timucin

hp confusion-cylinder heads

we all know that horsepower is directly related to airflow (cfm).I dont understand something why dont we just put a 400+cylinder head and receive more hp. why use dart 310's or 330's
isnt bigger better when it comes to heads?!?
why?
I know some heads suggest hi-comp(why?) like bb2xtra brodix;but they also work perfect with lower comps?


do you know any www related about heads?why?/what? stuff
or any thread ?
I have to learn more....

Tinkerer

Bigger is better only at high rpm. At low rpm ( up to 6000) the velocity is poor in the big ports. The fuel/air mix doesn't ram into the cylinder.

timucin

are you agree with : 300-320 cfm flowing heads are optimum for marine...500 to 540 cuin.

Tinkerer

Yes -- properly ported they will produce more HP and torque than big port heads even on a 540cube engine.

mcollinstn

In a port-injected motor, port velocity is a bit less important than in a carbureted or throttle-body injected motor cause the fuel charge is sprayed directly down the port to the intake valve (on a port injected motor). Low port velocity in a carb'ed motor allows the atomized fuel to "fall" out of the airflow and puddle in the intake.

High port velocity always works best but there are such tradeoffs. A port that gives high velocities at low engine speeds typically becomes a bottleneck at high rpm and does not allow sufficient flow for proper power production.

Aftermarket heads with optimized port designs can combine higher port velocities with high max flow numbers.

Large port volumes ALSO dampen the intake signal (or pulse) at the carb, which makes lowspeed jetting difficult to optimize. At very low speeds, the large port volume (and accompanying intake manifold volume) can oscillate and cause significant flow reversion throughn the carb which causes a horribly rich condition (since the air picks up fuel going in "both directions").

This is one reason that 4-valve motors can be so tractable while still retaining impressive topend performance. Two high velocity intake ports coupled with smallish valves can allow good lowspeed carburetion and high topend flow numbers. This benefit is enhanced even more with different cam timing on each intake when coupled with an intake butterfly that closes off the port to the aggressive cam port at low speeds...

Back to the original topic: Port volumes, shapes, and sizes are compromises in a street car or recreational marine application. Only in full-race applications can they be considered to be "optimized" (where part-throttle behavior is unimportant)...

timucin

I remember a company called arao ; they were making 4 valve heads for sm-chevy.
is there any company building 4 valve head for bb?