why pushrod?
why pushrod?
(OP)
In sweden last year one respectable race team owner said to me - "for drag race use pushrod engine only".
Since then Im searching for suitable block - my project is small cc alcohol engine, and this claim makes me wonder - why? Now I have to choose some old block, and actauly 100%remanufacture it rather than use OHC better design, some german engine trouble - free gadget.
If I understand right, OHC heads has less friction load, needs less lubrication, big valves and parts have more heat sink - but on the end it doesnt sound like something important.Any sugesstions on that claim? Thanks.
Since then Im searching for suitable block - my project is small cc alcohol engine, and this claim makes me wonder - why? Now I have to choose some old block, and actauly 100%remanufacture it rather than use OHC better design, some german engine trouble - free gadget.
If I understand right, OHC heads has less friction load, needs less lubrication, big valves and parts have more heat sink - but on the end it doesnt sound like something important.Any sugesstions on that claim? Thanks.





RE: why pushrod?
Pushrods offer no advantage over OHC engines except for simplicity of cam drive and no need to remove cam if you remove heads between rounds. Engine performance and durability of valve train both favour OHC.
With OHC and bucket type followers, you cannot run inverse flank cam grinds, but you can if you use a rocker arm and a roller follower.
Regards
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RE: why pushrod?
RE: why pushrod?
The flex in the pushrod is a bad thing and the trend is to stiffen all the valve train components.
So in the end you can have a much more aggressive valve action with it being amplified by the ratio.
I guess you could design a overhead cam with larger rocker ratios but you are definitely limited with a bucket over cam..
Cheers
I don't know anything but the people that do.
RE: why pushrod?
RE: why pushrod?
If there are real positive performance benefits from pushrod engines I'm all ears.
RE: why pushrod?
If you want an example ... take a look at a traditional Ford 5.0 V8, and compare it to the OHC 4.6 V8 "mod" motor that replaced it. The 4.6 is physically larger.
But, in general, pushrod engines have been on the way out for automotive applications. OHC has other advantages besides space that are becoming more important. Easier to implement variable valve timing, for one thing.
Pushrod engine layouts are mandated in NASCAR and drag racing because that's the way it has always been ...
RE: why pushrod?
One advantage of a pushrod engines is easy head removal if necessary. Many years ago (around '90 I think) there was a pit crew race for the Top Fuel teams. (Top fuel - long skinny cars with supercharged 500 cid engines) They had to:
- start the engine then shut it off
- take off supercharger, heads/headers, cam, clutch, pistons
- reinstall and restart
I think the winning time was about 8 minutes!
ISZ
RE: why pushrod?
RE: why pushrod?
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: why pushrod?
I've heard of folks in the import drag racing scene who are getting 600+ horses out of Honda DOHC 4-banger car engines, and you can get 500+ horses out of a 1300cc Suzuki Hayabusa DOHC 4-cylinder motorcycle engine. Yes, you have to change everything in order to do that.
RE: why pushrod?
To say that OHC engines have less friction is a gross generalization. It depends greatly on the type of cam drive, number of cams, number of valves, and valve actuation scheme being used. Same for "lower lubrication requirement". A gear driven cam with solid lifters will need less oil than a chain driven OHC w/hydraulic lifters.
As far as big valves - if both motors are 2 valve/cylinder then it is cam independent. If it is a 4 valve/cyl engine you will have smaller valves -- but the total airflow will probably be more than the 2 valve engine.
I have also read that the air flow advantage of 4 valve heads goes down some as the compression ratio goes up. If you have a hemispherical chamber you need a high dome piston to raise the compression ratio - and this has negative affects on flame travel and air flow. The other choice is to flatten the valve angle and leave the piston flat. This puts a sharper bend in the runners and increases valve shrouding - which reduces airflow.
If you want suggestions post where in the world you are. Of course opinions are like belly buttons ... everyone has one.
ISZ
RE: why pushrod?
1. Cost
2. Cost
3. Cost
4. engine packaging
5. Cost
RE: why pushrod?
I think the new Chrysler Hemi engine also does this, although with only 2 valves per cylinder and pushrods. Less included valve angle than the old Hemi, relatively flat-top pistons instead of the big dome that was on the old ones. It goes to show that the combustion chamber shape doesn't have to depend too much on the way the valves are driven, although certainly cost would favor having only 2 valves per cylinder and all of them in a straight line ...
RE: why pushrod?
RE: why pushrod?
RE: why pushrod?
RE: why pushrod?
Compare the 351 Cleveland to the small block chevy - the Cleveland had way more potential, but the SBC dominated most of the time for the same reasons as above.
RE: why pushrod?
RE: why pushrod?
Multivalve variable timing stuff is all common fuel related thing I believe.
RE: why pushrod?
Thanks for the bike info, I don't follow them too much. As for the intake angle, I agree with you but the packaging would be too much for most cars - especially with pedestrian stike concerns. The manufacturers need some space between the hood and engine to absorb the shock.
As for shrouding - a lower angle keeps the edge of the valve closer to the cylinder wall as opposed to a high angle. BUT (and I forgot about this before) the low angle does allow the other side of the valve to move away from the head more, which COULD end up a net gain. I was thinking of the SB Chevy head (granted not a OHC) and the fact that many people /shops keep going to lower valve angles.
ISZ (USA posters - don't forget mums day next weekend
RE: why pushrod?
When dealing with the huge HP these guys deal with, rarely does the one making the most HP win consistantly
RE: why pushrod?
Simply put there is a lot of garbage to make OHC work.
It is also more trouble to R+R the heads. Push rods are fast and simple. And like is noted. AA fuel dragsters seem to work okay with them.
Also take note the next time you take a OHC engined vehicle for service on such a thing as a tensioner or timing chain/belt driven water pump. Most average backyard mechanics either can't or don't want to deal with the hassel.