isn't a limitation of the camshaft position. Anyone have any other opinions or think it should be reworded? LrngCrv (talk) 04:23, 11 January 2008 (UTC)Reply

It might need to be reworded but not removed. Variable duration would be difficult if not impossible to implement on a pushrod engine, cam position is not the same as duration. Certain types of VVT systems would be impossible or extremely impractical to use on a pushrod engine. Multi-valve heads used in conjunction with a pushrod system is completely impractical for most applications unless they are being used as a retrofit for an older engine. Also something not listed in that section is that pushrod engines have very limited head geometry caused by the pushrods. The size and shape of the intake ports as well as the position of the valves are both limited in a pushrod engine. Both aspects are very important to engine performance. IJB TA (talk) 13:58, 11 January 2008 (UTC)Reply

Variable duration would not be impossible to implement on a OHV engine, please show me your evidence to the contrary. You say certain types of "VVT" systems would be impossible or impractical to use on a OHV engine but the same can be said for a OHC one. Please explain your comment about multi-valve heads being impractical because it doesn't make any sense or make any points. Your next point about head geometry can also be said about OHC engines, the cam in the head (esp. with VVT) is a huge limiting factor on head geometry. The rest of you comment is just random, unverified and baseless.LrngCrv (talk) 04:08, 25 September 2008 (UTC)Reply

Variable duration may or may not be possible with an OHV engine, isn't that what I said? My basis for saying that it may not be possible is the fact that all variable duration systems add to the size and mass of the valvetrain. OHV systems already have high reciprocating mass so adding to it would likely counter any benefit a variable duration system might add. Also there likely wouldn't be enough room near the cam to add such a system. As far as the types of VVT systems that could be used with OHC and OHV engines, I don't know of any system that is used on an OHV engine that is not also used on an OHC engine. Cam changing VVT systems (which incorporate variable lift, timing and duration) as well as continuously variable lift systems add mass to the valvetrain and again there is already considerable mass in an OHV system. Also cam changing systems are meant to extend the range that an engine operates efficiently into the higher rpm range. Emissions compliant production two valve engines have poor efficiency at high rpm simply because they only have two valves. Add to that the problem caused by the passage of the pushrods through the head restricting the size of the intake ports and you have an engine that is not going to gain much benefit from a VVT system designed to operate at high rpm. It's not that a cam changing system wouldn't benefit an OHV engine (they really need all the help they can get at high rpm), It's that the cost would likely outweigh the benefit. Multi-valve systems for OHV engines further increase the number of reciprocating components as well as the reciprocating mass of the valvetrain. Because of the mass of the valvetrain and restrictive ports, a multi-valve OHV engine would only benefit from such a system at low rpm and would not be able to take advantage of the multi-valve configuration at high rpm where it truly offers a benefit. Again the cost would very probably outweigh any benefit. In the case of multi-valve OHV diesel engines the valvetrain only needs to support operation at relatively low rpm so its mass is not a detriment to performance. As far as the cam in an OHC engine limiting the geometry of the head, that is total nonsense. The cam position in the head is determined by the placement of the valves. DOHC engines with any angle between the valves from 60 (or even more) to 0 degrees have been produced. IJB TA (talk) 00:23, 29 September 2008 (UTC)Reply

LrngCrv - the whole "Limitations" section could usefully be taken out. Pushrods obviously allow greater flexibility of valve positioning, valve quantity, VVT and all the rest of it. The fact that most of those options (eg moving rocker shafts, sliding cams, wedges etc) have never been used (or, we don't know of them having being used anyway) is neither here nor there. What are we doing comparing older technology directly to the newer stuff anyway? The OHV was around a very long time, in much more inventive times, and there's a vast amount more that could be included. Think of those rotary engines with a single push-pull rod, or a single rod operating the exhaust valve only. Mentioning the fact that some low-tech automobile manufacturers were still using this system long after it was superseded drags down a long and illustrious tradition. TomRawlinson (talk) 20:34, 29 October 2008 (UTC)Reply

Please provide proof of this: "Pushrods obviously allow greater flexibility of valve positioning, valve quantity, VVT and all the rest of it." Otherwise it's one of the most ridiculous things that has been said here. IJB TA (talk) 00:59, 2 November 2008 (UTC)Reply

I'm stating the obvious - adding another mechanical element (push-rod) between the cam and the valve (over and above the one we're getting from the rocker-shaft, which may or may not be present in an OHC engine) obviously adds another degree of freedom that wasn't present before.

In addition, there are many, many, interesting engines using some kind of OHV system which need inclusion - and are familiar to almost anyone who has been into a museum (or a University Engineering Department). TomRawlinson (talk) 21:26, 2 November 2008 (UTC)Reply

Wow, I thought you were joking. Actually the pushrod restricts the options that are available. Pushrods can't pass through cylinders or ports so they must be placed to suit the needs of both, the ports especially. Also the angle at which the pushrods pass through the block restricts the angle of the outside valve in the case of hemi and pentroof heads. There is absolutely no restriction of the placement of the valves in OHC engines because the cam is placed to suit the valve placement. Also the cams do not interfere with the ports or the block. There are parallel valve OHC engines with anywhere from 2 to 4 valves (Honda's i-CTDi engine uses 4 parallel valves, Toyota had some 3 valve parallel valve engines), pentroof engines with just about any angle between the valves you can imagine, there are Hemi OHC engines, the list goes on and on. Obviously there are more VVT systems in production for OHC engines, just read the VVT page. IJB TA (talk) 22:27, 3 November 2008 (UTC)Reply

OHC engines are a later invention that have virtually no relevance to this page. Ditto VVT, which could be applied to OHV engines but (presumably) never has been. However, there are large numbers of other OHV engines - such as rotary aircraft engines - which are highly significant and relevant and need mentioning. All this talk of OHC and near-obsolete car engines is an irrelevance. TomRawlinson (talk) 21:56, 5 November 2008 (UTC)Reply

I immediately noticed in this article that when comparing engines, some of the OHC engine benefits/examples were those using pneumatic valve springs, which are decidedly different than the standard coil springs used on the other examples. Can pneumatic valve springs only be implemented in OHC engines, or is it possible for an OHV engine to use them? None of the articles in question give any indication. If pneumatic valves are independent of valvetrain, I suggest the examples be removed from the article. If they are specific to OHC, I suggest that they be listed as a benefit, but set apart from standard coil spring OHC engines. Scott Paeth (talk) 10:22, 9 February 2008 (UTC)Reply

Pneumatic valve springs could be used in a pushrod engine but wouldn't offer any benefit. The use of pneumatic springs came about because metal springs begin to shatter at a certain RPM, well over the maximum RPM of any pushrod engine. Pushrods are limited by the high mass of the valvetrain, not the valve springs. IJB TA (talk) 06:03, 17 February 2008 (UTC)Reply

Thanks for the feedback. I'll edit the article to reflect this. Scott Paeth (talk) 05:09, 19 February 2008 (UTC)Reply

Not true, pneumatic valve return systems (PVRS) can be used in an OHV or OHC engine but wouldn't offer any benefit unless it is in a "race" type engine with current technology. Teams in Prostock wanted to use PVRS just like F1 does but the NHRA preemptively banned it unlike in F1. The use of it is because springs that are stiff enough to control the valve in those engines do not last long. It is pointless to talk about them in general terms between OHV and OHC especially if refering to production engines.LrngCrv (talk) 04:55, 25 September 2008 (UTC)Reply

You should really read my comments before replying to them. I just said almost the exact same thing as you. IJB TA (talk) 02:39, 27 September 2008 (UTC)Reply

While it is common to have only one cam in a pushrod motor I think it's worth noting that GM did design a twin cam VVT 3 valve pushrod motor called the XV8. The intent was to maximize the potential benefits of a pushrod motor. That is maximize the displacement you can get in a small total engine package size. The concept is described here: http://www.acarplace.com/brands/gm/xv8-engine.html and here with pictures: http://theautoprophet.blogspot.com/2005/06/redemption-of-pushrod.html —Preceding unsigned comment added by 129.59.246.28 (talk) 17:43, 28 March 2008 (UTC)Reply

So its the same size as a V6 and makes as much power as a V6, what are the benefits? IJB TA (talk) 06:31, 31 March 2008 (UTC)Reply

It's actually smaller than the average V6 and makes more power than the average V6. The benefits are in more than just the obvious. What's the benefits of a DOHC, it is larger than a OHV and makes as much power? It's a rhetorical question, I don't want a response it is just to make a point.LrngCrv (talk) 05:23, 25 September 2008 (UTC)Reply

It's not clear what the advantages are from the articles. It might be smaller than the typical 300hp V6 (usually a ~3.5L). It might be cheaper to make. It might have better power delivery. At low RPM displacement generally is better for torque. It may be the benefit is you get a V8 instead of a V6. At least in terms of power delivery adding cylinders usually makes things smoother. As it never made it into production maybe the hoped for benefits didn't pan out. I must say, it is cool looking. —Preceding unsigned comment added by Springee (talk • contribs) 04:35, 1 April 2008 (UTC)Reply

It seems to be ridiculously over complicated. A V6 with DOHC and VVT would have a considerably lower part count. IJB TA (talk) 05:11, 1 April 2008 (UTC)Reply

I don't see how your opinion is relevent, how does that even add to the discussion?LrngCrv (talk) 05:23, 25 September 2008 (UTC)Reply

I can only speculate but when it comes to cost, it’s not always the number of parts but the cost of the parts. Cam shafts are precision machined thus expensive. This is likely part of the reason many motors were SOHC 4 valve setups instead of DOHC 4 valve heads. This motor would also have only 2 cam phasers vs 4 for a DOHC setup (assuming changing both cams).

In the end it’s hard to say. GM certainly thought the design had merit or they wouldn’t have made the prototype. At the same time it’s not in production so it clearly had some limitation (NVH, cost, performance, little need, couldn’t get it in the right shade of red ;) ). I would be great if we could get the inside scope rather than just speculate. —Preceding unsigned comment added by Springee (talk • contribs) 07:10, 1 April 2008 (UTC)Reply

Why is there a engine comparison chart in the OHV article? This article is about the OHV/pushrod engine not comparing engine specs, although another page/article with these specs would be nice though. Not only that none of the mass produced engines on said chart have proper sources. It also does not specify if the engines are fully dressed or not. For example, if the weight of the engine includes the starter, exhaust, or even engines accessories such as A/C compressor, power steering, etc. Some manufactures even include the weight of the flywheel or a flexplate if said car has automatic which can alter the weight further.Planingspeed (talk) 19:31, 27 July 2008 (UTC)Reply

The reason the chart remained in the article was due to many edits claiming that all pushrod engines were lighter than all OHC engines. I will put this chart here so there are no future mistakes. IJB TA (talk) 02:08, 16 September 2008 (UTC)Reply

OHV engines are generally lighter than OHC engines. If you take an OHV engine and convert that same engine to OHC it will be larger and heavier. Posting a table with very randomly different engines does not prove your point.LrngCrv (talk) 05:29, 25 September 2008 (UTC)Reply

How does it not prove my point? Not all OHV engines are lighter than all OHC engines, end of story. Also the chart shows that OHC engines are capable of MUCH higher power to weight ratios than could ever be achieved with an OHV engine. IJB TA (talk) 01:36, 27 September 2008 (UTC)Reply

No one ever said that 'all' OHV engines are lighter than 'all' OHC engines. But a crate of feathers weighs more than a small ounce of lead, that doesn't make lead lighter than feathers. On an equal comparision OHV engines are more compact design than OHC ones, that was one of the major reasons for designing pushrod engines and their popularity during the "muscle car" craze in the '60s.132.61.176.7 (talk) 01:12, 30 September 2008 (UTC)Reply

I'm not arguing that pushrod engines are not compact, that's why that is still listed as an advantage in the article. Though compact engines hardly offer a performance advantage compared to an engine with a good power to weight ratio. Those muscle cars had plenty of room for an engine with large external dimensions, so it didn't really give them any advantage to have a physically smaller engine. Also OHC engines still have an advantage of small dimensions when it comes to very high performance engines. Consider an F1 engine or an LMP2 engine in the ALMS or the Powertec RPA V8 (Image), etc., all extremely compact engines for the amount of power they produce. IJB TA (talk) 14:07, 1 October 2008 (UTC)Reply

So you are saying more compact engines have no performance advantage in a production car? How about the fact that you can have a larger displacement engine and still fit it in an engine bay. That is completely obvious and displacement has MUCH more to do with engine performance than whether the cam is inside the block or outside of it. Go park a late model Mustang GT next to a late model Z06 Corvette and pop the hoods. That 4.6L 'Mod' motor is huge compared to the 7L LS7.

And quit telling people to look at an F1 engine when arguing about the basics of camshaft placement. Telling others to look at how well the performance of an engine that has hundreds of millions of dollars in R&D put into it every year doesn't prove any point you made. Why don't you go look at a top fuel dragster, they make a couple thousand horsepower and are OHV. That was rhetorical, I don't think you looking a the specs of the mopar motor used in a top fuel dragster is relevant to the discussion at all...LrngCrv (talk) 00:54, 23 October 2008 (UTC)Reply

OHC engines are capable of making more power from a given displacement, so there is no need to build OHC engines with huge displacements. Fact is: F1 engines would never make even close to their current power output if they weren't OHC engines, so an OHV F1 engine would never have an advantage, no matter how much money you pumped one. Also consider the other two engines I mentioned above, they don't have "hundreds of millions" behind their development (that money also goes into the chassis and things like building huge wind tunnels BTW). And what about the engines in motorcycles like the Yamaha YZF-R1 or the Honda CBR1000RR, etc., how much larger would they be if they were OHV engines? They seem to be pretty inexpensive too considering how they perform. Or how about F1 engines in the mid '80s? They were OHC engines and made nearly 1000 hp per liter, just like top fuel engines. They also lasted for about an hour (though, not while making 1,000 hp/liter) while top fuel engines have the life cycle of a few seconds, IF THEY'RE LUCKY. And one more thing, they weighed around 300 lbs. I don't see the advantage pushrod engines have here. IJB TA (talk) 23:54, 24 October 2008 (UTC)Reply

Comparison of naturally-aspirated engines for race and road legal track day cars

This article is overwhelmed by reference to the later OHC layout. The OHV motor has a long and honorable tradition which it would be interesting and valuable to document. The fact that it's now been (largely) superseded by something more modern is irrelevant - I don't see the horse article filled with references to how much better is the car. TomRawlinson (talk) 21:22, 9 December 2008 (UTC)Reply

Seconded. Conversely, I don't see much mention of OHV in the overhead cam article. Comparisons of engine types should go within internal combustion engine, or another article should be refactored to hold the comparison. Scott Paeth (talk) 12:52, 11 December 2008 (UTC)Reply

Here's the thing, though: with an overhead camshaft, where are the valves? In the cylinder head. Technically, an OHC engine is an OHV engine. It's the camshaft that's relocated, not the valves. Sincerely, SamBlob (talk) 00:47, 25 November 2013 (UTC)Reply

OHC engines are not OHV engines, even though their valves are obviously overhead.

These terms are defined by a process akin to COMMONNAME, not one of logical implication. It would be most unhelpful to have an overlap between the terms OHC & OHV, so convention is just that there isn't one.

If we were trying to define an ontology by some sort of propositional calculus, then we'd have an issue here. OHC & OHV are disjoint, yet as OHC engines have overhead valves, it is thus difficult to define "OHV" in a simple manner. However that's not what we're trying to do, we're simply reporting an engineering convention amd that convention is that OHC and OHV remain separate. Andy Dingley (talk) 01:30, 25 November 2013 (UTC)Reply

Wondering why this article is titled "Overhead valve" and not "Overhead valve engine" (which redirects here). The first sentence talks about "overhead valve engine" and not "overhead valve". Facts707 (talk) 08:22, 30 October 2009 (UTC)Reply

I don't understand that either. Maybe the article should be moved? Sincerely, SamBlob (talk) 00:47, 25 November 2013 (UTC)Reply

There are two basic principles for naming articles:

• Give them a good name as a stand-alone.
• Give them a name that is amenable to linking from other articles.

Overhead valve is much the better name for linking. It is rare to discuss overhead valve engines in situations where 'engine' is not already implicit and so unnecessary. Andy Dingley (talk) 01:23, 25 November 2013 (UTC)Reply

On the other hand, the title being "overhead valve" implies that it's about a kind of valve, like a ball valve or a needle valve, than an engine type, like a flathead engine or an IOE engine. Further, similar reasoning didn't stop straight-six from being renamed straight-six engine.

Otherwise, the article could be renamed OHV engine, with OHV as a desingnation and not simply an abbreviation. This would tie up not only this discussion's loose ends (that this article's title appears to the ignorant to be about a type of valve), but also the previous discussion's loose ends (that both OHV and OHC configurations have "overhead valves")

Sincerely, SamBlob (talk) 03:54, 25 November 2013 (UTC)Reply

In the introduction it implies that a 1949 engine was the first OHV engine. Well hardly. The curved dashed Oldsmobile produced well before WW1 and almost all Buicks ahd 'valve-in-head' engines. The first Chevrolet 'Stovebolt Six' came out in 1927 and the second in 1939.203.26.122.12 (talk) 06:33, 25 November 2009 (UTC)Reply

See Wikipedia:Articles for deletion/Cam-in-block Andy Dingley (talk) 10:18, 14 April 2012 (UTC)Reply

Maybe somebody can help me to understand the staggered valve arrangement? When I saw a picture of one side of a big block engine a few days ago, I didnt know about the staggered valve arrangement, Now that I know, it might be still not making sense. When you look at an overhead photo of the valves, you see six valves in the center, one valve by itself on one end, and one valve by itself on the other end. In total, eight valves makes sense: four intake valves, and four exhaust valves. But the thing Im not fully fully understanding is the staggered arrangement. It seems really, that of eight valves, only two are staggered. One valve at each end is set at an angle. Am I correct? The valves in the two middle cylinders are closer together, than each valve set in the end cylinders. Am I correct? Or am I missing some facts here? Marc S., Dania Fl 206.192.35.125 (talk) 13:50, 8 March 2013 (UTC)Reply

Do you have a photo or a link? Andy Dingley (talk) 13:55, 8 March 2013 (UTC)Reply

http://WWW.MOTORTREND.COM/FEATURES/CONSUMER/1301_CHEVROLET_CORVETTE_60_YEARS_AMERICAN_ICON_PART_1/PHOTO_17.HTML Marc S. Dania fl. 206.192.35.125 (talk) 19:42, 8 March 2013 (UTC)Reply

Looks like a perfectly straightforward valve layout. The valves are in pairs, per cylinder, and what's a little confusing is that the pair for one cylinder are the widely-spaced pair, whilst the closely-spaced pair are across two adjacent cylinders. All four of each bank, inlet (central) and exhaust (outer, towards the rear of the engine), are simply in line. Andy Dingley (talk) 21:00, 8 March 2013 (UTC)Reply

I propose that the section "1994 Mercedes/Ilmor Indianapolis 500 engine" be either deleted, or moved to a new page linked to this one. Arrivisto (talk) 15:01, 29 December 2014 (UTC)Reply

This component is broader in scope than a specific kind of engine, should be split off.--عبد المؤمن (talk) 13:06, 9 June 2019 (UTC)Reply

Agreed, but do we have any content in this article which would belong there?

Is there enough to start a pushrod article? What are the potential topics for pushrod outside the scope of OHV? Andy Dingley (talk) 13:50, 9 June 2019 (UTC)Reply

Of course there should be a Pushrod article, but it should not be split from the OHV article. There are pushrods for diesel engines, steam engines, sewing and looming machines, typewriters, and endless others. Nothing should be subtracted from the OHV article, merely a link to the pushrod article would be enough.MorganDWright (talk) 00:41, 29 July 2019 (UTC)Reply

There are many different types of pushrods, indeed. Would we just focus on OHV-related pushrods, or would we have a broader coverage of the subject? --Johannes (Talk) (Contribs) (Articles) 08:03, 29 July 2019 (UTC)Reply

If you think that there is enough scope in the basic mechanism, and its range of applications, to support a stand-alone article, then go for it. Personally I can't see it. Where are the "pushrods" in a steam engine? I've never heard the term (although there are a handful of "rods which are pushed", the term is unknown).

For OHV engine pushrods, where the term is certainly used, I see it as a better article to keep it here, and the content is hardly bursting out of here. Andy Dingley (talk) 09:30, 29 July 2019 (UTC)Reply

Merriam Webster Collegiate gives 1908 as the earliest year it has found, of a published usage of the term. So the point is moot, no need for a separate article.MorganDWright (talk) 23:08, 29 July 2019 (UTC)Reply

   This also compromises the spark plug location, since it is not possible to have a centrally located spark plug in an engine with two valves per cylinder.

This is plainly false. While most (possibly all) two-valve engines have the sparkplug offset to allow for maximum valve size, nothing prevents the designer from placing a sparkplug in the center and simply making the valves smaller. Of course, this effectively gives you about half the flow of a four-valve engine, but that's just inefficient, not impossible.

AI0867 (talk) 13:01, 27 January 2020 (UTC)Reply

A great deal of this article is false and unsourced, based on vague ideas from someone with no real breadth of knowledge of the subject. This is a typical example. Andy Dingley (talk) 13:20, 27 January 2020 (UTC)Reply

After a quick lookover, I think it's safe to say that there are more "interesting" things that might need fixing. The variable displacement explanation is written in a way that suggests that it is related to OHV; I'd say that it's "temporarily disabling fuel supply" rather than variable displacement, if I'd mention it at all. And OHV actually does allow variable valve timing (if you will): Many older Diesel engines have flip switches that change the outlet valve timing so that the valve doesn't close anymore. This significantly reduces the engine turning-over resistance and is used to ease hand-cranking all these engines that otherwise would only start with the help of a starter motor. Even bigger old Diesel engines used variable valve timing for adjusting intake air flow whilst starting up or for changing the crankshaft turning direction from counterclockwise to clockwise or vice versa. This is not really related to "modern" variable valve timing, but if it isn't varying the valve timing, what is it? --Johannes (Talk) (Contribs) (Articles) 00:01, 28 January 2020 (UTC)Reply