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Postby stang51d » Fri Feb 22, 2008 9:14 pm

It's all about how big of a prop you have and how hard/fast can you turn it. Everything else is just smoke and mirrors.

Two 140s, same size of prop, one has a 85 climing at 2500rpm and one has an o-200 climing at 2500rpm, you get the same climb. The ???? is not, what does some dyno say you have, but what will it swing.
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Postby gcgilpin » Mon Feb 25, 2008 12:24 am

The only functional difference between the C85 (with 0200 crank rods and pistons), C90 and O200 are the camshafts.

The C85 'stroker' ends up being very close to the stock C90. If a true comparison were done (ie. use two fresh engines, on the same airframe in the same weather... and choose an RPM limit ... ie 2575) you will not see any difference.

The 0200 cam is a 35 degree overlap camshaft while the C85 / C90 camshafts are 24 degree overlap camshafts. Less overlap / duration moves HP / torque to lower RPMS. C85 stroker / C90 outperform O200 up to about 2550 RPM. After that the O200 wins.

The C85 camshaft does have less lift than the C90 / O200 camshafts, however tests and calculations have shown that this is not necessarily a disadvantage but possibly an advantage. Camshaft design is not clear cut. Be careful what people tell you. Some people use the terms 'duration', 'overlap' or 'scavenging' causing the rest of us to assume they know what they're talking about. Don't be afraid to ask questions.

Oh and by the way... the old C90 530788 camshaft is not the 'end all be all'. It's torque / power curve falls somewhere between the C85 or newer style C90 (531076) and O200. Again... there are many people out there who will tell you that it is a magically camshaft... not true. It has less overlap than the O200 but more than the C85.

If you want an engine optimized for lower RPMs (ie. 2400-2550 RPM) go with C85 stroker or 531076 C90. If you want an engine optimized for higher RPM go with the O200.

Or if you just want to extract the most horsepower from your engine in a given situation (ie climb vs. cruise) pitch your prop accordingly.

From a guy who has spent way too much time researching this stuff... don't worry so much ... just fly your airplanes.
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Postby Randy Thompson » Fri Mar 07, 2008 11:44 pm

I have just overhualed two C-85's with the O-200 crank STC and I have to report that they are very strong. When doing the run in I found them to have a lot more thrust than regular C-85's. They seemed to be more reponsive too.
A&P IA. Holder of STC for O-200A's in 120, 140 & 140A's. Engine overhauls on C-85's ,C-85's with O-200 crankshaft, C-90's and O-200A's
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