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advice in this forum.]----
If I knew it was this easy to get you two guys going, I would have posted
that question earlier :-}
So in amongst this verbage another question comes to mind, is the C90 a 90
hp or a 95 hp engine, and where do I go to get these torque/horsepower
charts to compare one against another. If the C90 does infact produce 95
gross horses, all else being equal, then the change would seem to be more
economically inviting, however do either of you, or anyone else that has
manage to follow this thread this far, have any idea of the cost of just
regrinding the crank by 10 or 20 thou, as opposed to buying crankshaft, rods
and pistons.
Another question is if it takes so much to get from 85hp to 90/95 hp, how
come it is so simple to go from 75 to 85 with just the minimal change of
venturi size. Do the charts tell a different story, and was this just some
advertising mumbo jumbo ???
I guess the bottom line is that most of us can handle the cruise speed, but
most want better climb, especially on those hot summer days.
Regards,
John H.
----- Original Message -----
From: "John Cooper" <[EMAIL PROTECTED]>
To: "'Coupe-Tech'" <[email protected]>
Sent: Wednesday, February 22, 2006 6:42 PM
Subject: Re: FW: [COUPERS-TECH] C-85 STC, Performance,
Props-Correction/Addition
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advice in this forum.]----
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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