Pickup Trucks 101: Differential Cover Conundrum
For those looking to improve cooling for their pickup truck's rear axle and provide more strength, better performance and/or ease of use with a new differential cover, there are a variety of aftermarket options. But do they really perform better than factory covers? Let's dive into some of the thoughts and engineering behind aftermarket differential covers to see their strengths and weaknesses.
We will cover four basic issues: impact and torsional strength, fluid capacity, better cooling and ease of changing the differential oil.
Benefits
Strength: Having a stronger differential cover can provide extra safety for pickup owners with high-horsepower engines or for those who head off-road. Many aftermarket covers are thicker than factory covers, sporting additional cross bracing for improved strength and cooling. All of this helps keep the ring-and-pinion gear correctly aligned, provides more protection during impacts, and increases the rigidity of the axle and differential.
Fluid capacity: Increasing the amount of fluid in a differential has the potential to decrease the operating temperature of the fluid. However, the increase needs to stay within certain limits; adding more differential fluid than recommended by the manufacturer can potentially reduce fuel mileage, increase heat on certain parts and cause more overall stress over time. One of the easiest ways increase fluid in the differential is to buy a cover with more fluid capacity.
Cooling: Vehicles from the factory are designed to be used in a certain way. When things are changed on the vehicle — for example, adding oversized tires, hauling heavy loads or increasing power — or when the vehicle is consistently driven hard or in extremely hot environments, additional cooling might be needed. Keeping the differential oil cool will prolong the pickup's life and therefore prolong the life of everything the oil is protecting. Aftermarket manufacturers use expensive materials such as aluminum, which conducts heat quite well, to provide more differential oil cooling. They also incorporate more complicated design elements, such as adding cooling fins, to allow more heat to be dissipated into the air around the cover, thus reducing the oil temperature.
Ease of maintenance: How easy it is to perform maintenance on factory differentials varies by manufacturer. Some vehicles have a drain plug on the bottom of the differential, making fluid changes simple and straight-forward. Others require the differential cover be removed to drain the oil; in this case, the mounting surfaces will need to be cleaned and a new gasket installed.
Aftermarket companies have come up with a variety of helpful solutions to these challenges that are integrated right into the differential cover. The most common solutions are adding a dipstick to check the oil level, placing a drain plug in the bottom of the cover and adding a magnet to the dipstick or the drain plug to pull out metal contaminants. When a magnet is added to a dipstick it's easy to check for signs of damage or excessive wear without having to drain the oil.
Negatives
Fluid level: When driving forward, to provide proper lubrication and protection the ring gear must carry oil from the back of the differential, up and over the top, and to the pinion bearing and pinion gear. Aftermarket and vehicle manufacturers have designed their differential covers and designated proper fluid levels to promote a precise amount of oil flow to accommodate the most common usage parameters. The level of the fluid may vary depending on how the axle is set up; therefore, aftermarket and manufacturers using the same axle have been known to require different fluid levels for work-duty variations. The bottom line is that all differential covers should provide owners with a way to check fluid levels; right now, that's not the case.
Capacity: Aftermarket differential covers create issues with the recommended fill level and the total amount of fluid. Filling above the manufacturer's recommended level can cause negative effects such as increased heat and reduced efficiency and oil life due to the extra work being done on the oil — not to mention the possibility of overstressing seals. Most aftermarket differential covers have the same fill level as factory covers, but some may not, so be sure to check aftermarket against factory fill levels when considering a differential upgrade.
Fluid dynamics: More oil will reduce heat and prolong the life of the differential oil and everything it touches. However, fluid dynamics are more complicated. Manufacturers design differentials around a specific amount of oil so that once the gears are moving and up to proper operating temperatures, everything is properly lubricated without having an excess of oil. The extra oil capacity of an aftermarket differential cover might have the same negative result as over filling the differential, reducing efficiency and creating higher temperatures. Of course, that depends on the specific design of the differential gears and differential cover.
Shape: Ideally, aftermarket differential covers should maintain the relative internal shape of the factory cover to promote appropriate fluid flow. Unfortunately, our research indicates that many aftermarket differential covers differ in shape from their factory counterpart. For instance, having sharp angles along the path of fluid flow will require more sheering force on the fluid which could prevent the fluid from reaching where the places it needs to go. This will reduce efficiency, decrease oil life and likely increase heat buildup. Some companies such as ARB USA, American Expedition Vehicles and TeraFlex all closely mimic the shape of factory differential covers but offer more strengthen and other benefits. Other big-name brands such as Mag-Hytec, Nitro Gear & Axle and AFE Power focus on adding fluid capacity, features for cooling and features to make maintenance easier rather than the cover's shape.
In Summary
Ultimately, choosing a differential cover replacement is much more involved than it appears. Knowing what the vehicle is going to be used for, how long it's going to be used in that way and other modifications made to the vehicle will help you choose the best differential cover for your pickup truck. For most vehicles that are used normally, the factory cover is likely best, but for special applications like hard off-road use, heavy trailer towing or extreme power outputs, the benefits of an aftermarket cover will likely outweigh the negatives.
Gale Banks, president of Banks Power Engineering (an aftermarket performance equipment manufacturer), has done extensive research on rear differential covers and we think it's worth a look. He shares some of that information (look for all three parts) in this YouTube video.
Cars.com photos by Mark Williams
