427street

I just replaced the rear suspension. While the diff was out, I took off the cover to check for wear, but everything looked fine, so I replaced the side yoke seals and rear gasket and put it back together.

I drove about 20 miles tonight, then crawled underneath checking for loose fasteners, leaks, etc. and found the differential is hot, not burn-your-fingers hot, but definitely hot.

To be honest, I had not checked it before I took things apart, but there was no indication that anything was wrong. How hot should it get during normal operation? I expected it to be warm, but not this. What am I looking at here?

Oh yeah, there is fluid in it - 2 bottles of GM gear lube and one small bottle of GM positraction fluid.

Jvette73

If you try the synthetic Mobil1 gear oil it should cool it down some. Maybe as much as 20 to 30 degrees cooler. The synth is that much better at reducing friction. And the friction is whats building the heat.

427street

I didn't think about synthetic at the time. Do you still need to add the small bottle of positraction additive or would it be incompatible with synthetic.

In any case though, is it normal for the diff to get hot?

Shark Racer

The Mobil1 additive claims to work just fine w/ LSD, so I would assume no additive necessary. I added Mobil1 to my friend's 1990 300ZX (LSD) and it did just fine, no chattering.

-Steve

boredsoIstrokedit

Use the posi additive if you use Synthetic or not. What Mobile 1 means is that the lube will not hurt LSD. :cheers:

ddecart

Probably nothing to worry about. The heat that is. The diff is going to get warm under normal use just from the internal friction warming things up. If it gets HOT hot, I'd be more concerned.

btw, I'd still put the posi additive in regardless of what fluid you have. But damn that stuff smells BAD! :U

ROD NOCK

:iagree: totally.

FeedVaal

Unless you have side pipes, the exhaust runs underneath the differential. That tends to warm things up. Doh! You do have sidepipes. NEVERMIND....


[Modified by FeedVaal, 12:44 AM 8/13/2003]

Chuck Harmon

Yes, the differential will get hot during normal use. This is one of the reasons flywheel horse power and rear wheel horse power are different. The 12-15% power you loose between the two measurements is typically lost in heat. Consider how many BTU's of heat equal the approximately 80hp I lose from flywheel to the rear wheels. If you were to really work the engine, both the trans and the rear end will get very hot. Racing cars often use oil coolers for their rear ends. I believe my ZO6 also has an oil cooler for the rear.

Chuck

pws69

I rcently changed my '69 over to Mobil 1 and I DID use the bottle of GM additive. Made a huge difference - really quieted things down (also didi my 4 speed at the same time - shifts real nice).

427street

OK, after the feedback from everyone, I began to think I was overreacting so I took it for a longer drive tonight - 1/2 hour drive at highway speeds. Now its HOT HOT. Too hot to touch. This can't be normal can it? And what would cause it?

I am getting no unusual vibrations or noises - at least not yet.

ddecart

I think one or more of us needs to go for a drive and feel the diff when we get home. I really doubt that you have a problem if everything otherwise seems fine.

Chuck Harmon

I know the transmission under the console of my Z makes anything in the console compartment above very hot. I can imagine how hot the transaxle must be, even in normal driving. Energy cannot be consumed or expended without heat. Even a 5 hp drive train loss produces heat equal to almost 13,000 BTU's per hour. This is just about what it would take to boil 100 pounds of water (12 gallons) starting at room temperature.

Chuck

ROD NOCK

This heat does sound extreme, but I may be wrong. Two low cost methods I would use would be to wrap the exhaust in the diff area, and change the diff lube out to M.1.

gkull

Mine gets so hot during spirited driving that you can't touch it for a long time after shutdown.

I've considered adding a oil cooler. My problem is 4.11 rear gears lots of hp and big tires. The pinion runs at very high rpm (7000+rpm). I use valvoline synthetic Multi weight. something like 100w 130. one tube of posi and 8 ounces of Energy Release friction inhibitor

427street

I tried a few things this week and thought I would post an update. I finally got a chance to compare my car with another vette.

You guys were right. The diff just gets that hot during normal driving - I never noticed before. In any case I did change to Mobile 1 synthetic. It still gets hot, but the synthetic makes a noticable difference. I did not add the tube of positraction additive, but I am getting some chatter, so that is my next step. Otherwise everything appears to be OK.

Thanks for the feedback.