A C85 should be marked for 4.5 quarts, not 4.0 The reason is based on a certification requirement. An engine must be demonstrated to have adequate cooling capacity with 1/2 of the total oil capacity. A C75 apparently can cool adequately with 2 quarts and is therefore marked at 4 quarts. The C85 requires a little more. The oil capacity is not a limit, so it is legal to fly with less than a full tank.
There is no technical explanation for a properly configured C series engine to "blow out the first quart". The engine design is a "semi dry sump" and the oil in the sump is unaffected by anything the engine is doing. The only explanation anyone has been able to come up with that makes any sense is a leak at the filler cap. This could cause a pressure differential between the sump and the crankcase as long as the level of the oil is sufficient to completely cover the filler neck where it enters the tank. Once the level falls below the top of the tube any pressure difference bleeds off and things return to normal. Just a theory, though... I have a C90 which I fill to 4.5 quarts. It uses 1/2 quart in 4 hours or 1 quart in 8. Doesn't matter if I add a half quart in the middle or not. The first pint goes no faster or slower than the second. I try not to let it get below 3.5 quarts. Oil is cheap insurance. John Cooper Skyport Services 4996 Delaware Tnpk Rensselaerville, NY 12147 518 797-3064 www.skyportservices.net
