Hi Paul,
Good general rule of thumb is that with PROPERLY TIGHT baffling, indicated oil temp. should be roughly 100ºF above ambient.
Many coupes over the years have "lost" the small baffle (hard to see, see 17-47, p. 22 of Parts Catalog) between the right air intake (facing the nose bowl) and the crankcase which closes a considerable gap between the engine and the nose bowl. Others have a generally poor baffling seal after well-meaning (but uninformed) try to "level the engine baffling (it's SUPPOSED to slope forward). Sometimes the little between-cylinder baffles (17-44) are not there. Sometimes the breather has been rerouted leaving an unplugged area between 17-45 and the crankcase.
Cooling air takes the path of least resistance. If it can escape through gaps in the baffling easier than going down around your cylinder fins (and cooling them), it will. Open your cowling when it's dark all around and shine a flashlight or place a work light under your baffling. Any gap of 1/8" or more needs attention. The flexible material intended to seal between your side baffles (17-42 and 43) and the side cowl skins, and the back baffles (17-39, 36 and 40) and the top cowl eventually becomes stiff and must be replaced.
The area behind the front cylinder air intakes and above the horizontal baffles is supposed to be "high pressure. The cool air is intended to be forced by a pressure differential down around your cylinder fins, etc. into an area of (relatively) "low pressure" and then exit the engine compartment between your side cowls/bottom cowl and the fuselage. The more the pressure difference, the better the cooling efficiency. If there is a gap between the carb air box and the nose bowl, it can raise pressure in the "low pressure" area and greatly reduce cooling efficiency.
That having been said, I have seen high oil temperatures for a time climbing out from a desert strip in 100ºF+ ambients into the cooler air up high.
I try to hit a balance in "cruise-climb" for 85-90 mph forward speed and a reasonable rate of climb. There is no reason for concern for such TEMPORARY, understood and accepted higher temperature operation.
C-75/85 recommended temperatures are from a time when aviation oil had few effective additives. The load bearing lubricant film began to break down at a lower temperature than the oils with additives wehave today.
Most circumstances allow one to reduce power. The engine's ability to get rid of heat is not reduced as you reduce the heat produced (fuel burned), so your oil temperature should go down as a result. The worst situation is departing a high-altitude airport (like Albuquerque, NM) under high density altitude conditions in the heat of the day. Been there, done that (at 11am and full gross). DON'T DO IT! You need more power than may be available to get out of the superheated air at the surface and stay 1000'+ above the rocks. If you aren't off by 10am (if at or near gross under such conditons), stay overnight and depart early the next am.
Tip: If you see buzzards, manuver into a position below them. The thermals they ride will speed your climb into cooler, denser air.
Once any temperature problem is resolved, have your mechanic restore your carburetor mixture control to proper operation. It works fine if the disc openings are clean, used correctly. There is no benefit to wasting fuel and increasing 100LL deposits when operating above 5000'.
You also can't lean for best power on high-altitude take-offs, which I see as a safety issue.
A disabled mixture control could void one's insurance (aircraft no longer conforms to the aircraft type certificate as approved) if there is no FAA-approved paperwork for such major powerplant modification.
Regards,
William R. Bayne
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(Copyright 2002)
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On Jul 12, 2004, at 7:27 AM, Paul Mays wrote:
Hi everyone,
I recently purchases ercoupe 94170 a C model with a C85, and I want to thank all of you for your help with advice on how to find one. I am mostly happy with this aircraft but have one concern. Having come from a glider and ultralight (2cycle) backgound, I'm somewhat obsessed with engine operations and in particular engine temperatures.
My problem is that this engine registers (I think) high in all categories with 400-450CHT, 1500EGT, and oil temps typically 125 degrees above ambient at cruise. According to what I've heard and read in the manuals these values are all on the high side. Is this right?
I realize that with these thermocouple based instruments there maybe a 50 degree instrument error, but given that they all seem high, it seems to me that it is running to lean, even tough I've been reassured by my mechanic that the carb is wired full rich.
I'm not sure what I should do, if anything. Am I worried about nothing?
The engine has about 120 hours on a major overhaul and I'd like to keep it fresh as long as possible.
Any Infromation of advice will be greatly appreciated
Thanks in advance.
Paul Mays
Chattanooga
