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At 05:54 PM 2/22/2006, WRB wrote:
The existing prop should allow the engine to
reach 2475 rpm (tweaking pitch at maximum diameter), at which engine speed
horsepower produced increases from approximately 85 hp (per Model C75 & C85
SLPC) to approximately 93 hp (per Model C90 SLPC)! So you DON'T "...get the
same power..." even though "...your rpm limit doesn't change."
I'm not really sure what you're trying to say here, but...
Forget about the modification for the moment. When does a C85 produce
85 hp? Under exactly one set of circumstances: When you are
operating at full throttle at standard conditions at a steady 2575
RPM. Nothing else (prop, airframe, attitude, etc.) matters.
How many of you fly the plane under those conditions? Any other
conditions (except exceeding red line) produce less power.
Things would be different if we had a variable pitch prop and a
manifold pressure gauge, but we don't.
Now, assume your prop allows exactly 2575 RPM at sea level at full
throttle in level cruise. You're making 85 hp. Now, have the engine
modified. Fly at full throttle at sea level in level cruise. Oops,
you exceed red line. Game over. Pitch the nose up until you are
steady at 2575. For an instant, you are making more than 85 hp, but
as you climb your manifold pressure drops and the power drops
off. (How many of you fly like this?)
The reality is we all fly and climb at RPMs considerably less then
2575 or altitudes higher than sea level and the reality of that is
that you will not make more than 85 hp, not even close under most
circumstances. You will, as I said before, have more of that 85 hp
at your disposal under more circumstances after the mod than you did before.
One caveat. If you repitch your prop so you once again can cruise
level at 2575 at full throttle, you can then exceed 85 hp under those
circumstances. But, who flys like that?
One last thing: if it takes 60 hp to turn your prop 2300 RPM at 100
mph, it doesn't matter if you have an A65 or an O-720, it'll still be
making 60 hp at 2300 at 100 mph.
Maybe I'm not making my point any clearer than you've made
yours... ;) What I'm trying to say is that if you have a "Horse
Power" meter in your plane, it will under almost all legal conditions
read less than 85 regardless of whether you have an O200 crank or
not. The few circumstances where it reads 85 or greater are so far
out on the fringe that you have to be working at that and not at
flying to get it to happen. You will, however, have higher average
readings post mod than pre-mod, just not higher than 85.
YMMV
John Cooper, A&P
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865
www.skyportservices.net
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