Frank, Tim,
Apparently, my assertion that engines should be compared on an equal cfm
basis needs some further explanation. What I said earlier was that a 400 ci
motor at 2000 rpm, to a zeroth order approximation, equals a 200 ci motor at
4000 rpm. Let's break that down, piece by piece. Can the carburetor tell the
difference? Basically, not; it's flowing on average the same for each motor.
There may be some subtle differences in the time variations in flow, but
that's it. Also, the same average cfm flow goes through the tail pipe, again
only with small differences in the instantaneous flow pattern. What about
flow past the valves? Here it gets a little more important which motor it
is, but still, on average, the valves are flowing the same cfm; it's just
that the smaller ci motor takes smaller, faster breaths, while the bigger
cfm takes slower, bigger breaths (same valves, cam, etc.). One can speculate
which is more efficient (best volumetric efficiency), but the difference is
relatively small. The main point is that only when these differences are
compared at the same cfm can they really be attributed to the any real
effect of engine displacement (or bore/stroke ratio), whereas comparing two
engines of different displacement at the same rpm, that is at different flow
rates, is simply comparing apples to oranges. Should one really expect
engines flowing at different cfm rates, all else being equal, to have the
same horsepower output? Of course not.
BTW, I think Tim's "diminishing returns" means the bigger engine grenades
sooner.
Bob Palmer
rpalmer@ucsd.edu
rpalmerbob@adelphia.net
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