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1986 BMW 325 with 383 Chevy conversion

1K views 13 replies 5 participants last post by  stevej66 
#1 ·
Have vibration in driveline at 3500 rpm and up. I'm running a 383 Chevy V8 (400+ HP), a 83 Camaro T5 5 speed, a balanced GM driveshaft with a manufactured adapter for BMW diff, and a 4:10 posi BMW differential. Everything's fine in all five gears until I reach 3000-3500 rpm and that is where it starts a fine vibration that gets worse as the rpm's increase and is really bad at 5000. I was sure at first it was the motor but I can rev it up to 5500 rpm out of gear with no vibration. Now I wonder if the misalignment of motor/trans to differential through the driveshaft could cause the vibration.
Thanks,
Steve
 

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#2 ·
Hard to say, I built a couple of 240 Z's with SBC, one had a bad vib. at freeway speed and it turned out to be one of the 4 driveshaft bolts had worked it's way out (stupid me for not tightening it). Since you ran your engines rpm up and got nothing ???? Could be driveline, unbalanced tire, lots of stuff to look at. Almost worth pulling the driveshaft and making sure it's still balanced properly. Good luck with your search.
 
#4 ·
I was told yesterday by a local speed shop that the vibration has to be coming from the engine because of the rpm issue at 3500 in all five gears. They also said the reason it dosen't happen in neutral is because there's no load on the motor.
Thanks for all the help/info,
Steve
 
#5 ·
Sounds strange to me that an internal vibration to the engine is sensitive to whether there is a load on the crankshaft. If this is an imbalance on the crankshaft assembly, and that's easy to do on a 383, it should be there whether the engine is under load or not. You may feel it more whit load because the engine is reacting movements though the mounts into the body but it ought to be there all the time. So are they saying based on observation or from your description of what's happening.


Bogie
 
#7 ·
That could very likely be where the problem is. A 383 can be built either externally or internally balanced. I tend to build them internal but most are probably built external.


The flywheel and damper need to match the way the crank is balanced. If the crank is internally balanced then the flywheel (or flexplate) and damper are neutral balanced. If the crank is externally balanced then the flywheel or flexplate and the damper have an small offset weight and are not neutrally balanced.


Look at the damper if the backside has what looks like a notch then the crank is externally balanced and needs a flywheel in your case or a flexplate in the case of an automatic that has a piece of offset weight on it.
However, if the damper's backside is a smooth diameter then the crank is neutral balanced and so must be the flywheel/flexplate.


One can generally say that which ever type of balance is on the engine the flywheel that replaced the flexplate is not the correct balance type.


Bogie
 
#10 ·
I'm with Bogie on this.

If there was no vibe with the flexlate for the automatic trans but now there is after a change to a manual trans flywheel....then the flywheel does not have the same balance as the flexplate it replaced.

Two-piece rear maen seal or one-piece rear main seal block and crank??
 
#13 ·
Based on this I'd highly suspect the pilot bearing is gone. With the trans in neutral or the clutch disengaged the load on the input shaft is pretty small. But with the trans in gear moving the vehicle the input shaft will run the extremes of its clearance in the pilot which will shake the trans input bearing and shaft. You might also find the big bearing is damaged and where the output shaft rides inside the input shaft that bearing may also be damaged.


Bogie
 
#14 ·
That makes sense to me. Also this trans is an 83 and is suppose to use 90w gear oil but I used ATF because it shifts better with ATF. There's a big controversy on line about using ATF in trans that are suppose to use 90W but I used ATF in my last T5 and when shifting with 90w it would grind on a fast shift but no problems with ATF.
Thanks,
Steve
 
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