If you’re still concerned about your diff, you might want to pose your question on a Subaru-specific discussion board, where you’re likely to get more people with direct experience on your make/model. This is part of the reason for the relative efficiency of front-wheel-drive vehicles: because their engine and drive axle are on the same axes, they can utilize an epicyclic or spur-gear differential - no hypoid - so the differential has much better efficiency than a right-angle-drive rear diff would.īottom line, a rear diff can get pretty damn hot, and it’s OK. Sometimes you can see on pickup trucks that the diff cover actually has cooling fins on it to help it dump waste heat. It might take 15-20 horsepower from the engine to keep your car moving down the road at 25 MPH, which might mean ~1-2 horsepower (750-1500 watts) being lost to the diff as heat. So it’s reasonable to expect a rear diff to get pretty warm after cruising at high speeds. This typically involves a hypoid gearset, which has a lot of sliding motion between the meshing gear teeth, which limits efficiency. Not so much because of the differential aspect of it, but mostly because of the right-angle drive that takes the mechanical rotation of the driveshaft and puts it on a different axis. What do you guys think is going on and should my rear be so hot and bothered?Ī rear diff is inefficient. I plan to lift and crawl under the car over the weekend for a better inspection. From this data, and the sounds he heard while performing the ride, crawl, and listen session he concluded that it was a bearing and not the gears making the noise at the rear diff. I crawled under there to confirm and found that it was indeed slightly hotter on the left, and was overall cooler than in my previous temperature-sampling event. He went on to say that he found the left side of the ham was warmer than the right. He did and reported that while it was warm, it wasn’t as hot as I had described it in my ad. When we got back, I asked him to feel my rear end and tell me how hot he thought it was. So I mentioned that I had my buddy ride around w/ me, and we were only gone about 8 minutes at average speed of ~45mph. I can’t keep my hand on it after my 30-minute 50mph average (max ~ 70 mph) commute… it’s freaking hot. Sure, they are warmer than ambient, but that’s about it. I (stopped first) got under and touched both rear brake backing plates and calipers and neither of them are hot. It gets more-pronounced as it gets warmer, too. The sound is unaffected by gear selection or brake application, and the only effect that accel/decel has is to speed-up/slow-down the frequency of the sound. It gets progressively faster as the car moves faster, and I can even hear it as I come to a stop (down to ~10 mph) when it sounds like a soft grind or rub. I had my buddy ride with me and crawl around the inside to confirm that it’s coming from the rear differential area. There’s a significant whining/whirring sound coming from the rear of the car that was barely-noticeable about a week ago. How hot should the rear differential get after a 30-minute highway ride (~65mph)? Should it be so hot that I can’t touch it, or should I be able to rest my hand on it and enjoy the warmth it offers?Ģ003 Subaru WRX, stock (no modifications)
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