Hi TGO,

I recently finished a t56 swap in my gta with a spohn crossmember and crossmember mount adjustable torque arm. I've been trying to chase down a low RPM and throttle bucking that develops in the driveshaft. Between 800 and 1700 rpm the rear end starts oscillating in a feedback loop that gets worse until i accelerate harder (>20% throttle). I can't figure out if the driveshaft is bottoming out or what exactly, but if its the rear end oscillating that's hammering the rest of the drivetrain i need to fix it before damage happens to all the new bits i just put in.

Some notes:

• 1987 gta l98 with 94 t56 swap
• re-used 700r4 driveshaft with t56, as it is the same length
• adjustable torque arm has not been adjusted. Crossmember mount side thread is extended 1/2"
• oscillation starts under 20% throttle in every gear before developing a feedback loop that hammers the entire drivetrain
• rear end is fine, transmission is fine, engine is fine. Oscillation is located to front half of driveshaft near tail housing on transmission (can feel it at low speed with your hand on the tunnel when it starts bucking)
• wear pattern on driveshaft shows 1" exposed wear pattern on yoke when vehicle at rest on a loaded suspension
• all parts in good shape and torqued to spec
• u joints in good shape, no binding, steel driveshaft, no vibration
• oscillation (bucking) was not there before the swap

Is it possible for the 3rd gen driveshaft to bottom out in this swap? If it's the same length as the 4th gen driveshaft, is something else going on? It must be something simple i'm missing...

This thread had the same issue and it was never resolved: https://www.thirdgen.org/forums/tran...ml#post2478197

Any thoughts would help!

 

My t5 kind of does this too, I end up slipping the clutch slightly to 'reset' it. Mainly when it's cold in my case. It soemtimes does it when warm in too higher gear. I don't worry about it.

 

I would expect it around idle and just shift down, but up to 2000 rpm? That's more than 1/3 the useable range off the engine, lol. It happens when it's cold or hot. Something else has to be going on. Any thoughts?

 

I had a similar issue after I bought my car. It was lowered and the lower control arm angle was way off. Some LCA relocation brackets fixed most of it. I would check your torque arm adjustment.

 

Hmm, good info. Would the torque arm angle have that much of an effect? I know it has to be -1* to the pinion and it's definitely a bit more than that at the moment.

Edit: Posed some questions to Spohn about it and there's a definite possibility it's the torque arm angle being out by a lot. Going to crawl under the car tonight and see if I can get in spec.

 

It could cause some issues like that. Take pictures of everything and post them. You will probably need a magnetic angle gauge.

 

Alight guys, for anyone in the future that searches out this thread, the issue was the driveline angle. The true pinion angle was -5*, which is way out of spec and it was causing the ujoints to bind at part throttle. I reset the torque arm to -1* as Spohn suggests and it completely eliminated the problem.

A quick search on google will help to understand how to set the angle (driveshaft angle at rear of transmission - pinion angle = true pinion angle), which should be -1* for a stock fbody car. Both may be negative when you start, so basic order of operations math applies.

Thanks again for the help guys!

 

Glad you got it worked out