[1971455HO]

1971 Trans Am (LEAF SPRING CAR), not coil spring.

I swapped the original 400 trans to a 4L80E (overdrive) to make my car more derivable on the interstate. Bought the correct crossmember for the new trans, bolted right up in the original bolt holes. The trans is about 4" longer so I had a professional drive shaft company build me a shorter driveshaft. Here's my problem:
Engine & Trans are at 5* negative slope (down at rear) but my driveshaft is now at 1* positive slope (Up at rear). The driveshaft is literally running uphill to my rear differential. That makes the front joint operating angle 6*. Way to much!

My numbers front to rear are... output shaft 5* negative/down, driveshaft 1* positive/ up.... pinion is neutral @ 0*.

Figuring axle wrap at 2*-3* normal driving mode (Not Drag Racing) will just make it worse.

I need the drive shaft to be down at least 3* if not 4*. I'm hoping someone else has been here before and can tell me what I should do to correct this.

I lifted the rear of the car hoping the axle would drop from the weight as the car went up and get the driveshaft to go down, but Trans Am suspension is so stiff it did nothing, maybe 1* with the fender wells 4" above the rear tires. I need advice...
Thanks

[Cardo]

These shims will probably take care of that. You'll need to figure the correct angle / degrees but you'll need a pair shims to correct your pinion angle. https://www.amazon.com/Warrior-Produ.../dp/B004JARX70

[grivera]

I have a 4L80e in my 69 - didn’t have to monkey with the pinion angle but did have to shim the tail of the transmission up and it sits at 3 degrees sloping to rear. The drive shaft is on a downward slope towards rear.

[Scarebird]

I have a set of shims (not as nice as Cardo's) you can have for 20 USD shipped.

Note you may need to use a longer centering button.

[1971455HO]

Thank you guys. All great answers. Thank you. Not to whine to anyone, but I paid an exorbitant amount of money for what I thought was gonna be a pretty straight forward job to the local speed shop pro's where I live! They did not deliver. So I'm having to redo a lot myself. I'm getting old, but I'm better than those guys. Converted to Holley Sniper EFI, New tank w/in tank pump, new fuel system, yada yada.
First dilemma, built engine with roller cam, needed taller valve covers so instead used 1" billet spacers so I could use my factory valve covers, they barley clear the vacuum booster. So, I cant lift the trans tailshaft high enough to achieve the much needed 3 degree slope.

It needs lower/compact roller rockers to get rid of the 1" spacers and of course probably will need different length pushrods. Hoping to avoid that, but probably can't. Engine builder told me to buy only steel roller rockers, so I sourced them from Butler. Didn't know they were that tall. Aluminum Scorpions and others are much more compact...my bad.

Just for the heck of it, my car sits about an inch too low in the rear. considered raising it at the suspension, but still don't think I'll gain that much if I did.

Thanks for all of your help, ideas and comments.

[Schurkey]

You can be just about guaranteed that the rear leaf springs are broken if they're OEM GM springs--never mind the actual original springs the car was born with.

Firebird, Camaro, NOVA (Nova, Omega, Ventura, Apollo) rear springs are infamous for breaking where they're clamped to the axle housing--invisible. The Summit replacement springs I bought are acceptable, not fabulous--but that's OK, they're discontinued now. They each needed a spacer (NOT included, I had to fabricate the things) or the rubber pads won't properly clamp the spring, and the first time you nail the brakes the axle will shift rearward and squash the transverse muffler. I learned this the hard way.

BEFORE you screw with pinion angle, get the rear ride-height where you want it--which may or may not be the OEM ride-height.


Photos from a '77 Nova, as an example, attached.

There's no reason to use tall valve covers with roller rockers. Stud girdle, sure. Roller rockers, no. Get shorter polylocks, and potentially modify the stock valve cover baffles.

[Scarebird]

You can also go manual brakes for clearance.

But Shurkey is right - get the axle springs sorted out first.

[JLMounce]

This issue starts presenting itself as the car gets lowered. I had to deal with it myself after a 4l80e swap in my 69 bird. Same situation, the driveshaft travels upward to the rear end.

You'll likely have to shim both the transmission mount up a little while also shimming the differential yoke down to account for axle wrap with the leaf springs. If you have very tall tires in the rear, going to a shorter tire will lower the rear end in relation to the engine which will also help.

If you just can not get any of the angles right, you can do a CV joint in place of your offending U-Joint (Likely at the transmission output). Then you only need to worry about the rear u-joint working angle. Kind of expensive though.

[Scarebird]

Originally Posted by JLMounce

...If you just can not get any of the angles right, you can do a CV joint in place of your offending U-Joint (Likely at the transmission output). Then you only need to worry about the rear u-joint working angle. Kind of expensive though.

In the end the cheapest and best solution - if you chased this issue like I did.

[Singleton]

this is what you want for angles. If your trans is 5 degrees, then you want the pinion and the same parallel angle, as the picture shows.

[JLMounce]

Originally Posted by Singleton

this is what you want for angles. If your trans is 5 degrees, then you want the pinion and the same parallel angle, as the picture shows.

Accurate, but with a leaf spring rear-end, you have to account for axle wrap. The pinion moves a lot more under power with a leaf spring setup than it does with a 4 link like you see in the A-body.

Under normal driving the pinion will rise around 2-4*, under heavy load it may rise as much as 6* depending on the leaf pack.

[Dragncar]

Originally Posted by 1971455HO

Thank you guys. All great answers. Thank you. Not to whine to anyone, but I paid an exorbitant amount of money for what I thought was gonna be a pretty straight forward job to the local speed shop pro's where I live! They did not deliver. So I'm having to redo a lot myself. I'm getting old, but I'm better than those guys. Converted to Holley Sniper EFI, New tank w/in tank pump, new fuel system, yada yada.
First dilemma, built engine with roller cam, needed taller valve covers so instead used 1" billet spacers so I could use my factory valve covers, they barley clear the vacuum booster. So, I cant lift the trans tailshaft high enough to achieve the much needed 3 degree slope.

It needs lower/compact roller rockers to get rid of the 1" spacers and of course probably will need different length pushrods. Hoping to avoid that, but probably can't. Engine builder told me to buy only steel roller rockers, so I sourced them from Butler. Didn't know they were that tall. Aluminum Scorpions and others are much more compact...my bad.

Just for the heck of it, my car sits about an inch too low in the rear. considered raising it at the suspension, but still don't think I'll gain that much if I did.

Thanks for all of your help, ideas and comments.

You could run a 9" duel diaphragm booster and it will get you more clearance and the same braking power as a 11" single diaphragm booster.
https://www.ebay.com/itm/30524503382...Bk9SR8LEku-8Yw

[JLMounce]

He could actually run a dual 8", gain slightly more room and actually have a bit more vacuum assist. A dual 8" diaphragm booster has 100.54 sq in of area while a single 11" diaphragm booster has 95.03 sq in of area. Within 5% and likely unnoticeable compared to his brake assist now.

The dual 9" booster would increase brake assist by over 30%.

[Scarebird]

Dual 9's are good down to 9 in Hg.

[1971455HO]

Originally Posted by Scarebird

In the end the cheapest and best solution - if you chased this issue like I did.

Yes...CV Joint...Thank you so much, That's the fix I'm gonna go with.

I appreciate all of you guy giving me your help. I really appreciate it.

[1971455HO]

Originally Posted by JLMounce

This issue starts presenting itself as the car gets lowered. I had to deal with it myself after a 4l80e swap in my 69 bird. Same situation, the driveshaft travels upward to the rear end.

You'll likely have to shim both the transmission mount up a little while also shimming the differential yoke down to account for axle wrap with the leaf springs. If you have very tall tires in the rear, going to a shorter tire will lower the rear end in relation to the engine which will also help.

If you just can not get any of the angles right, you can do a CV joint in place of your offending U-Joint (Likely at the transmission output). Then you only need to worry about the rear u-joint working angle. Kind of expensive though.

Thank you for sharing your experience with me. I like the CV Joint alternative. Going with that. Thank you.

[Schurkey]

I wouldn't be inherently opposed to using a fancy U-joint or two on the driveshaft. I think you'd be better-served by trying to correct the driveshaft-angle problem before covering the problem with more-complicated U-joints. Or at least correct the angularity issue as much as practical, before resorting to fancy U-joints.

ESPECIALLY true if part of the problem is failed rear leaf springs. Aside from the actual springing, they're also an axle-locating device; when they crack they lose the ability to properly locate the axle in high-demand situations.

[1971455HO]

This is pretty long, please forgive my wordy explanations.
I guess I should clarify that my rear height is not the product of old, bad, weak or broken rear leaf springs. I bought new leaf springs from Eaton last year. I thought by virtue of them being 54 years old that they might be the reason for my rear being down a little lower than the front. I replaced them with stock factory height springs. I worked my butt off swapping them out this past summer only to find that the new Eaton springs left it 1/4" lower than the factory ones were....No Cigar!
Anyway, turns out Herb Adams (Firebird Suspension Guru) designed the Trans Am to be slightly lower in the back.

My engine/trans is at 5* negative slope. My stock valve covers with 1" spacers contact the vacuum booster when I raise the transmission tailshaft about 1/2". That puts the engine at 4* negative slope. I think I could get it to 3* if I could lift it higher.
The first thing I gotta do is yank the valve covers off, remove the 1" aluminum spacers, remove the too tall PRW steel roller rockers. A member here informed me he has a higher lift roller cam than me and it all fit under his factory covers. He is using Harland Sharp roller rockers. If I can lower these valve covers I should be able to get my transmission a little higher up in the tunnel (hopefully leaving a little room without contacting the floor pan, it's pretty close). If I can lift it up to negative 3* with my driveshaft @ negative 2* I would be in the ballpark. I then could simply to set my pinion allowing for axle wrap to wherever I need it. The car drives and handles well right now. at 50mph there's no vibration. I want to be able to drive it as fast as I want to. I just don't want to hurt the front U-joint. It's at 6* sitting still, I know it's probably at 8 or 9 cruising on the highway at 50mph. Hope this explains my dilemma. Thanks to everyone trying to guide me in the right direction.

[nas t eh]

I have PRW steel roller rockers with stock valve covers, I only needed a 3/8” spacer on the valve covers to clear the rocker/polylocks. I have a solid roller cam with 0.583 lift.

So I think you can run shorter valve cover spacers and raise the engine and trans a bit.

However getting rid of the PRW rockers should also be on your list of to do items, they have a horrible track record of failure at the roller tip pin and I wish I had never wasted my money on them.
I have a set of Crower Enduros to replace mine as soon as I get a chance.

[62posbonny]

On my LS swapped Cutlass i had driveline vibration due to u joint working angles. That was with only 3° downward tilt at the trans, but lowered car made the driveshaft run uphill like you describe. I tried and tried to get it figured out moving my pinion angle but in the end having a custom CV joint driveshaft made was the only solution. Now it is smooth at any speed, so certainly found and fixed the problem.