Hi Nick,
The decision of "what rpm to cruise at" is today less a technical one than a personal one. It relates to one's "comfort zone".
We can access experience and knowledge entirely unavailable to the engineers making decisions back when the Ercoupe and its Continental powerplant were designed and built. Making decisions by referring to information long superseded, such as leaning only when operating above 5000' MSL, denies a pilot the benefit of many advances that have taken place over these many years.
The same basic engine design carried forward from the C-65 into the improved C-75 and C-85. We now know that the longer-stroke higher horsepower C-90 and 0-200 easily and reliably cope with the operating stresses associated with cruise rpms up to 2525 rpm and maximum sustained rpms in excess of 2700 rpm. Common sense suggests the older, shorter connecting rods of the C-75 and C-85 should handle the lesser reciprocating loads at such rpms if materials, fabrication or testing methods used do not meaningfully differ.
Accordingly, for whatever prop is fitted, I would want a C-85 (or one with the 0-200 Crank STC) to just be able to reach 2575 rpm in cruise at gross weight at 2000 MSL on a hot summer day. Anything less adversely affects climb performance. I would point out that a "strong" C-85 will turn a more aggressive (coarser) prop pitch than one that is "weak" (for various possible reasons).
Some operating from higher elevation airfields whose priority is climb performance should seek a prop pitch that allows the engine to reach 2575 rpm in climb on a hot summer day. What's so "magic about 2575 rpm? That is the rpm at which the camshaft grind was intended to allow a C-85 to develop "rated" horsepower. Accordingly, in the considered opinion of a long time performance automobile enthusiast, that should be a "sweet spot" in terms of a lot of things that go on in an engine doing what it is supposed to do under load.
Flying cross country I routinely cruise at 2574 rpm with the lesser reciprocating stresses at 6,000' - 10,000' MSL (and higher density altitudes). I do not believe either reliability or longevity of a C-85 in good condition to be reduced in the slightest by such practice.
The plate on your motor is likely the one affixed when it was originally assembled as a C-75. The conversion of a C-75 to a C-85 must be "...in accordance with Continental Service Bulletin M47-16, dated 6-7-47" per item 110 of ATC 718 [p. 4] or ATC 787, Note 4.(d) [p. 11]. Quoting from p. 3 of said Service Bulletin:
In order to complete the conversion it will be necessary to order from this office and install a new Engine Identification Plate which is properly stamped according to the new model number." Perhaps one of our certificated mechanics (I'm not one) can speak as to the sufficiency of overstamping the original plate, or the installation of a non-Continental plate with the appropriate information stamped thereon in lieu of having followed the exact procedure specified by Continental, each of which I have observed in the field.
I'll leave it to others more knowledgeable to help you with your header tank problem.
Regards, William R. Bayne .____|-(o)-|____. (Copyright 2009) -- On Sep 12, 2009, at 14:17, nick.king88 wrote:
What turns a 75 into a 85. If I understand, it is just mods. My log books make reference to my C-85 , but also mentions theC-75. The plate on the motor says 75. I need to know so I know what rpm to cruise at. I have a 50 pitch prop. Also , my header tank leaks (cracked at bend in rear)How do I remove it.
