Talk:Overhead camshaft

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Peugeot history[edit]

History (or introductory) part is somewhat incorrectly written. Peugeot developed four valve DOHC engine in 1910-1911. Engine was powering, winning Peugeot L76 racecar, at the ACF Grand Prix in 1912. It was the first four valve headed DOHC engined car (so it was not Fiat nor the Pugatti). Four valve equipped Peugeot's won in the USA at the Indianapolis in 1912, 1913 and 1916. Peugeot was using 'some Harry Miller' services at the States (L76 and/or L45 car's were stored there during the war years). There (at the Harry Miller's shop) worked 'some Fred Offenhauser', who was one of the 'Offy' engine originators (which is derived and 'somewhat a copy' from the L76 race engine).

In 1922 Peugeot introduced 5-valve head race engine, with three camshafts, but the idea was abandoned, as the engine was failure. Late 20's and early 30's Henry Ford sold power adding kit's to their engine, which was known as the 'Peugeot Head'.

So, may I succest, some corrective edit. Regards BR —Preceding unsigned comment added by (talk) 01:52, 26 November 2009 (UTC)

OHV vs OHC[edit]

Removed the following (speculative and subjective without references):

"This arrangement is less complex, usually has fewer parts and requires less engine power to operate. It relies on a timing belt or chain to drive the cam(s), and allows for greater valvetrain flexibility."

Discuss? CJ DUB 16:34, 8 May 2006 (UTC)

When compared directly between two valve cam in block engines (OHV) and two valve overhead cam (OHC) engines the OHC system will be less complex. An OHC system only requires a drive system such as a belt, chain, or gear drive, a cam shaft and some type of cam follower. An OHV system requires all of the components of the OHC system but would also require the addition of pushrods and tappets. The same can also be said for four valve OHV systems compared to four valve OHC systems. In engines with more than one bank of cylinders the OHC system becomes slightly more complex but still requires fewer parts. An edit was made to include those instances. IJB TA

  • Not good enough I'm afraid. You know the pushrod page says exactly the same thing? I would say for sure OHC is more flexible, for VVT and multivalves, even intake geometry, but "less parts" is pure speculation. Even the section about higher engine speeds; speculation and hearsay. Some modern pushrods in cars can spin up to 8000 rpm. In race applications they can reach 10,000 rpm. I'm gonna edit the pushrod page too. CJ DUB 16:02, 11 May 2006 (UTC)

I have not read through the pushrod page but if it is written by the same person it is also very likely wrong. I understand the capabilities of pushrod engines but the maximum speed of the engine does nothing to determine the number of parts in the valve system. If you want to argue the engine speed capabilities consider that the rotational speeds of an OHC F1 engine approach 20,000 rpm. If you consider what I have said to be speculation then you are clearly not qualified to edit these pages. IJB TA

  • Nice. This is a discussion not a dictatorship. "I heard of engine that does 20,00 rpm, if you haven't heard of that then you shouldn't be editing." Gimme a freaking break. Anything is open to discussion. You haven't presented one single reference yet. There are several facts in the list of the abilities of the OHC that are speculation, but not ALL. My biggest issue is the less parts and the less engine power statement. You said it yourself, it's more complex=more parts. Maybe OHC does spin faster. But thinking of these things in your head and quoting some F1 number does not make a strong argument. Cite it. If you can't cite then you shouldn't be editing. 8-) My opinion is that you should discuss the advanteges and disadvantages of OHC like is done in pushrod engine. CJ DUB 13:36, 12 May 2006 (UTC)

This article is NOT a discussion. I am listing fact only. Do the research yourself, I will not be teaching you things you should already know if you are going to be editing these articles. Concerning the number of RECIPROCATING parts, just look at any OHC engine and OHV engine and count the number of major reciprocating parts. As far as references, here are some links I happen to have already. I am getting a second opinion on the "less power to operate" subject, I did not include that statement in my last edit. - Most if not all F1 teams have engines that rev to the 18,0000 rpm to 19,0000+ rpm range. - Here is a Skyline GTR that can rev to 13,000+ rpm. Just about all OHC Super Sport motorcycles will rev to 12,000+ rpm. IJB TA

  • K, I'm gonna try this one more time. THIS is the DISCUSSION relating to the CONTENT of the ARTICLE. I can discuss whatever I choose. You'll also note I haven't done any changes without consulting. I kept the edit on the discussion page. for future. When we come to a decision about something, then we change the article. Hopefully with more than 2 of us. I removed the material that was unreferenced staright away, however. You, on the other hand don't even have a frigging login other than your IP address. Now lets see some references for more power & less parts. Also, you may want to think about reading the pushrod page as there are good arguments for OHC on there. CJ DUB 22:59, 12 May 2006 (UTC)

There is nothing to discuss, these are the facts and nothing more. I am not responsible for your ignorance on this subject, it is your responsibility to make sure YOUR facts are correct. If you question the facts that I have given then it is your responsibility to do the research to determine the validity of the statements I have made. THIS IS NOT AN OPINION PAGE, so keep them to yourself. Also this site does not require me to register, that is my choice. Lastly I am not going to dig up every book I have ever read just because one person has an OPINION about this subject. Like I said before, do the research yourself, I have done mine. IJB TA

I have not said "more power" anywhere in this article, this is the last time I will say that. The cam-in-block page does not have any credible reference either, how can you say with any certainty that it is correct? IJB TA

  • Haha. All you have to do is read it and realize it was written by somebody with automotive experience and first hand technical knowledge. We need somebody like that to read this page and decide. This page was written by somebody who read something on OHC, once, basically. I suspect that neither you nor I are qualified to provide definative answers on the properties of OHC.

Fiat Dohc[edit]

“Fiat is credited as the first car company to use a belt-driven DOHC engine across their complete product line, in the mid-1960s.”

How can this be? Fiat X1/9 mid-engined sports car (1972-1980) came stock with a single overhead cam 1290 cc. engine. GT —Preceding undated comment was added at 06:34, 2 August 2008 (UTC)

That sentence is speaking mid 60s, not 70s.... but eg Alfa Romeo has been using dohc engines since 20s.... --— Typ932T | C  07:15, 2 August 2008 (UTC)

The first statement is correct. FIAT utilized a belt driven DOHC engine beginning in the mid 1960s beginning with the 124 Spider and 124 Coupe. The X1/9 utilized a SOHC engine, and was built from 1973-1988. —Preceding unsigned comment added by (talk) 00:14, 9 December 2008 (UTC)

I call bullshit on this one. The Fiat 500 was in production in the mid-60's and had a pushrod OHV engine. (I know because I stripped one.) Fiat may have used DOHC engines in the mid-60's but certainly not across their complete product line Rocknrollsuicide (talk) 04:21, 6 September 2009 (UTC)

The Fiat 500 started was produced from 1957-1975. It is a carry over product and not within the scope of this discussion. ([[User talk:Turnsix])) —Preceding unsigned comment added by (talk) 21:18, 21 October 2009 (UTC)

Poor engineering on display.[edit]

Several obvious faults in this article, and I don't know where to start correcting them.

  • OHC is not more complex than OHV, quite the reverse. It's "relative fragility" comes about because of the difficulty of keeping it lubricated, nothing to do with number of parts.
  • Article refers to "I-Head" engine instead of OHV - it's near enough impossible to find "I-head" elsewhere in the encyclopaedia, the article diverts to OHV. (Though it also appears under "Cam-in-Block"). It's use seems intended to confuse.
  • No mention needs to be made of there being two SOHC or 4 DOHC in a V8, it's plain confusing to readers.
  • I'm surprised to learn it was Fiat who first rushed into DOHC engines. Even if true, it's unencyclopaedic. TomRawlinson 15:21, 5 May 2007 (UTC)

Re:"obvious faults"
  • The drive system for an OHC valvetrain is more complex, the rest is less complex.
  • I'm not really sure what to do about the whole I-head/pushrod/OHV thing. Every piece of material I have ever read on the subject has named the pushrod engine as an I-head, no one seems to call it that anymore. Then some people seem to not want to call it a pushrod engine while others don't want it to be called an OHV engine. When I find the time I'm just going change it all to I-head and see what happens.
  • I will simplify the explanation of the number of cams.
  • The mention of the first production DOHC systems seems plenty encyclopedic to me, it is in the history section. IJB TA 07:31, 15 May 2007 (UTC)

"I-head" was only used to differentiate between those and "L-head" engines. Since the flathead is no longer around, I don't see the point. "OHV" is the standard terminology, it's what magazines use when listing specifications. It matches with "DOHC" and "SOHC". "Pushrod engine" is an informal way of saying "OHV". Most people are not going to have a clue what "I-head" means. .45Colt 14:39, 4 January 2014 (UTC) — Preceding unsigned comment added by .45Colt (talkcontribs)


I noticed all the "limitations" listed under the pushrod article, but that section wasn't listed. There really are no limitations? I always figured more complexity would render a better chance for wear and tear, at least I always thought cams wore down over time. Huh, guess I was wrong, there isn't such a thing as "cam wear". Oh well. Zchris87v 15:37, 29 May 2007 (UTC)

Oh, I almost forgot - don't OHC engines require higher RPMs to produce optimal power? This is at comparison with an OHV, obviously, because they produce optimal power at lower RPMs. Keep reading...
These pages [1] at compare two cars - one mine, one belonging to a friend of mine. Mine is an automatic 1992 Chevy S-10 Blazer, his is a 5-speed 1999 VW Jetta. Mine's producing just over 200 HP (195 stock), and his 115. Engines are, respectively, a pushrod 4.3L Central Port Injected (injects fuel to each cylinder all the time, not efficient), and a 2.0L overhead cam engine (standard injection I assume). So, that's a difference of 2.3L on the engine size. Now, stock gas mileages have his car getting 21 MPG in the city and 28 on the highway, whereas mine is listed as 14 city and 20 highway. Now my only argument is that these numbers aren't 100% accurate for the style of driver or anything. I'll give you that his Jetta is more efficient, but for having over half the engine size and 80 HP less, it probably wouldn't move anything remotely the size of what I drive. For him to realistically drive, he has to shift much higher than normal, whereas my engine shifts at 2000-2500 RPM. His is about 3500-4000 to drive in traffic. On the highway, I can cruise at 60 MPH and be sitting around 1250-1500 RPM, and his is a good bit higher, high 2000's to around 3000. Now at 45 I'm resting comfortably at 1000RPM, where he doesn't like to let his engine get below 2500RPM in any gear. Why? The engine bogs down and won't power it below that. But I can still manage to drive the same stretch of interstate and manage a decent 20-something MPG (with an air intake system I made) and he gets somewhere around the stock estimate.
I don't know how you can compare this all, but all I really mean by it is that there ARE limitations to the OHC engine. Great, the engine can have a high redline, but the peak HP/TQ is at a high RPM. So to drive around in a vehicle with this engine would require a higher RPM to cruise, correct? That's all I'm trying to say, these things aren't absolutely flawless. In fact, don't most of the vehicles on this [2] list use OHC engines? And as far as reliability is concerned, my engine's at 210,000 miles and it hasn't had any valve adjustment or anything. In fact, just semi-regular oil changes and that's it. I know Hondas can see some high mileage, but most that I've seen need valve adjustments at least. Even so, you can't dismiss the venerable Chevy 350 as being "crap" by any means, it's a true testament to the pushrod engine. Zchris87v 15:37, 29 May 2007 (UTC)

OHC engines are less complex, they almost always have fewer moving parts. OHC engines do not have to run at higher rpm to produce optimal power, but most are tuned to produce peak power at a high rpm. There are plenty of OHC engines that produce more power at lower rpm than many pushrod engines. Cam lobe wear? From what? Pushrod engines have greater loads on the cam lobes due to the much greater mass of the valvetrain. I drive an OHC vehicle and the engine very rarely exceeds 2500-3000 rpm under normal driving. Comparing two vehicles simply does not provide nearly enough information to back up what you are implying. Please read the discussion here. IJB TA 21:35, 1 June 2007 (UTC)

twin cam and dohc[edit]

is there any difference at all between twin cam and DOHC engine configurations? or are those merely two different names for the exact same thing? Cirilobeto (talkcontribs) 15:14, 3 April 2008 (UTC)

Yes there can be difference, twin cam doesnt tell where the camshafts are and dohc tells they are overhead. --— Typ932T | C  15:23, 3 April 2008 (UTC)
Ok, so location could be different. how about the cam themselves? is there any difference in them as to size, valve opening or anything like that? Cirilobeto (talk) 17:12, 3 April 2008 (UTC)
While the terms DOHC and Twin-Cam are commonly used interchangeably, as said previously, Twin-Cams are not necessarily overhead. Additionally, in any engine with two banks (like a V-engine or a Boxer) a DOHC will actually have four camshafts. For an inline engine with two overhead cams, both terms apply.

As far as the difference between the cams themselves, it's extremely uncommon to see any two engines using different configurations to have identical cams (sorry I don't have any references on hand ATM), but then it's also rare to have interchangeable camshafts between one marquee and another. -- (talk) 03:44, 14 August 2008 (UTC)

For a long time, all twin cylinder Triumph motorcycles used twin camshafts. They were still OHV engines, not OHC. They ranged between 350cc and 650cc and that was the only design used (by one of the world's very biggest manufacturers?) from the mid 1950s to the 70s. (1970s three-cylinders models the same). Both camshafts remained in the crankcase, with pushrods etc and for all purposes were regular OHV engines, just slightly more complex. TomRawlinson (talk) 23:54, 28 October 2008 (UTC)

Honda CRX picture[edit]

Please pardon my ignorance, but isn`t that picture claiming to show an SOHC head really a DOHC? Aren´t there two cams visible? If I am mistaken, please correct me. --328cia (talk) 18:27, 15 April 2008 (UTC)

No, the photo is in fact of a 12 valve SOHC head. The 2 shafts which you are mistaking for camshafts are rocker shafts. The camshaft is connected to the centre of the cam sprocket, and is obscured and not visible in the photo. Also notice there is only a single cam sprocket, meaning there is only a single cam. (talk) 10:46, 23 April 2008 (UTC)
Apologizing for the delay, I do thank you ever so much for correcting my errors. --328cia (talk) 02:50, 26 October 2008 (UTC)
I think you've just pointed out something very useful - all the photographs are useful - except this one. I'm almost tempted to say it should come out and replaced with a better example when we've got one. In fact, I'd go further, photographs of multi-cylinder engines simply confuse people. TomRawlinson (talk) 18:00, 1 November 2008 (UTC)
How is the photo not useful? How do photos of mutli-cylinder engines confuse people? Please explain. IJB TA (talk) 01:56, 20 January 2009 (UTC)
people can easily confuse the visible shafts on multi-cylinder heads with the cam shaft which in turn makes the engine appear to be DOHC. Besides which (and correct me if I am wrong) isn't this a 6 cylinder engine? If so then it is not from a Honda CRX as can be varified easily at the Wikipedia article on the CRX. — Preceding unsigned comment added by Kingfrito 5005 (talkcontribs) 19:36, 15 May 2013 (UTC)

Stop rabbiting about OHV[edit]

This article treats OHV as if it's an inferior competitor of OHC - which is historically nonsense and must rate as a basic fault of article writing. It's particularly misleading to the reader in this case when OHC = OHV! Let's have every mention of OHV out of the lead, and a section on "Development" explain how the OHC builds on what went before. TomRawlinson (talk) 14:42, 13 December 2008 (UTC)


The first picture is a double overhead camshaft cylinder head. Those are hydraulic tappets, in place of the more common shim and bucket. Those are in fact cam lobes. An engine with over head cams and a rocker shaft does not have tappets on the valve springs.

An overhead cam can have almost as many parts. The minimum missing is pushrods. Many are as shown, with very few moving parts, and less complexity. Some over head cams even have roller followers on the rocker shaft.

The hydraulic tappet takes up valve lash. The shim and bucket style found on high performance engines requires check and adjustment, such as a high performance solid lifter cam. OHV engines do not wear the camshaft more. Engines with one intake an one exhaust valve require more lift and duration to get the same airflow as an engine with multiple intake and exhaust valves, and subsequently more spring tension to close the valve as a result, Something a multi-valve engine isn’t faced with. The engine gets more airflow at lower lift, from more total valve area. Being OHC or OHV has nothing to do with it.

The unrestricted intake ports of certain OHC engines do in fact yield higher performance with out the pushrod interference. The valve layout can be the same in an OHC or OHV engine. Both can have canted or angled valves, for better airflow, as turning 90 degrees to enter the cylinder inhibits airflow. An example of that would be what Chrysler calls the "Hemi", for hemispherical combustion chamber. DOHC engines of the same layout are usually called pent roof, but have the same cross flow layout. Japanese especially call it a pent roof combustion chamber, as the chamber is not as rounded as the Chrysler design.

Push rods have no effect on engine torque. Cam timing, manifold design, and bore and stroke effect engine torque. Horsepower is a function of engine torque and rpm. A small Honda engine will have less torque from less displacement compared to a larger displacement engine. If you need an explanation on variable cam timing, let me know. I am sure others can write that article also.

I have a degree in Automotive Technology, am ASE Master Certified in Automotive, and in Medium and Heavy Trucks. I work as a Field Service Representative for Navistar (International Truck and Engine Company).

The other comment is correct. This is not an opinion page.

Corrections for the corrections. The question concerning the picture of the SOHC head was directed at the second picture, the CRX head is in fact a SOHC. OHC engines always have fewer parts when comparing systems with the same number of valves and cylinders (4 cylinder 2 valve OHC engine to 4 cylinder 2 valve OHV engine). Though it is true that some OHC engines do come close to the high moving part count of an OHV engine it is very rare. The Honda SOHC C series V6 engines for example had a pushrod and two rockers just for the exhaust valve. Yes OHV designs can have some of the same configurations as OHC engines but OHV engines simply do not have the same versatility. There are some SOHC engines that have true hemispherical combustion chambers (LOM and Ranger inline 6 aircraft engines are two examples). Hemispherical heads are different from pent-roof heads, it's not just a different name for the same thing. There are no automotive engines with true hemispherical heads in production today, they simply wouldn't meet the current emissions or fuel economy requirements. Your point about a 2 valve engine being able to match the airflow of a 4 valve doesn't make much sense, at similar lifts the 4 valve will always have superior airflow. This is why at the very best 2 valve engines can only achieve volumetric efficiencies around 100% and 4 valve engines near 140%. The pushrod itself does not affect torque but it does affect the basic design of the engine including port design and valve area, both of which effect torque, and both suffer as a result of that design. 4 valve engines almost always make more torque for their specific displacement so a smaller 4 valve engine can match the torque output of a larger 2 valve engine. Also, I wrote much of the VVT article. IJB TA (talk) 02:37, 28 February 2009 (UTC)

DOHC - correct terminology[edit]

'DOHC' is an official abbreviation for Double Overhead Camshaft and NOT Dual Overhead Camshaft. "Double Overhead Camshaft" is a well-established term, and is clearly referenced in the following academic/professional textbook - "Hiller's Fundamentals of Motor Vehicle Technology, 5th Ed, ISBN: 0-7487-8082-3, page 79". Please respect offical terminology for encyclopaedic content, and not some wrongly-informed myth from some unregulated internet forum. (talk) 17:31, 1 April 2009 (UTC)

Looks like someone still can't read - they reversed the above IP editors corrections. Now re-instated correct encyclopaedic terminology, alon with inline academic text-book citations. (talk) 10:41, 29 August 2012 (UTC)

Article Split?[edit]

Just wondered everybody's thoughts on splitting article into two articles Cam-in-head and Overhead camshaft. The difference being how the valves are actuated; indirectly actuated valves vs directly actuated valves, respectively. VX1NG (talk) 14:02, 20 November 2013 (UTC)

  • Oppose because it's completely opposite to industry practice and naming. If you have to (which is a bad idea), then make OHC a disambig and have Cam-in-head (no objections to that as an article) and invent a Wikineologism of "bucket tappet" or something for the directly actuated valves. Andy Dingley (talk) 14:10, 20 November 2013 (UTC)

Confused paragraph.[edit]

What's with the second-to-last paragraph in the middle section?

It says: "SOHC designs offer reduced complexity compared to overhead valve designs — when used for multivalve cylinder heads, in which each cylinder has more than two valves. An example of an SOHC design using shim and bucket valve adjustment was the engine installed in the Hillman Imp (four cylinder, eight valve); a small, early 1960s two-door saloon car (sedan) with a rear mounted aluminium-alloy engine based on the Coventry Climax FWMA race engines. Exhaust and inlet manifolds were both on the same side of the engine block (thus not a crossflow cylinder head design). This did, however, offer excellent access to the spark plugs."

First sentence is about reduced complexity on engines with more than two valves. Why does the next sentence give us an example of a two-valve engine, and why does it mention that it's a bucket-and-shim engine? No one said anything about bucket-and-shims. It doesn't explain what that even means or what it has to do with a OHC engine. Then, although I'm sure the Hillman Imp was a very interesting car and all, I'm not sure we all need to know so much about it. I'm also not sure what the fact that it was not a crossflow engine has to do with it's being an OHC engine, and the fact that the sparkplugs are easily accessible is irrelevant. That is true of most non-crossflow engines, which can be either OHV or OHC. This is information that belongs on the page about crossflow engines, not OHC engines.

It seems like there is several entire sentences missing from this paragraph, ones that would make these different topics blend together better. — Preceding unsigned comment added by .45Colt (talkcontribs) 15:01, 4 January 2014 (UTC)