Inlet vs exhaust flow

[superbike]

If I were to find out the percentage of inlet to exaust flow how do I do it ?

Do I just test inlet at full lift vs exhaust at full lift or inlet vs ex bare port flow ?

What percentage do I need? Some one 'put my fire out' by saying the 75 percent rule was made in Nascar with engine revving to 9000 only.

What do I need for a motor spinning to 12,000 rpm ?

I haven't tested many exhaust seat combinations but would we be guessing by saying an exhaust seat with a 60 throat would flow less than a 75 throat due to the 75 making the seat wider ?

All comments welcome

[Guy Croft]

You're right about the throat angle, because the bigger the throat diamter the more gas it will flow. The throat or the port may be the controlling section, ie: the controlling section is the part that flows the least. This is true of the inlet side too.

The E/I ratio ( a term I coined for brevity) is the ratio of exhaust flow to inlet on the flowbench. You can measure it at the max valve lift the cam is going to give with valve in or bare port. I suppose doing it with the valve in has more validity than just bare port flow really, though I rarely bother with valve-in ex flow tests. If the ex/in valve area ratio is over 70% you're going to be fine. Eg: 29mm ex, 33mm inlet gives a ratio of valve head diameters of 77%, so you know the ex valve per-se is not too small (the thing to watch is that the ex port flows enough on bare port flow test, otherwise your generous ex/in valve area ratio is not going to help much).

If, say the bare port flow of ex is 100cfm at 10" and the inlet is 140cfm you have a ratio of (100/140) x 100% which is about 72%. I like to see at least that nomrally aspirated and pref 75% and that's what my big valve 270bhp + spec Vauxhall XE heads have with 33/29mm valves. Anything over that is overkill really. Below 70% and you might well have a pumping loss problem. On turbo engines I like to see 80% +

During supertuning for big top-end power you need to maintain an E/I ratio of 75% by progressively increasing the ex flow to keep the ratio constant. And if you can - the thing that determines how much power the engine gives and whether it's high enough up the rev band for your needs will mainly be the inlet flow and the camshaft characteristics. You need a whopping lift integral and a lot of overlap to get any n/a engine to produce power at very high rpm and it always means losing it at the bottom end.

GC