I'm a new owner of a 1981 resto with a zz4 crate motor, r700 tranny, BTE torque converter and 3.73 posi rear. The car handles nicely with my foot on the gas. However when I take my foot off the gas the rear end of the car seems to sway and I beleive that I can hear a whine from the gears turning in the rear end.

Any thoughts?

 

The C3's IRS is certainly better than a solid axle for handling BUT has limitations since it was designed and installed in C2/C3's starting in 1963. Without more info, the following areas could be the problem:

1. Trailing arm bushings
2. Strut rod Bushings
3. Rear wheel bearings with too much play
4. U joints worn/axle yokes
5. Differential mounting bushing shot
6. Alignment

Any unwanted movement in the rear suspension will cause the rear to feel unstable.

I have replaced the rear spring (360 composite with poly cushings since 1986), Shocks (Bilstein sports), 7/16 inch rear oem bar with 3/4 inch OEM type bar with poly bushings, tires (255/50/17 ZR's) which made the car much more stable than the 255/60/15 tires, and the one change that made a very big improvement to the rear end stability, competition adjustable strut rods with heim joint end links- NO bushings. I have not changed the rear Trailing arm bushings (needs to be done when the TA's are rebuilt with new rear wheel bearings), u-joints, or the differential mounting bushings (have a poly one waiting to install when I do the TA's). The car has 66,000 miles on it and is very stable but I am sure will benefit from the other changes. Hope that helps!

 

NOW is as good a time as any to do the pre-purchase inspection up on a lift.
everything jb said plus look for a rusted out frame.

 

What jb78l82 said.



The only thing I might disagree is that it may not be necessary to install high rate springs. I live in NJ, where 85% of the roads are terrible. A softer spring allows the suspension to follow the bumps, rather than crash into the bumps.
And the softer springs are easier on the frame, as well.
Years ago, I had the 460 lb/in springs installed on the front suspension of my '74 smallblock convertible. During an oil change, I noticed the frame was cracking around the spring towers. Had it welded up, and reinstalled the stock rate springs.
If you look at the Chevy Power book, they recommend using the higher rate springs (for racing), and then say paint the frame white so that when the frame cracks, the cracks will be easier to spot!

 

My '76 did almost the same thing... Shift left or right when accelerating / decelerating... New T/A bushings stopped it....

 

Ditto

 

I hope to never do trailing arm bushings again. If you decide to do it yourself, you are going to need a tool like this one. Mine is going to be for sale as soon as I can figure out how to sell it.

I usually don't sell tools, I collect them, however this tool is a grim reminder of the part of my life I will never get back while trying to get the bolts and shims out.

 

You should check all your bushings...

X2 on the heim-joint strut rods. Bad strut rod bushings will make your rear end surprisingly squirrelly even if everything else is in reasonably good shape. I really noticed the difference in handling...

 

johnt, I would be interested in buying the T/A bushing tool, depending on how much you want for it. Please send me a PM if you decide to sell it, thanks