why pushrod?

[ebola]

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.

 

[IceStationZebra]

BrianPetersen - " the trend in motorcycle engines has been towards a very low included valve angle and either flat-top or slightly dished pistons. The bend in the intake port is reduced by having the intake runners at approx 45 degree inclined angle relative to the head. It's easier to get decent squish-bands in the chamber with a low included valve angle without shrouding the valves as much. The slight theoretical reduction in valve area by doing this doesn't seem to be an issue.."

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 )

 

[wgknestrick]

This statement is correct (with respect to top fuel) / tear down after run style drag racing. It is mainly due to assembly time as mentioned above. OHC always out performs push rods when comparing equal displacement.

When dealing with the huge HP these guys deal with, rarely does the one making the most HP win consistantly

 

[dicer]

The cam drive is much less complicated on a push rod engine.
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.