Why you should avoid flat-back diff covers

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Gale banks rear flat back differential Banks Power

Gale Banks is back with the next installment of his fascinating series on rear differential science. We’d highly recommend watching Part 2.1, as this is where Banks first introduces us to the brilliance that is the clear diff cover. This transparent lexan unit provides us a killer view of the chaotic churning fluids that live beneath your car.

The most recent episode—Part 2.2, as Banks is calling it—dives deep into the realm of aftermarket “flat-back” differential covers. These lids are designed to increase the oil capacity and raise the static fill level within the rear end, which, along with finned aluminum construction, are said to dissipate heat better. But what if these covers are have an unintended impact on the flow of oil within the housing?

Banks sets out to test this theory by modifying an existing flat-back cover. He and the team machine the flat portion, adding a lexan viewing window along with LED lighting, but leaving the flow physics unchanged. And when the Dodge Ram fires up on the lift and goes into gear, the differences between stock and aftermarket covers become apparent.

The cover’s two additional 90-degree bends (versus the smooth contour of the stock setup) greatly alter the path of oil over the ring gear and to the pinion. Banks worries that this type of cover starves the ring/pinion gear interface, due to the more randomized dispersion pattern and noticeable increase in oil aeration. He also hypothesizes that the newly introduced sharp bends in the lubrication path could lead to increased heat within the fluid.

Here’s the question: is the increased oil capacity of a flat-back differential cover worth these tradeoffs? Banks doesn’t think so, going as far as calling the whole thing a “fool’s errand.” And in the the next episode Banks Power will test multiple options on a chassis dyno to measure their true impact. We can’t wait to geek out with Gale when he does.

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