Trying to find rational info on why car makers are using OHC vs OHV. Would seem that OHV would be a simpler valve train, but everyone is going to OHC...what are the advantages of OHC motors, and how do they stack up against OHV motors?
Trying to find rational info on why car makers are using OHC vs OHV. Would seem that OHV would be a simpler valve train, but everyone is going to OHC...what are the advantages of OHC motors, and how do they stack up against OHV motors?
I would think that packaging alone would make for a good reason. Instead of pushrods needing to be packaged the entire length of the motor, a timing chain or belt is placed at one end.
If I remember right, making 4-valve heads work with an OHV setup is an absolute nightmare. Not sure if it's a packaging or complexity issue, but the benefits of multi-valve heads are well documented.
OHC generally means a lighter valve train (you lose the weight of the cam follower and pushrod) and lighter is accepted as better, less chance of valve float. Pushrods will also flex like hell at high RPM and big valve spring numbers unless they are made of really nice stuff and have guide plates, etc. It's also much easier to do separate variable valve timing for intake/exhaust with a separate cam for each.
4 valve pushrod heads for Ford Windsor motors:
http://www.themustangnews.com/content/2011/05/4-valve-heads-for-your-windsor-small-block-mustang/
Pretty cool. Pretty damn pricey too, like starting at $4995.00.
I believe one reason GM has stuck with a conventional V8 is packaging - a non-overhead cam V8 generally takes up less space than an OHC version. Plus, since they can meet emissions requirements, get decent mileage and still make 400 hp or more, why not stick with what they know works.
SlickDizzy wrote: If I remember right, making 4-valve heads work with an OHV setup is an absolute nightmare. Not sure if it's a packaging or complexity issue, but the benefits of multi-valve heads are well documented.
I put 100 miles on Hondas 4-valve pushrod setup today. Seems like a pretty simple solution to me:
As much as I don't really care for GM these days, you have to admit, they're getting some very respectable numbers out of an antiquated OHV engine design.
Shawn
I remember reading a quote from someone from GM before the C6 came out. They said they were perfectly well aware that they were leaving power on the table by going with a 2V OHV configuration, but that the weight savings and packaging advantages were very desirable. They seem to be able to obtain reasonable fuel economy and the engines can rev fairly high.
mistanfo wrote: I would think that packaging alone would make for a good reason. Instead of pushrods needing to be packaged the entire length of the motor, a timing chain or belt is placed at one end.
Actually, an OHV engine is a lot easier to package than an OHC. Look at the physical size of a Ford 4.6/5.0 versus an LS1 (5.7-7.0L).
Trans_Maro wrote: As much as I don't really care for GM these days, you have to admit, they're getting some very respectable numbers out of an antiquated OHV engine design.
There's nothing "antiquated" about the OHV LS1. OHC was actually invented at about the same time, if not earlier.
Curmudgeon wrote: 4 valve pushrod heads for Ford Windsor motors:
Chambers look very similar to a Toyota 4AF head...... 
packaging, assembly, timing, and even the price is easier on OHV. And to be honest, it is a more reliable and rebust design.
But there is a reason that most high revving highoutput production engines are OHC and preferably DOHC..
Trans_Maro wrote:SlickDizzy wrote: If I remember right, making 4-valve heads work with an OHV setup is an absolute nightmare. Not sure if it's a packaging or complexity issue, but the benefits of multi-valve heads are well documented.I put 100 miles on Hondas 4-valve pushrod setup today. Seems like a pretty simple solution to me:
I work on a lot of 4 valve pushrod industrial engines and they all use something similar.
mistanfo wrote: I would think that packaging alone would make for a good reason. Instead of pushrods needing to be packaged the entire length of the motor, a timing chain or belt is placed at one end.
Exact opposite. Instead of sticking the cams and stuff on top of the valves, most of the stuff is nestled in the otherwise unused space in the valley.
OHV V-engines are awesome for packaging compared to OHC, especially DOHC. DOHC V-engines are generally a lot wider than a pushrod lump.
GM uses a lot of pushrod engines, but note that they make no pushrod inline engines - there's little to no packaging benefit since the width comes mainly from the manifolding anyway, and variable cam timing is a lot easier to accomplish. It's not impossible to make variable timing OHV - the Viper has a bizarre concentric camshaft layout - but it's not pretty, either.
I expect that what will kill pushrod engines is direct injection, which favors a certain combustion chamber shape, and it's very convenient to stick the pump on one of the valve covers and drive it off of a camshaft.
the next round of GM V8's will have pushrods and Direct injection. Watch for the 5.5L engine coming to trucks and Corvettes.
Interesting. Where does the pump go?
I suppose that it's possible to use another pushrod and drive it off of an eccentric, but it'd have to be one beefy pushrod, no?
In reply to Gearheadotaku:
That sounds sexy
I'm really sorry that GM hasn't made a 6 cylinder variant of the LSX motor. They got good numbers out of the final version of the 60 degree pushrod v6. Don't know why they never Made a shorty LSX.
And to be honest, it is a more reliable and rebust design.
I disagree. With less moving parts, the OHC setup will be more reliable, and most OHC top ends that I know of are far more robust than OHV.
not really that many less parts. Your typical chain driven OHC engine has a longer chain that can stretch, not to mention tensioners and guides that will need maintance.
I can only go by industrial and commercial engines. most of them use pushrods.. and this includes Airplanes where reliability is a serious concern
kreb wrote: I'm really sorry that GM hasn't made a 6 cylinder variant of the LSX motor. They got good numbers out of the final version of the 60 degree pushrod v6. Don't know why they never Made a shorty LSX.
It's called the Series II/III 3800.
(also, inster standard rant about the LSX being a non production engine)
mad_machine wrote: not really that many less parts. Your typical chain driven OHC engine has a longer chain that can stretch, not to mention tensioners and guides that will need maintance.
VW/Audi have severe problems with guide wear, GM has severe problems with chain wear. 10,000 miles before the chain needs to be replaced on the 3.6 sometimes.
Belts are bad why, exactly? They're too quiet and too easy to service or something?
Knurled wrote: Interesting. Where does the pump go? I suppose that it's possible to use another pushrod and drive it off of an eccentric, but it'd have to be one beefy pushrod, no?
That's where I would put it. The valley cover would be a perfect palce to put it- with a LOT of advantages to NVH to have it so buried. Easy access to side injection, too. Not so easy for top, if they go that way- but at the moment, both ways have their pros and cons.
Oh, and not nearly as beefy as the valves. Way more effort goes into opening and closing the valves compared to the fuel pump.
Other than that, this thread is entertaining to read.
When I was looking to buy a Datsun Roadster a fellow owner gave me a tutor on buying the OHC 2000 or the OHV 1600.
As I was first starting out I bought a 1966 1600 that had plenty of power to start with. Today OHV 1600 engines are cheap (in California)
i like the simplicity of a cam in block OHV V8.. they are easy to work on and the technology has been in such widespread use for so long that they are deadly reliable these days.. yeah, and OHC engine will rev faster and higher, but an OHV engine will make more power at a lower rpm- even if it puts out less power per unit of displacement. a 6 liter OHV V8 that makes 400hp at 5500 rpm will be an all around better and more livable engine than a DOHC 2.5 liter I4 that makes 250hp at 7300 rpm.
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