Some driveline vibration questions...

[GTJake]

I'm back for spring break, so I decided I would rip off the balancer and check it out because I've been getting a jackhammer vibration through the shifter above 3.5k rpms in first, second and third gears. This all started right after I installed the Romac too. Well, anyways I forgot a breaker bar, so I'll be headed back to where my cars stored tomorrow to check it out.

While I was there I rev'd it up while in netrual well passed 3.5k rpms and the balancer looked great. Smooth and zero vibration that I could see coming from the engine. What could be the problem? Some people say it's not the DS because it's rpm dependent and not speed dependent. Could the stock balancer have been cancelling out a vibration from the tranny and then when I got the Romac on there, the vibration from the tranny showed up? Also, if it was the balancer, wouldn't it vibrate in all gears and not just first, second and third? What sort of things in the tranny would cause this kind of vibration?

Sorry this is so long, just trying to kinda figure this problem out before I dump $3k into my engine.

Jake

 

[VibrantRedGT]

Well if doesn't vibrate while sitting and only when moving it could very well be the driveshaft. If it's all the time then the balancer is suspect. I had severe vibration after 70MPH in the shifter. Aluminum driveshaft cured that issue.

 

[GTJake]

The vibration is rpm dependent and not speed dependent though...

It only does it in first, second and third above 3.5k rpms. An aluminum DS is a cheap option though. I may try that before anything else. Otherwise I think it may be the tranny falling apart, although there are zero symptoms besides this vibration.

Jake

 

[VibrantRedGT]

You will need the aluminum DS either way. Hopefully it's not the tranny.

 

[Rootus]

You're positive it doesn't do it in 4th or 5th -- depending on your gearing those are harder to verify. I could see the logic in that, since 4th and 5th are a different setup than 1st, 2nd, and 3rd. It's interesting that the shifter is taking so much abuse -- it's on the tail end of the transmission, not up where most of the action is. I had a problem with my shifter vibrating, but it was directly related to speed not RPM (and turned out to be a bent mainshaft).

Dave

 

[GTJake]

I'm positive there is no vibration in fourth or fifth. I've had it above 3.5k rpms in both of those gears and there was no vibration. How do you mend a mainshaft? Will the tranny still work perfectly fine with a bent mainshaft?

Jake

 

[GTJake]

So what parts could it be if it only vibrates while in gear? DS, rear end, tranny?

Jake

 

[Killercanary]

I don't know if I mentioned this to you or not, but one thing to try is to remove the tranny and clutch fork. The start the car and rev it up with the clutch and pressure plate attached. If the vibration is still there, remove the pressure plate and clutch disc and repeat... if its still there then the problem is internal or in the flywheel/balancer. One way to chekc the balancer is to hold a screwdriver up against it until it just touches the balancer, if it is "wobbling" you'll surely feel it this way. If you do all of this and it doesn't vibrate, but does so with the tranny in the car, its a tranny problem either in the input shaft or something else internal. This is the only way I know of to know for sure.

 

[GTJake]

The problem though, is there is no vibration when the car isn't moving, so taking the tranny out wouldn't really tell me anything. How could the input shaft get damaged? I'm worried that it's the balancer, because all this vibration started right after it, but the balancer looks perfectly fine and there is no vibration when the car isn't above 3.5k rpms in first, second or third. Your almost positive it isn't the DS though, Paul?

Thanks for the reply.

Jake

 

[Killercanary]

Jake, if its rpm dependent, it can't be the DS as the DS turns with the tires. Think about it this way. Turb the car off on a hill and put it in neutral... the car will start rolling down the hill and the DS will turn with the tires... the motor will not.
Here's what is rpm dependent:
balancer
rotating assembly (crank, rods, pistons, valvetrain, cam, etc)
flywheel
pressure plate and disc
input shaft

MPH dependent:
Mainshaft
driveshaft
ring and pinion
differential
axles
wheels/tires

If you have a RPM dependent vibration, definitely look at the things that spin with the motor, if its a MPH problem look at things that turn with vehicle speed. Do you still have your old balancer? Maybe throw it one. Did you triple chekc the part number on the balancer with romac to make sure they didn't send a 28oz or neutral balance in a 50oz box? I'd also call romac. Maybe you have a defective one. Romac makes a great product, but everyone messes up every now and again.

 

[Killercanary]

Jake, did you check out that link I sent you on driveline vibration? That article made me think twice about aluminum DS's... I still have one though for weight loss. It changed NOTHING in terms of vibrations in my car.

 

[Joes95GT]

Assuming no mistakes were made on the part of Romac, I would rule out the balancer for the simple fact that it is brand new and it's of much better quality than stock. Sure you never had/noticed a vibration with the stocker, and when you switched to the Romac, you did get the vibration, but why would the stock balancer cover up a vibration that the aftermarket one does not. It's just not logical to me unless the balancer was installed incorrectly, but I don't think that would be the case because you would know if it was.

Have you done simple things like fill the trans up with fluid and that type of thing?

This is a guess with some theory behind it:

We know that acceleration (not speed), is greater in first gear than in fourth gear, and because of this, the rate of acceleration of the driveshaft will be faster in first gear than in fourth gear. This can also be said about second and third gear as well. Because the revolutions per minute of the driveshaft are increasing at a much faster rate in first gear, do you think it is the driveshaft because it is being spun at such an abrupt rate that is not noticeable in the higher gears because the rate of change of revolutions is not as great or abrupt in the higher gears?

Just a guess......

Joe

 

[GTJake]

I threw out the stocker....so I'm SOL there. I need to call a machine shop to see if they can check the balancer. I don't know if it's possible, but it's worth a shot. Thanks for the replies guys.

Jake

 

[Mustang78]

hey killercanary, how would you start the car with no tranny?? lol the starter would have no place to mount, right? i guess you could just bolt up the bellhousing...still to me all that work wouldent be worth it..also are your tires balanced?? it couldent hurt to check the weights

~Mark~