Pushrod vs OHC?

[chuntington101]

also people are forgetting that with DOHC (the evolution of OHC)it enables you to alter the intel and exhaust timing indapendently of each other! then there is things like veriable vale timing!

Chris.

[lanciachris]

Are there engines that have dual cams in the block to allow the same thing?

[chuntington101]

not that i know of. it would be difficult as you would have a crossing of pushrods and be very complex. may as well move to over head cams, like ford did with thier V8s.

Chris.

[AliasX1]

So far, there have been 2 pushrod engines with vvt. Cadillac's Sixteen concept car, and GM's Vortec 3900 V6 (as found in the Pontiac G6).
www.ai-online.com/issues/article_detail.asp?id=352

For most people in everyday driving situations, the design limitations of either type of layout will never be an issue.

Not sure how much older OHV pushrod is (if at all) compared to DOHC. Does anyone know? I know that Duesenbergs of the 20's had DOHC, and OHC has been found on Peugeot race cars as early as 1912, if not earlier.

[denisb]

One really major advantage of OHV -

It doesn't take me over eight hours to time the cam(s)!!!

I am never ever ever going to attempt the cam timing on my quad cam V6, a twin cam I4 is bad enough!

[FesterNath]

Lower centre of gravity with pushrods?

[ph'er(zzr1200)]

busa_rush said:

Pushrod is a bad idea, I can't see any benefits to a pushrod design unless the cam is driven by a gear rather than a chain, but almost all pushrods seem to use chains. Even gear driven it has very little going for it. Isn't it just a case of "that's the way the first bloke did it so we've always done it like this." ?

I think the ford kent ohv engine is a good example to compare OHC and pushrod design against as the standard engine was converted into the Lotus twincam etc in the 60's, the thing here is that both types of valvetrain could produce the same amount of power, about 180bhp the main difference being that the head on the twin cam is bigger, taller etc. the pushrods themselves if upgraded can easily rev to 9000rpm as they do in the Hot rod engines.

So perhaps it is really am matter of cost and public perception, a bit like not really needing 16 valves in a modern engine but the public wanting it because it soundds like a good idea?

Paul

[havoc]

A 16v engine WILL give better fuel economy and better emissions, purely through more even combustion and better airflow.

If it wasn't a benefit, why would manufacturers have largely turned to 4v per cyl OHC, despite the greater weight and cost? The main Euro/Jap mfrs that still use 2v or 3v per cylinder are GM/Vauxhall and Merc...neither of whom are particularly known for their economy OR their output (excluding putting effing big blowers on!). ALL other mfrs use 4v per cyl OHC virtually exclusively, with a few forays to 5v.

[wheeljack888]

havoc said:

A 16v engine WILL give better fuel economy and better emissions, purely through more even combustion and better airflow. If it wasn't a benefit, why would manufacturers have largely turned to 4v per cyl OHC, despite the greater weight and cost? The main Euro/Jap mfrs that still use 2v or 3v per cylinder are GM/Vauxhall and Merc...neither of whom are particularly known for their economy OR their output (excluding putting effing big blowers on!). ALL other mfrs use 4v per cyl OHC virtually exclusively, with a few forays to 5v.

Thats not strictly true, because it really does depend on the application. I agree there are emission benefits to having a central spark plug (or injector if diesel). But a benefit of having ONE inlet valve is that at slow engine speeds and light loads the air rushes into the cylinder at a higher speed than would with two valves. This aids charge mixing through tumble effect in spark ignition and swirl induction in diesel (sometimes a bit of both for each).

4 valves generally are more efficient at higher speeds and higher loads. Please see the Honda Vtec-E version (E for economy) which operates effectively one inlet valve per cylinder at slow speeds and opens up to 2 inlet valves at higher revs. Link is here

[havoc]

OK, had forgotten about VTEC-E (have been driving Honda's for a while, know most of the tecchy-stuff).

[ph'er(zzr1200)]

wheeljack888 said:

Thats not strictly true, because it really does depend on the application. I agree there are emission benefits to having a central spark plug (or injector if diesel). But a benefit of having ONE inlet valve is that at slow engine speeds and light loads the air rushes into the cylinder at a higher speed than would with two valves. This aids charge mixing through tumble effect in spark ignition and swirl induction in diesel (sometimes a bit of both for each). 4 valves generally are more efficient at higher speeds and higher loads. Please see the Honda Vtec-E version (E for economy) which operates effectively one inlet valve per cylinder at slow speeds and opens up to 2 inlet valves at higher revs. Link is here

And the ford duratec v6 which uses butterflys to shut of one inlet valve below 3000rpm and by using different tract lengths gives better low down power and economy.

[lanciachris]

Wow. technology amazes me

[sjoerd1980]

Like Marquis_Rex tried to point out to us, it all depends on the application. Both have their pros and cons.

If you want power, OHC or DOHC is the way to go. This is because of the large valve area and the reduced moving mass (although rotating mass might be more, moving mass on the valves is less). A high revving engine is not to good for a heavy street car, more for a light thingy. The engine will be a bit heavy and big. Engine response can be very good on these engines.

If you want torque, OHV can be nice. The engine concept somewhat limits the revs and therefore the horsepower. However the engine is compact and light. Good for the heavier car or just very nice to drive (torque curve). Response will be a bit worse than the (D)OHC.

[wheeljack888]

zzr1200 said:

And the ford duratec v6 which uses butterflys to shut of one inlet valve below 3000rpm and by using different tract lengths gives better low down power and economy.

Much underrated engine that one. Built like the proverbial brick outhouse, but with the right fettling it can be up there with the best of them.

[havoc]

Which Morgan and Noble both appreciate...