Have rebuilt the entire suspension, new motor etc.. Running Spohn torque arm and crossmember and an Energy Suspension poly-pro transmission mount (700R4).

At 70-80mph in overdrive, engine at 2500RPM I get a wierd "beating" noise, a pulsing vibration. If I put the car in neutral at this speed the noise goes away. I assume this clears the driveshaft - which is new and high speed balanced.

At 70-80mph in 3rd, engine at 3500+RPM, no vibration.

Under hard acceleration I may get a bit of it.

I have a B&M holeshot TC. I am guessing that the engine speed and the TC speed are close but not the same - this might create the pulsing.

Has anyone run across this before? Any ideas?

Thanks

 

It sounds like driveline angle. The poly mount has a plate that goes between the trans & the mount. This makes the trans tailshaft too high in some cars, increasing the driveshaft to trans angle. More than 3 degrees can cause vibration. The same could be true at the rear of the driveshaft. The Spohn arm is adjustable & yours may not be quite right. You ideally want the tailshaft up 2-3 degrees in relation to the driveshaft & the pinion nose down 2-3 degrees.

 

I agree about the ES mount, If you installed the plate with it try removing it. I kept ripping the stock rubber ones. So I installed the ES mount and got vibrations at certain road speeds. I took out the preload plate and the vibrations went away. With out the plate the ES mount is really close to the same hieght as the stock one I pulled out. I didnt measure it, but side by side they look the same.

 

Thanks for the info. I did leave the plate in the ES mount and had the same problem with the stock rubber mounts - they would shear into two pieces after a month or so.

I set the pinion angle at -1.5 deg. but the accuracy is questionable due to the resolution of the gauge - it's not that great but I really haven't found a good one. I have a range of pinion angles from about +1 to the current setting. -1.5 deg. did reduce the vibration quite a bit, it now only happens at very specific speed and load conditions.

 

OK, double checked pinion angle - set it at 0 just for a test. Vibration still there, next step I'll remove the plate on the trans mount. I'll keep posting on this thread until I figure it out.

 

0 degrees can cause a vibration as well. When engine torque acts on the drivetrain it changes the driveline angle. That is why you need 2-3 pos at trans & 2-3 neg at the pinion. when the engine torques, the trans goes down & the rear comes up, zeroing both out.

 

if your getting a vibe at certain rpms and you put it in neutral and it goes away forget about the drive shaft and pinion angles, it's something else, i have a vibration in mine at 2300 rpms i have a manual push in the clutch goes away, try this sitting in the driveway put it in neutral rev it to those rpms and see if the vibes are there, if they are it's something between the motor and tranny

 

Not to be difficult BUT I had the same exact problem. If i'd put it in neutral the vibrations would go away. In my case the driveshaft was bent and missing one of the balancing weights. I know yours is new but if nothing else works...........

 

Again, thanks for the help. The latest episode:

Checked driveshaft, torque arm adjustment, driveshaft/tailshaft angle - all good.

Road test: (one of many)

80mph in overdrive - beating vibration, frequency about 1-2 per second. Put in it neutral - it's gone. While still in neutral (@ 80) bring engine revs up to 2600-2700 - it's back but reduced in intensity.

3rd gear - sets in (a slightly different sound) at about 2700 RPM, same deal with neutral and engine RPMs.

Under hard acceleration in 1st or second - not watching the tach but the vibration appears and I run through it quickly - just a couple of beats.

My guess: Engine and/or torque converter out of balance at certain speeds. Vibration wasn't noticed until I rebuilt the suspension with all polyurethance on body side and metal Heim everwhere else. The hard connections are coupling the vibrations to the car.

Talked to my cousin that used to race, he says "drive it, this is normal for the way you have set your car up."

Any other ideas?

 

I had a vibe at 2500 and it went away when I pulled off the AC belt. However, now I have vibe at 90 MPH that seems to be a rear end issue. When you check the pinion angle make sure the top of the axle to the bottom of the frame is 119 mm or ~4.65 inch. Then validate the pinion angle. The service manual says you may need to add weight to the car to meet the 119 mm. I have my rear at -2.0 degrees, never checked the front, but plan to do all this to see if my 90 MPH vibe goes away.