Quote:
Originally Posted by mwebb
..."(1) the actual mass movement of the exhaust-gas "slug," at about 300 fps,"...
i always assumed the speed of the exhaust "slug" column would change based on RPM and load ...... are you saying the speed is a constant ? 300fps more or less ?....
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...no, the
optimum velocity is ~300 fps(*) at the engine rpm (or just slightly beyond) where you want the
HP to peak.
...and, yes, you are correct, cylinder displacement and engine RPM are the two parameters that most affect which pipe diameter to use to achieve the desired mass slug velocity of 300 fps(*) at HP RPM.
...basically, you're exchanging cylinder volume and piston velocity for pipe volume and pipe velocity. Piston velocity is a function of engine RPM while pipe velocity that you want is basically a constant 300 fps(*) at that same engine RPM:
Vg/Vc = Ag/Ac, or:
Vg = Vp*Ag/Ac
where:
Vg = gas-slug velocity, fps
Vc = piston velocity, fps
Ag = area of pipe, sq-in.
Ac = area of piston, sq-in.
...hence, obviously, an engine RPM of 6,000 (for HP) will dictate a larger diameter than an engine RPM of 2,000 (for FE)...which is the point being made.
(*) ~300 fps for race engine, 80% of that (~240 fps) for street vehicles.