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Old 05-19-2017, 12:39 PM
OC_ OC_ is offline
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The dumbest thing i ever found wrong with my car.

I have an '89 firebird thats been in the family for a long time, since about the mid 90's. Ever since I have had this car it has always had a bad vibration at speeds starting at 80mph. For years I examined everything to try to find the cause of it and never found anything. I figured it was something to do with u-joint angles and i tried everything to fix it. I noticed i could adjust the angles and make it worse, but i could never get it better than a certain point. Finally, i noticed that the entire engine is not mounted square in the engine bay, and that GM must of screwed up when they made my car. The car is a genuine lemon, i have no idea how it could be this messed up. It turns out that one mount is farther forward on the engine cross member than the other, and this has upset the entire alignment of the engine and trans which has, in turn, messed up the angles of the u-joints.

I am now doing work to straighten all of this out, and wow, its brutal work...
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Last edited by OC_; 05-19-2017 at 12:46 PM.
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Old 05-19-2017, 01:27 PM
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Driveshafts have resonant frequencies. A driveshaft isn't balanced or not balanced; it is tuned to not vibrate in a given RPM range. It was not designed to run at 80mph, therefore it might vibrate there.

Regarding the engine placement; it was built on an assembly line, to drive rich highschool kids around and its a series of weldments...this wasn't built on an F1 chassis jig.

You might think about putting in an LS1 aluminum d/shaft if you can find one, it might bump the RPM ceiling for your vibration up higher; aluminum driveshafts will also cover up some resonant frequencies it seems
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Old 05-19-2017, 01:43 PM
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Quote:
Originally Posted by AutoGear View Post
Driveshafts have resonant frequencies. A driveshaft isn't balanced or not balanced; it is tuned to not vibrate in a given RPM range. It was not designed to run at 80mph, therefore it might vibrate there.

Regarding the engine placement; it was built on an assembly line, to drive rich highschool kids around and its a series of weldments...this wasn't built on an F1 chassis jig.

You might think about putting in an LS1 aluminum d/shaft if you can find one, it might bump the RPM ceiling for your vibration up higher; aluminum driveshafts will also cover up some resonant frequencies it seems
The car has an aluminum driveshaft behind a T56. I have tried 3 different driveshafts in the car, all aluminum, and and none had any difference. 80mph is nothing, no car should have trouble at that speed, especially with a 3.42 rear end gear.
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Old 05-19-2017, 01:54 PM
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When you put a longer transmission in than the car was designed for, and thereby shorten the driveshaft; you change the range at which the driveshaft would want to vibrate, nevermind that the shaft speed of a double overdrive transmission is way higher than most people realize. Irrespective of that, 3.42 rear gears has literally zero to do with my point, or your situation. It'd be interesting to know what your bellhousing runout measured when you did your install.

Good luck.
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Old 05-19-2017, 02:44 PM
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Quote:
Originally Posted by AutoGear View Post
When you put a longer transmission in than the car was designed for, and thereby shorten the driveshaft; you change the range at which the driveshaft would want to vibrate, nevermind that the shaft speed of a double overdrive transmission is way higher than most people realize. Irrespective of that, 3.42 rear gears has literally zero to do with my point, or your situation. It'd be interesting to know what your bellhousing runout measured when you did your install.

Good luck.
The car was designed for this. The 3rd gen and 4th gen f-bodies share the same dimensions and the drivelines swap between the 2 cars. My driveline is from a 4th gen, its all OE stuff and the driveshaft is OE length. But I recall the car vibrating even when it had the factory 3rd gen drive line in as well.
Furthermore, if you make a driveshaft shorter, as long as the u-joint angles are correct, this pushes the critical speed higher.

Its hard to measure the bellhousing runout when the bell is integrated into the transmission case. This is a factory t56 from a LT1 4th gen.

I dont understand why anyone would think that having different angles at the front and rear u-joints is somehow not the problem?
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Old 05-19-2017, 02:46 PM
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Quote:
Originally Posted by OC_ View Post
i noticed that the entire engine is not mounted square in the engine bay, and that GM must of screwed up when they made my car. The car is a genuine lemon, i have no idea how it could be this messed up. It turns out that one mount is farther forward on the engine cross member than the other, and this has upset the entire alignment of the engine and trans which has, in turn, messed up the angles of the u-joints.

I am now doing work to straighten all of this out, and wow, its brutal work...
The trans output shaft must be parallel to the pinion on the rear end.

Rather than mess with the motor mounts, have you looked a the trans mount? I suspect there is enough flexibility in the motor mounts that you can simply shift the back of the trans sideways to align the output and pinion shafts. Should be a lot less work (like just slotting the holes in the trans crossmember). This should better align your torque arm also.
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Old 05-19-2017, 03:03 PM
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Quote:
Originally Posted by joe_padavano View Post
The trans output shaft must be parallel to the pinion on the rear end.

Rather than mess with the motor mounts, have you looked a the trans mount? I suspect there is enough flexibility in the motor mounts that you can simply shift the back of the trans sideways to align the output and pinion shafts. Should be a lot less work (like just slotting the holes in the trans crossmember). This should better align your torque arm also.

Yeah, this was the first thing i tried. My trans mount, which is an aftermarket unit, has slots on it for an adjustments like this, but even when i loosened everything, even the engine mounts as well, I was unable to straighten it out. The way the engine connects the engine mounts is such a way where it doesn't allow the engine to be able to rotate like this. i even put slots on the engine calmshells to make it adjustable, and while this allowed me to adjust the engine left and right, it still didnt allow me to change the angle of the engine.

The 2 engine mounts create a line between them which is the plane the engine is mounted on. If one mount is more forward or back, which i believe is whats wrong in my case, this will determine the angle of the engine.

I saw that summit makes these slotted mounts for LS swaps.
https://www.summitracing.com/parts/sum-7731100
I plan on slotting my cross member to be able to act this way.
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Old 05-19-2017, 03:06 PM
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GM went to aluminum driveshafts to cure vibration problems on the later F-bodies. I use to keep one in stock 10 years ago when changing gears in a Gen3 was getting done almost daily.

Many engines are not in the center of the chassis, old C3 Corvettes,esp BBC, are offset a bunch.

As to how the engine mount location could get off on a car made in the last 40 years, I am amazed.
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Old 05-19-2017, 03:15 PM
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Originally Posted by lmsport View Post
GM went to aluminum driveshafts to cure vibration problems on the later F-bodies. I use to keep one in stock 10 years ago when changing gears in a Gen3 was getting done almost daily.

Many engines are not in the center of the chassis, old C3 Corvettes,esp BBC, are offset a bunch.

As to how the engine mount location could get off on a car made in the last 40 years, I am amazed.
The engine does not have to be in the center of the chassis, thats fine and its a good thing for weight balance to have it shifted over a bit to offset the weight of the driver. But, the engine has to be perpendicular with the rear axle, which mine is not.
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Old 05-19-2017, 06:14 PM
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Quote:
Originally Posted by OC_ View Post
The engine does not have to be in the center of the chassis, thats fine and its a good thing for weight balance to have it shifted over a bit to offset the weight of the driver. But, the engine has to be perpendicular with the rear axle, which mine is not.
I agree. Straighten it out and see what happens.
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Old 05-19-2017, 07:04 PM
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I used to call that the 80mph death wobble cause mine did the same thing. The upside is it would go away at about 90 but return at 110. I never did figure it out either. I eventually crashed it and bent the front subframe and the rear subframe in opposite directions.
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Old 05-20-2017, 08:10 AM
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Some vehicles use left and right-hand motor mounts, maybe yours was assembled with two lefts or two rights? That could be pushing the alignment out...

Russ
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Old 05-20-2017, 11:38 AM
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If I was to guess, the crossmember is not square in the car. If the mount locations on the cross member were off much at all, there would be no getting the clamshells down on them and the bolts through.
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