Quote:
Originally Posted by lust4speed
There is also a lot more heat generated in the rearend with the low ratios. The heat problem with 3.90's and lower wasn't much of a factor with the old Muncie because we worried about what the extended term RPM was doing to the engine. While we might have bumped speed up for a few miles with the lower gearsets, we didn't get on the freeway and go for an hour or more at high speed. We tend to forget that that little pinion is madly spinning away since the engine now loafs along in overdrive.
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Which is no different than most trucks that are 4.10 and numerically higher, but a truck differential is usually larger in overall construction. Allowing for at least twice the oil volume of a standard car differential. The smaller overall construction only allows for so much oil in the available space.
Ford style 9 inches used in nascar have fluid coolers and pumps, because of the sustained time they are used, (roughly 3 hours) even though the ratios are mostly 3.50 and numerically lower, they still build heat running in a high heat environment, exhaust proximity, and black asphalt that routinely can be in excess of 140 degrees.