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I think the new Chrysler Hemi engine also does this, although with only 2 valves per cylinder and pushrods. (First they gave up 5 valves on the MotoGP bike, and now on the 2007 R1.) Yamaha just recently gave up on 5 valve per cylinder heads because 4 valves per cylinder works better. The slight theoretical reduction in valve area by doing this doesn't seem to be an issue. It's easier to get decent squish-bands in the chamber with a low included valve angle without shrouding the valves as much. The bend in the intake port is reduced by having the intake runners at approx 45 degree inclined angle relative to the head. IceStation, the trend in motorcycle engines has been towards a very low included valve angle and either flat-top or slightly dished pistons. ISZ RE: why pushrod? tbuelna (Aerospace) 3 May 07 23:36 Of course opinions are like belly buttons. If you want suggestions post where in the world you are. This puts a sharper bend in the runners and increases valve shrouding - which reduces airflow. The other choice is to flatten the valve angle and leave the piston flat. 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. I have also read that the air flow advantage of 4 valve heads goes down some as the compression ratio goes up. 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. A gear driven cam with solid lifters will need less oil than a chain driven OHC w/hydraulic lifters.Īs far as big valves - if both motors are 2 valve/cylinder then it is cam independent. Same for "lower lubrication requirement". It depends greatly on the type of cam drive, number of cams, number of valves, and valve actuation scheme being used. To say that OHC engines have less friction is a gross generalization. Ebola - ".OHC heads has less friction load, needs less lubrication, big valves and parts have more heat sink."