Rear end sways when givin gas on a corner

In my  71 Mach 1  I have  a new engine  based on a  429 and being now 501 cid with ca  650 hp/ 850 Nm.

There is  aluminium flywheel and McLeod  RST clutch and Toploader behind it.

At the rear  there are  Caltracs, mono split leafs and adjustable shocks  (  adjusted to #6). Tires are Nitto  555 R semi slicks.

When  cornering  and giving mildly gas in the middle of the  large radius  turn to the left  the rear end  sways in the left right direction  ( counter clock wise)  - you can feel it on your seat. The tires are not spinning ( loosing grip) anyway.

There is no sway bar at the back. I have  checked the  torques of the U bars to 65 lbs inch  as well as the rear attachment of the leaf spring  to 35 lbs  inch as wel as the Cal Tracks pre load - should be correct

Any ideas what to check more and what could cause that phenomenon ?

I don't know if it fits but I remember that mine did something similar and it was the stock locking rear axle that released. But it was a small swift shift.

Felt a little like if something in the trunk had slid sideways into the quarter panels (without the sound of course).

I now notice that it hasn't done it since I have the new True Trac rear.

What type of differential do you have? Open? Limited slip? Torque biasing? Locker?

You are probably feeling the car unload from one side to the other. A good sway bar and maybe add a panhard bar to keep it centered up better. You can also add limiting cables or chains to each side to limit the amount of lift what can happen in the body.

Check with some of the performance suppliers I am sure some makes everything you need.

Big block cars will never handle like a small block, weight balance can never be right. Just have to make as good as possible with what you have.



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What type of differential do you have?   Open? Limited slip? Torque biasing? Locker?

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Hi,

 the differential is  Yukon Grizzly 9"

That would be normal behavior for a locker.

Bentworker is correct. I have a Detroit Locker and things can get very exciting when turning and I'm on the power and my C6 shifts from first to second (has a hard shifting kit). Something like a Detroit Truetrac is better suited for the street. All non-open differentials have idiosyncrasies, though.

I don't know if it fits but I remember that mine did something similar and it was the stock locking rear axle that released. But it was a small swift shift.

Felt a little like if something in the trunk had slid sideways into the quarter panels (without the sound of course).

I now notice that it hasn't done it since I have the new True Trac rear.

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HI  and thanks of your comment.

yes.... actually you got it - that 's how it feels - not much but you feel it and the differntial is a brand new  Yukon Grizzly 9 ".

What do you mean by " it was the stock locking rear axle that released ".... I do not quite get it...

Previously I had a Detroit locker 9" and I did not feel it - as I can recall....

Why did you pick a locker?

A locker is good for the dragstrip with the kind of horsepower you are making, but it will have horribly unpredicable street manners like you are describing.  If you don't want to deal with it loading / unloading in the corners the only fix it to remove the locker and have a diffrent carrier installed.  For a street driven car I'd suggest a torque biasing (gear type limited slip) differential like a Detroit TrueTrac or a Quaife.  They are strong and don't wear like a clutch type limited slip.  They also don't require friction modifier like a cluch type limited slip.

Hard to explain what I meant with that. It felt like the differential locked when hitting the gas and immediately opened up (released) as there was no traction problem and the car was turning. It felt like something would let go or open up in the rear axle hence my use of "release" for lack of a better word.

The clutches in the stock locking differential (posi-traction) break loose under load and when turning.

"Rear end sways when giving gas on a corner".

I'm biting my tongue on this one; it is soooo hard not to crack a joke...just a few more bites and I swear I'll do it!

I really need to get my mind out of the gutter.

"Rear end sways when giving gas on a corner".

I'm biting my tongue on this one; it is soooo hard not to crack a joke...just a few more bites and I swear I'll do it!

I really need to get my mind out of the gutter.

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It says "giving" not "passing"! 

Butt I see what you mean.

The clutches in the stock locking differential (posi-traction) break loose under load and when turning.

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This is what I was trying to say! 

My English skills are starting to rust.

This is a similar situation but my ‘72 fastback seems to shift left when I accelerate on a straight line.  It happens when I hit the gas aggressively but straightens out once i let off. It isn’t a feeling of “out of control”, more like the rear axle assembly shifts or is somehow loose.  I have checked, rechecked, had a fellow gearhead check and everything is tight and as it should be. I’m running a moderately built 351C with a T-5 grafted in and 3.50 gears on a a posi rear end.  Tires are 235/60-15 BFGoodrich radial T/A. Any advice is appreciated.

Do you have subframe connectors. The engine torque and twisting will twist the entire body. Did you check tire pressures? How about traction bars? Weak springs on one side can cause it if the springs on one side wind up more. Are you sure the posi is working correctly, and doesn't have worn out clutches?