On Sep 28, 2004, at 10:32 AM, Skyport Services wrote:
At 11:05 AM 9/28/2004 -0400, LLoyd Perkins wrote:This is the second time I have heard that
changing the venturi in your carb (on a c75)will make the engine an
85.......Please explain this one to me. I thought there were a lot more
differences than the size of the venturi??????
As far as Continental is concerned, the only difference between the C75 and the C85 is the carburetor. All other parts are identical. Oh, yeah, the oil filler cap has to be remarked to say "full" mat 4.5 quarts. Of course, the RPM limitation (Red Line) is different and there are necessarily airframe specific issues. Prop, trim tab and baffling, in the case of the Ercoupe.
Rich:
The C85 is eligible for installation on the C and CD, so there is no LSA issue. It may look and smell like a C85, but it won't perform like one if you don't have the prop repitched.
John Cooper
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Hi all,
While no one has offered any inaccurate information in this thread as of yet, it's still kind of a jumble. The motivation for going from C-75 to C-85 is to get more usable power. The regulatory gobbledegook is a distraction from our "bottom line" goal(s).
Fred Weick stated many times that with his original selection combination of engine and Sensenich 74FC propeller it was possible in a clean, light, well rigged steel gear Ercoupe (thru SN 812) to cruise at rpms sufficiently above the 2275 "maximum" of the C-75 that you pretty much HAD 85 HP with a C-75 (for all practical purposes). He also acknowledged that "he was all wrong...people wanted to "officially" HAVE a C-85 (presumably for "bragging" purposes).
So, the devil is in the details. By the way, the 415-C and CD are NOT required to have to have the extra baffles required on the C-85 in an A-787 coupe operating at higher gross weight.
As to the prop, it's MORE than just re-pitching. Since most coupes now have McCauley metal props, going from C-75 to C-85 means the allowable diameter range of 71-73" and static (wide open throttle) r.p.m. not under 1950 and not over 2100 changes to 69.6-71" and 2025-2225.
Longer propellers are more efficient than shorter ones, other things being equal. Fred "wrote the book" on propellers while at NACA. The "well-rigged" C-75 would have had a 73" diameter prop pitched to make 2100 r.p.m. static to perform as Fred described. The factory got 127-129 m.p.h. with new planes in 1945 thru spring of 1946.
To change above installation C-85 specificatiion, you chop 2" off the prop diameter and pitch it for 2275 r.p.m. in cruise a bit below full throttle (out of the carburetor's "automatic enrichment" area of operation) for lower fuel consumption. A propeller so modified is less efficient than one originally manufactured and shaped to be 71" diameter because the longer prop is thicker at the hub and on out. Do this and you may not get much more usable power, if any!
If you had a 71" "cruise" prop on the C-75, then you might have to re-pitch it to get into the new static r.p.m. range. Depending on how bad a match this prop was before, you might see improvement in performance by re-pitching (per the preceding paragraph).
The "well-rigged" C-85 would need its 71" prop pitched per the above. Such a prop would almost certainly have a static r.p.m. within the 2025-2225 range allowed. Only after a coupe is so "optimized" (and operated in summer and winter) should the average owner seriously consider additional "climb" or "cruise" pitch changes.
So, is a "well-rigged" C-85 coupe faster, or does it climb better than when it was "well-rigged" as a C-75? It was as close to a "sore point" as I ever found with Fred. He ALWAYS managed to avoid a straight answer that question, as if sworn to secrecy. As Ed Burkhead says, "Results may vary" (and one day I intend to properly document such a conversion from start to finish).
Every engine-propeller- airframe combination is a compromise, giving some here to get some there. You must thoroughly evaluate what you need as to where and how you fly before you are likely to understand it sufficiently to achieve it. Again, the devil is in the details!
You, who have done this, would you share your "before-after" observations?
Regards,
William R. Bayne
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Copyright 2004
