Vibration - Not the Good Kind

[EddieE]

I'm getting a vibration between 3100 and 3600 RPM's when I run the car hard.

Here's the specs:
340 stroked to 416 and internally balanced by Shady Dell Speed Shop
A833 just rebuilt by Jamie Passon with 3.09 1st gear
New McLeod (Borg Beck) clutch set from Passon
Denny's Driveshaft made to fit - all new yokes and u-joints
3.23 Sure Grip with new chrome moly yoke from Denny's
New OEM style motor and trans mounts (insulators), but they are the $10 parts store type
Original crossmember (it's old)

First off, when I run the car easily or at highway speeds there is little or no vibration.  So let's focus on when I'm running it hard.

The vibe is hardly noticable in first, a little worse in second, and VERY noticable in third.  By the time I hit fourth I'm usually not accelerating that hard so it's not an issue.  Again, at highway speeds with low load it's smooth.

I don't notice it when the engine is revved, with or without the clutch out.  It's not the clutch set or trans, as I just replaced both and the vibe hasn't changed, or if it did it was very slight. Vibe was there with previous A833 close ratio trans and McLeod Street Pro Kit (which slipped so I junked it).

I spoke to Ryan Johnson at Shady Dell and he suggested that it may be alignment related, particularly the trans mount.

Any help would be appreciated.  I'm thinking maybe the crossmember is off center, although it fits easily into place with the four bolts to the body.

[fantum]

I know you said Denny's built the driveshaft, but the symptoms sure sound like the problems I used to have until I had my driveshaft rebalanced.  Just my 

[blknout]

maybe a bad tranny mount. did you try getting it up to that rpm on the highway? because if its a drive shaft it would vibrate under a load or not

[EddieE]

I'm thinking it's the trans mount relationship to the drivetrain and body.  We all know the drivetrain is not centered in the car.  Does anybody (aka Chryco) know what the correct measurements would be from specific reference points on the body/frame componenents that would indicate that the tailshaft is centered in the car?  My mount has enough adjustment room (1/4" or more each way) in the mount-to-tailshaft area that I could pry it one way or another then tighten them up.  Crossmember doesn't have any play where the trans mount is bolted to it, and all of the four holes in the crossmember to the body line up with no problem.

[Chryco Psycho]

Sorry I do not have the measurement you need for center .
Do you have a pinion snubber , what spring are you running , under load the pinion gear climbs the ring gear & can create a horrible angle between driveshaft & yolk , you pinion should b 5-7* nose down from the driveshaft , using a spring with a stiff front section really helps to keep the pinion down .

[EddieE]

Sorry I do not have the measurement you need for center .
Do you have a pinion snubber , what spring are you running , under load the pinion gear climbs the ring gear & can create a horrible angle between driveshaft & yolk , you pinion should b 5-7* nose down from the driveshaft , using a spring with a stiff front section really helps to keep the pinion down .

The rear is the only thing I haven't touched, other than changing the gear oil and adding the additive for the Sure Grip.  It has the rear sway bar and the bushings all look very recent.  There's no pinion snubber installed, but the bracket is there.  It's a 742 housing.  I don't get any wheel hope at all if the tires spin (by accident).  I think the springs are original because the car sits slightly low and is level with the front end.

[EddieE]

Here's the stance.  The height looks a lot like your Challenger.

[Chryco Psycho]

Great looking Cuda 
if the front section of the spring is soft it will raise the pinion & cause a vibe through bad pinion angle & thet seesm to match the times when you get the vibe , when you are on the throttle heavy so that is the first place I would be looking
 you can use a short square top U bolt to clamp the front spring section to stiffen it , if that reduces the vibe you know you are looking in the right place

[EddieE]

Thanks.  Any suggestion on how far up the spring I should place it, and where I might find one?

[sadil340]

Been trying to resolve a driveline vibe for many years - will be watching this one. My  would be the yoke rising under load. Makes sense with it getting worse the higher the gear because of the greater torque effect.

[EddieE]

I don't have the knowledge or the the tools/dial indicator to determine if the stock aluminum bellhousing is exactly right on the engine.  Could a tiny bit of misalignment be in the mix?

[Chryco Psycho]

I put  a clamp about 1/2 between the spring eye & diff
 Standens was the company I used , you may be able to buy online , they were local to me .
stock belhousing are generally OK as I have never seen offset dowels from the factory

[EddieE]

Good info on the bellhousing.  Love conclusions that don't involve me getting more tools or smarts.

I guess the next step is the springs.  Would I be dumb to assume that the springs are at the end of their lifespan after 38 years, so it would be better to replace them?  If  so, what spec/supplier would your recommend to keep the rear of the car at stock level, and also to handle the HP and torque that is close to twice what it had in 1974?

[Chryco Psycho]

I would try clamping them first , just to be sure that kills the vibe .
 Espo springs & things seem to have a good product reasonable priced , the problem is getting the height the same , Brad might sell youhis springs but they sit higher than stock

[sadil340]

I don't have the knowledge or the the tools/dial indicator to determine if the stock aluminum bellhousing is exactly right on the engine.  Could a tiny bit of misalignment be in the mix?

One of the steps I went through when trying to solve my vibration was to index the bellhousing to the crank using a dial indicator and offset dowels. It was quite a bit off. Centering the bellhousing did not solve my vibration but it is sure easier to R&R the trans now!