Bill wrote:
>Supercharged piston engine aircraft had short pipes for the same reason
>that drag motors do--to get the exhaust gases out of the way as fast as
>possible. they weren't looking for a few inches of vacuum at the exhaust
>port to help scavenge the engine. some supercharged motors have exhaust
>pressures in excess of 500 psi, whereas conventional engines might reach 70
>psi at the valve and average 10 psi in the pipe.
Bill, I beg to disagree. Super/turbo/super-turbocharging on military
aircraft was to help them operate at their ceilings. There is not a lot of
air at 35,000 feet and even with boost, they never performed as well
as at sea-level. I think it was Michael who suggested that weight
was the reason for the short exhaust stacks, but I do not buy into that
100%. If longer primaries bought more hp, then the extra weight of long
primaries would be easily compensated for. Remember, these motors
(including the water-cooled vees of Merlin and Allison) were only turning
2,500 rpm at fast cruise. Propellor blade design was a much greater
factor in determining 1940s performance than primary length.
My thanks to everyone who responded to this thread, and I apologize
if I did not write off-list to each of the responders individually as I
usually do. I have also carried this discussion to other forums and
friends, and can make the following general statement:
Opinions on primary lengths are just that; opinions, and are based
on anecdotal evidence. This is especially so with normally aspirated
motors. That some designs work better than others is largely based on
luck, and then refined on the dyno. This may offend some folk, but
I can guarantee that for every long-primary success design, I can find you
an equal short-primary success design.
Exception: For our TR motors, Kas is the only one who has done extensive
quantitative work on the dyno. Even though Kas now suggestively refutes
his early findings in the face of new "anecdotal" designs, I think Kas will
agree with me that we have not yet seen any quantitative comparisons of
the new header designs vs the old.
Shane Ingate, sticking with Kas' recommendations, in Maryland
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