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View Full Version : Can a Ford 9" slip under load??


vette427-sbc
09-20-2019, 12:48 PM
working on a 55 chevy with a stock posi unit ford 9"... At 40+ mph, something will slip under a load, similar to a clutch that cannot hold power. I get no burnt clutch smell and the clutch is new with only break-in miles on it (~500 city miles of no hard driving). I have checked TOB engagement to ensure that it is not putting pressure on the pressure plate with foot of the clutch. The motor is nothing special, a stout 350, maybe makes 425 hp.

So my test is this:
I have an optical tach reading the driveshaft speed in 4th (1:1) gear. As I roll into the throttle and power band comes in, the driveshaft speed matches the engine rpm. There is a very obvious RPM increase as if something is slipping (its not breaking the tires loose either). Ive had some local guys tell me its possible with the stock style (cone?) clutch pack. I have never heard of a rear end slipping before? Whats going on here?
badazz81z28
09-21-2019, 04:52 AM
A posi can slip, but never heard of such a thing while cruising. Additionally, one tire is locked to the carrier, you can’t have such a slip that no tires move. A posi simply turns into a one leg drive.
badazz81z28
09-21-2019, 04:57 AM
A posi can slip, but never heard of such a thing while cruising. Additionally, one tire is locked to the carrier, you can’t have such a slip that no tires move. A posi simply turns into a one leg drive.
TheJDMan
10-10-2019, 01:19 PM
There is no possible way that the driveshaft can slip in the differential unless you have broken spider gears and in that case you would not be moving at all. Clutch smell or not your clutch is slipping guaranteed!
vette427-sbc
10-10-2019, 03:10 PM
We are going to pull the center section out of the rear this winter... Every time we test, it shows a driveshaft speed WAY higher than tire (vehicle speed) when the slip occurs. Spec Clutch also says that ALL of their clutch materials absolutely make an odor when the friction material burns. No carpet, or even shifter boot on this car yet and I cant even smell a hint of burnt clutch.
I was ready to pull the clutch out of this car until I put the rear up and noticed that his rear was no longer a "posi"... Thats the only reason I even started looking at the rear and investigating the Ford style clutch packs. I will be just as surprised as everyone else if its the rear...
TheJDMan
12-24-2019, 07:15 PM
Please update this thread once you find the problem.