Pushrod vs OHC?
Discussion
So yes. Why is OHC considered better than pushrod engines? is it actually? from my perspective
ohcs have longer timing chain or belt requiring more maintenance, whether its replacing tensioners or belts, or both.
ohcs are harder to get decent lubrication to
ohcs are easier to access.
pushrods are likely to have more things to replace if something breaks (thinking bent pushrods / damaged rocker gear).
ohcs have longer timing chain or belt requiring more maintenance, whether its replacing tensioners or belts, or both.
ohcs are harder to get decent lubrication to
ohcs are easier to access.
pushrods are likely to have more things to replace if something breaks (thinking bent pushrods / damaged rocker gear).
I think one big benefit is you don't have to make room for the push-rods to come up through the head, so less constraints on your inlet and exhaust ports.
Also the old pushrod valve gear with rockers had more inertia and thus imposed a lower safe rpm limit on the engine than an OHC design.
However, OHC with solid lifters make setting your clearances a bastard job, measure clearance, remove shim and measure, buy new shim, etc. But once set up the settings stay OK for ages, compared to the easily adjusted rocker clearances.
Thank god for hydraulic tappets....
Also the old pushrod valve gear with rockers had more inertia and thus imposed a lower safe rpm limit on the engine than an OHC design.
However, OHC with solid lifters make setting your clearances a bastard job, measure clearance, remove shim and measure, buy new shim, etc. But once set up the settings stay OK for ages, compared to the easily adjusted rocker clearances.
Thank god for hydraulic tappets....
lanciachris said:I would suggest that in addition to this an overhead cam engine may waste less energy due to the reduced number of moving parts.
So yes. Why is OHC considered better than pushrod engines? is it actually? from my perspective
ohcs have longer timing chain or belt requiring more maintenance, whether its replacing tensioners or belts, or both.
ohcs are harder to get decent lubrication to
ohcs are easier to access.
pushrods are likely to have more things to replace if something breaks (thinking bent pushrods / damaged rocker gear).
This may (or may not) have the side effects if making overhead camshaft engine quieter, more powerful, more fuel efficient and generate less heat in the engine bay.
hadnt considered valve location stuff. Cool.
Isn't it because of the expansion in the pushrod.
I thought the main reason for moving over to OHC was that you didn't end up with aggrevated wear on the engine, when cold.
Obviously on a pushrod engine if you set the clearances when cold, they are reduced as the engine warms and the pushrod expands. If you're not careful the valve may not fully close when hot.
On the other hand if you set the clearances when hot, not only do you get rattle when the engine is cold, but you also get a certain ammount of wear associated with the rocker rattling between the pushrod and the valvestem.
The problem still exists to a certain degree in an OHC engine, but without the pushrod acting as a "thermal amplifier", the problem is significantly reduced.
I thought the main reason for moving over to OHC was that you didn't end up with aggrevated wear on the engine, when cold.
Obviously on a pushrod engine if you set the clearances when cold, they are reduced as the engine warms and the pushrod expands. If you're not careful the valve may not fully close when hot.
On the other hand if you set the clearances when hot, not only do you get rattle when the engine is cold, but you also get a certain ammount of wear associated with the rocker rattling between the pushrod and the valvestem.
The problem still exists to a certain degree in an OHC engine, but without the pushrod acting as a "thermal amplifier", the problem is significantly reduced.
love machine said:
Well, you can have the valves as 8 8 8 8 instead of oo oo oo oo which is a good thing for sizing and port shapes. You can also have a semi-pent roof chamber shape which is a good thing, etc, etc.
Chrysler hemi engine is a pushrod. And not just the one from the sixties, I mean the one they've just launched.
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." ?
Isn't it just a case of "that's the way the first bloke did it so we've always done it like this." ?
busa rush, i have noticed this alot on US forums, they hate the idea of changing from pushrod to overhead cams (be it single or twin) for some strange reason! even when i asked about what people thought of a 4 vavle per cyclinder LSX engine (be nice on the LS7) they said that you wouldn't get anymore power than what you can get with a 2valve per cyclinder (yeah right)! i disagre, but then i was talking to people that are still using single, 4 barreled carbs! i notice that Webcon have come out with a Quad downdraught kit for V8s that is said to offer good power gains, but still bet some in the states will disagree with them!
just the way the world is i geuss. if only some enginers from over here could get hold of the things for a bit. bet Cosworth could make a blistering engine from a LS2.
thanks Chris.
just the way the world is i geuss. if only some enginers from over here could get hold of the things for a bit. bet Cosworth could make a blistering engine from a LS2.
thanks Chris.
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." ?
Explain. Why is a chain a bad idea? it works, the chain run would be too short to require and tensioners, so nothing there to break...
The benefit is that you can whip the head off without having to bugger about with belts and also, the head has less height.
As a silly aside, does anyone run OHC heads on a Rover V8 block? With Mini engines, there are SOHC and DOHC varieties available and people butcher allsorts of heads to "bodge on", including BMW bikes, Corsa 16V heads, etc. I was wondering if you could bodge something else on.....K series?
As a silly aside, does anyone run OHC heads on a Rover V8 block? With Mini engines, there are SOHC and DOHC varieties available and people butcher allsorts of heads to "bodge on", including BMW bikes, Corsa 16V heads, etc. I was wondering if you could bodge something else on.....K series?
Just to elaborate on what nel said.
The reason why ohv's have a lower rpm than ohc's is because the valve spring has far less weight to control and so can be either softer or tolerate a far more aggrassive cam profile.
Likewise the cam profile can be more aggressive because it has less mass/spring force to worry about.
And to say nothing of component flex in the valve train.
The reason why ohv's have a lower rpm than ohc's is because the valve spring has far less weight to control and so can be either softer or tolerate a far more aggrassive cam profile.
Likewise the cam profile can be more aggressive because it has less mass/spring force to worry about.
And to say nothing of component flex in the valve train.
lanciachris said:
Explain. Why is a chain a bad idea? it works, the chain run would be too short to require and tensioners, so nothing there to break...
Sorry, didn't mean it to sound like that. A gear driven cam will be more accurate than chain driven cam but with a short chain the difference will be almost negligible.
Expansion is the big killer as well. Pushrods expand considerably and the clearances required will be sometimes bigger. OHC's are more simple. Can have direct actuation or rockers.
The inertia on the system is huge on a pushrod engine. To reduce it, a compromise between weight and valve spring efficiency is needed.
The Corsa 1.2 16V head has been used by someone out there on an A series, instead of the stupidly priced KAD head.
The inertia on the system is huge on a pushrod engine. To reduce it, a compromise between weight and valve spring efficiency is needed.
The Corsa 1.2 16V head has been used by someone out there on an A series, instead of the stupidly priced KAD head.
Some very worthy reasons given for OHC but I feel I must stick up for the venerable old pushrod design:
Package, package & package (Engine height and width on V's is a killer)
Potentially less overall engine mass (esp V engines)
Less camshafts, less friction
Very simple & robust camshaft drive (no need for tensioners)
The pushrods don't get that hot and also clearances can be kind of solved with hydraulic lash adjusters. Pushrod isn't that bad and its certainly reliable. And engineering is really about being FIT FOR PURPOSE.
Some points maybe missed on OHC or rather DOHC. The benefit of a central and upright spark plug for clean uniform combustion (no cold bore-walls to interfere). You're controlling inlet & exhaust camshafts seperatily so then you can introduce effective cam-phasing. Also paricularly with multivalves more effective bore area to valves area ratio usage, and smaller lighter valves.
Package, package & package (Engine height and width on V's is a killer)
Potentially less overall engine mass (esp V engines)
Less camshafts, less friction
Very simple & robust camshaft drive (no need for tensioners)
The pushrods don't get that hot and also clearances can be kind of solved with hydraulic lash adjusters. Pushrod isn't that bad and its certainly reliable. And engineering is really about being FIT FOR PURPOSE.
Some points maybe missed on OHC or rather DOHC. The benefit of a central and upright spark plug for clean uniform combustion (no cold bore-walls to interfere). You're controlling inlet & exhaust camshafts seperatily so then you can introduce effective cam-phasing. Also paricularly with multivalves more effective bore area to valves area ratio usage, and smaller lighter valves.
Interesting one.
Europeans ( and I DO include us Brits in that) seem to often have an unfair bias against pushrods.
As already reflected:
1) An OHC engine design allows a stiffer lower inertia valvetrain-which allows rapid flank accels on the cam design and typically allows higher revs.
2) OHC design allows more freedom in packaging- allowing not only decent port layout for breathing but also good combustion chamber design.
This layout doesn't lay claim solely to a cross flow design (cross flow layout encourages scavenging)- as Gavin pearson rightly pointed out: There are land mark pushrod designs out there, like both the old and new Dodge Hemi engine.
3) OHC allows much shorter cam ramps to take up the slack/lash in the valve train system. This should NOT be underestimated- the long ramps of a really "floppy" unconstrained valve train arrangement such as the Rover V8 is responsible for it having so much in cylinder residual. It's this same under constrained rocker shaft arrangement of this engine that leads to the effective cam periods closing up at higher engine speeds (the worlds first variable cam timing engine?). This isn't a problem with a well designed pushrod layout such as the Corvette motor.
Pushrod:
1)Allows lower friction- ESPECIALLY on a V8.
On a V8 this is the case because one cam can be used instead of 2 in between the banks. Aside from this, due to lower temperatures of the block away from the combustion chambers- the pushrod cam operates under a more favourable regime of lubrication (Elasto-hydro dynamic where as an OHC engine will typically run under boundary- I THINK (it's been a while since I've looked at a Stribeck curve!))
2) Can be easier to make more refined-less valvetrain noise intrusion to concern oneself about.
3) Shorter cam drive
4)Lower mass- a well designed pushrod engine- especially with a V configuration- can be designed to be very light
On a low revving truck-type V engine (where there isn't a great need for valve area) the arguments for a pushrod engine become more compelling.
Europeans ( and I DO include us Brits in that) seem to often have an unfair bias against pushrods.
As already reflected:
1) An OHC engine design allows a stiffer lower inertia valvetrain-which allows rapid flank accels on the cam design and typically allows higher revs.
2) OHC design allows more freedom in packaging- allowing not only decent port layout for breathing but also good combustion chamber design.
This layout doesn't lay claim solely to a cross flow design (cross flow layout encourages scavenging)- as Gavin pearson rightly pointed out: There are land mark pushrod designs out there, like both the old and new Dodge Hemi engine.
3) OHC allows much shorter cam ramps to take up the slack/lash in the valve train system. This should NOT be underestimated- the long ramps of a really "floppy" unconstrained valve train arrangement such as the Rover V8 is responsible for it having so much in cylinder residual. It's this same under constrained rocker shaft arrangement of this engine that leads to the effective cam periods closing up at higher engine speeds (the worlds first variable cam timing engine?). This isn't a problem with a well designed pushrod layout such as the Corvette motor.
Pushrod:
1)Allows lower friction- ESPECIALLY on a V8.
On a V8 this is the case because one cam can be used instead of 2 in between the banks. Aside from this, due to lower temperatures of the block away from the combustion chambers- the pushrod cam operates under a more favourable regime of lubrication (Elasto-hydro dynamic where as an OHC engine will typically run under boundary- I THINK (it's been a while since I've looked at a Stribeck curve!))
2) Can be easier to make more refined-less valvetrain noise intrusion to concern oneself about.
3) Shorter cam drive
4)Lower mass- a well designed pushrod engine- especially with a V configuration- can be designed to be very light
On a low revving truck-type V engine (where there isn't a great need for valve area) the arguments for a pushrod engine become more compelling.
I think the lower friction thing might be overestimated, on a OHC the valve springs can be very low poundage, I would guess (??) that OHV needs much stronger springs ? More mass to move too.
My main reason for liking DOHC is that it gives a far better combustian chamber shape, valves at good angles, plenty of valve area and you can adjust intake and exhaust cam timing separately, all things that help with making good power.
My main reason for liking DOHC is that it gives a far better combustian chamber shape, valves at good angles, plenty of valve area and you can adjust intake and exhaust cam timing separately, all things that help with making good power.
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