Talk:Overhead valve engine

Pushrod engine merged here

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)

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)

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)

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)

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

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

Should we add Wankel Rotary Engines?

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)

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)

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

OHV engine more complex but more reliable than OHC

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

Comparison of engine configurations and types wording

"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).

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

Umm...No

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)

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 would suggest removing the article point citing service intervals as an advantage. This point is not intutively obvious and does not cite a source.

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)

Limitations - Limited Cylinder Head Design Flexibility

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)

• 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)

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)

Use of Pneumatic Valves unbalanced?

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)

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)

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