Inlet port sizing - effect on flow

[superbike]

Say we have one port one valve and find that after prepping the seat area we find the port now does not supply the valve with enough air even at max lift.

We then carefully enlarge the port step by step and we gain flow at most lifts. At some point we will get to the point where enlarging the port even more will not gain us any more flow on the flow bench with valve fitted.

If we reached this point and the car was still lacking high rpm power would enlarging the port any more do anything worth while? I would personally at this point in time say NO but I would be really interested in your opinions please.

Chris

[4v6]

I would be inclined to suggest that the port should be made only as big as it needs to be.

If it's made larger - then velocity would suffer, so although the port is capable of large flow figures it won't work as efficiently as the smaller port, because it's flowing more slowly for the same time the valves are open.

That's my understanding at the moment anyway.

[Guy Croft]

Yes, true.

There will be a controlling section in the port, could be any part of it. In this case the valve is that thing, because we know it will not flow any more even if the port is made bigger.

It can be the other way round, where port enlargement does continue to yield gains with the valve in. Can equally well be the inlet manifold, the carb, throttle body, rampipe, airbox etc etc.

Certainly likely to impact on velocity - as discussed - if you go bigger on the port than the valve 'needs', and thus cylinder filling and torque are upset. It is a very common mistake to get the ports - valve and port flows should be closely linked.

If you want more power beyond that which valve and seat work will offer, the secrets lie in the cam profile. A lot of overlap (narrow lobe centreline angle) and big lift integral with very high rate of acceleration, long dwell at full lift and a lot of lift around tdc is the way to go, but extremely aggressive cams put a lot of strain on the springs/valve train and unfortunately this is more often than not the limiting factor. All that said, if you go to those lengths you would be well advised to get a simulation run to optimise the whole engine package, otherwise you might end up with a mismatch. For example, find that the intake event was so effective that the exhause port could not cope (pumping loss).
Asymetric inlet cams, if you can find anyone capable of doing them, with high rate opening and slower closure can yield benefits too.

GC