ebola - "...OHC heads has less friction load, needs less lubrication, big valves and parts have more heat sink.."
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
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