Bob, thanks. This resolves some of the difference but not all. If I
run three different cases, all the same displacement but with different
rod and stroke combinations, then the HP and Torque curves track up to
about the torque peak but then diverge with the higher bore to stroke
ratios making more HP and torque.
I have read that larger bore engines have an advantage because the
larger the bore the less valve shrouding and therefore have an advantage
with respect to breathing but I doubt this software program is
considering that. Also, we know that the long stroke engines have more
frictional forces in the cylinders, maybe the program is accounting for
the higher frictional losses of the long stroke motors which would show
up more at higher RPM. I kind of doubt the software is looking at that
either but maybe.
Frank
> -----Original Message-----
> From: Bob Palmer [mailto:rpalmer@ucsd.edu]
> Subject: RE: Desktop Dyno Fun
>
>Here's how I think it should be
> analyzed. To a first approximation, engine horsepower scales
> as displacement times rpm. That is, a 400 ci engine at 2,000
> rpm will have about the same horsepower output as a 200 ci
> engine at 4,000 rpm (but half the torque of course). In your
> case, if you proportion the rpm scales for relative
> displacement between a 306 and a 331 engine, then the two
> curves should be very similar if not identical. For example,
> take the HP curve for the 306 engine and divide the rpm scale
> by 306/331 = 0.924. Then the HP peak will shift down by about
> 8% and of course will fall off faster too, which matches the
> behavior you describe.
> Bob
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