QuestionVehicle Specifics: 67 coupe 110, PG, 3:27 axle gear; the converter has been modified for a slightly higher stall speed. The transmission is using no fluid (so I doubt it is passing fluid into the diff).
Symptoms: After an extended run at highway speeds (70-80 mph), upon parking, and looking under vehicle the next day a puddle of diff lube can be found under the vehicle. The puddle size varies, but most recently it was about 10-inches in diameter. Reaching up and feeling the top cover of the diff, diff lube is found on top of the cover. As lube level lessens, amount of blow out decreases. After a period of vehicle use, gauging level of diff lube, by sticking digit into fill hole, no lube can be felt: add more diff lube and repeat cycle.
What I've Tried: Modified underside of diff cover, welding an additional baffle shield under OEM port leading to cover vent... no appreciable reduction in blow out. Replaced diff lube with synthetic lube treated (50%) with Lucas Oil Stabilizer. Following this, blow out seemed much reduced, but still present: lowered lube level approximately 1" below lube fill port, and blow out appeared to cease... until yesterday.
AnswerI'm really not very familiar with Corvairs, so I'm flying a bit blind here. I assume that the transmission has a separate lubricant reservoir (and what you say seems to support that idea), and if so, there should be no confusion about the possibility that the fluid puddle is transmission fluid, because if it were, it would be thin, red fluid which has no strong odor, and is easily washed up with water (Transmission fluid is vegetable oil, not petroleum based). Differential grease is not red, has a horrible odor - one you cannot get off your hands for days after you touch it, and is much thicker.
If you have the wrong grease in your differential, that might be your only problem. I advise you to return to the original differential grease, which is GL-4, SAE140W (NOT GL-5, and not 85W140 weight - that is too thin!!). Since you do not have that grease in there now, treat yourself to a whiff of the awful odor and drain out the synthetic oil; then put the right stuff in there.
The only time a differential will expel lubricant out the vent tube is when it is overfull and/or overheated. From your description, I suspect yours is overheating. If you smell burnt fluid, or see smoke from the vent after a hard run, that is proof positive that it is running hot.
Inspect the differential vent to make sure it is not damaged or modified from original - it is possible that it has been changed in some way so that it is not as "spill proof" as the original. Some of those vents came with a tube that went up much higher on the pumpkin (or even higher) than the place where the vent orifice goes through the housing - if someone has removed the extension tube, there will be much more tendency to leak out. I'm not personally familiar with the vent tube on the Corvair, so this may not be an issue with your car, but be alert for the possibility.
Mine wasn't a Corvair, but I bought a new Chevy pickup in 1985, and kept it for almost 20 years and over 300K miles. Every single time I drove it hard pulling a trailer, it would overheat the differential, smoke would come out the vent, and it would expel lubricant all over the underside of the truck from the axle back. It did this from new, and I had it back to the dealer to "fix" it, and their solution was to drop the grease level to about 1 inch below the fill plug. That greatly reduced the amount of trouble, but it would still do it during severe usage. I suppose the axle was running hot due to some production mistake (probably pinion bearing crush sleeve set too tight), but I just kept driving it, slowing down when it began to smoke - it never did fail on me.
I don't know if these thoughts will lead you toward a solution - if you figure it out, I'd like a follow up message from you to tell me what you find.
Dick
