The new motor is up and running but I've got a vibration that only seems to occur in a specific RPM range. In park, if you bring the revs up slowly, it occurs between 1500-2500...any lower seems fine, and any higher seems fine?? The vibration is bad enough that you can see the car mirrors bouncing so much that you can't see an image in them.

A LOT has changed in the formula, so I'll try to hit everything. Any and all suggestions welcome! Most of the items were added at the same time, so I can't say for sure that anything is exempt from suspicion.

Basically I had a forged 383 built for my A4 95Z. The builder supplied an SFI flexplate that was too large (168 tooth) which is supposedly balanced like stock and the motor was balanced to this flexplate. Since it wouldn't clear the starter, I picked up a B&M 20238, 153 tooth with weight, late model SFI. They looked similar except for size, and both the builder and a local machine shop said it would be interchangeable and not need to be rebalanced. We suspect that since the vibration is not constant, and doesn't appear to get worse with higher RPM, this is NOT the cause.

Second, the tranny had a Transgo kit installed...can't see why this would cause it either, although I did install the firming washers and tighten the belt pretty good. I don't think that spins in park? <shrug> I may need to drop the trans/torque converter to see if the problem goes away. Also, I had picked up a used Vigilante and had it checked and rebuilt by Precision Industries prior to install though, so I ASSuME it's fine. The car has an aluminum 1LE shaft already, and since the problem occurs in Park, we know it's not the problem.

A BMR k-member with poly mounts went in. The tranny has a 1LE rubber mount...can't remember if I just installed it, or had it in for awhile. I only mention these since isolation has changed.

I put a new 7" pulley for the Vortech on, but pulled the belt and the vibration is still there. Last night I installed the old smaller pulley and will check tonight if anything changed.

A completely new exhaust system, consisting of SLP 1-3/4" headers & y-pipe, custom intermediate pipe, carsound cat, and Flowmaster muffler is on the car. The exhaust tips seem to dance quite a bit, but we assume that's just the cam (224/236-114) causeing it. At this point, we're leaning towards some sort of harmonic in the exhaust possibly???

I have the stock flexplate, and the one the builder supplied. I've thought about taking them down and having them tested to see if they are the weighted/balanced the same (size is different)?

The car was dead smooth before all these mods went on. Any ideas on what to check and how to check it?

 

What crank do you have?
Was it internally balanced on both the front and rear?
What harmonic balancer are you running?
What flexplate was the crank balanced with?
Were you running the blower before themotor rebuild?

 

It's a Trickflow 4340 one-piece rear main seal crank I believe, I don't have the specific number handy.

Not sure on the balanced front/rear...however, I did send the blower pulley and stock harmonic balancer to the builder and was told the motor was balanced like a stock LT1 so that if I ever needed to replace something externally, I would be able to. All the weights in the stock harmonic balancer are removed...not sure what that indicates? The harmonic balancer is one with the inside machined so that the Vortech pulley sits inside it.

The crank was balanced with the larger flexplate that I had to remove, but again according to the builder and the machine shop I bought the new one from, it did not need to be rebalanced to it.

Yes, was running the blower before the rebuild...that's the cause of the rebuild actually. The blower was rebuilt while the new motor was being installed, but we've disconnected the belt to rule it out.

 

soooo... If you were running the same balancer/crank pulley before...
then the options i see are...
The front of the crank my be externally balanced like an older 350 (mabye the engine builder didn't know LT-1 were internal in the front from the factory.
or
The larger flexplate is not balanced the same or is put on backwards or with the pin in the wrong spot
or the TC is not balanced correctly.
Those are the possibilities I see
Talk to the guy who did the balancing, thats the eazy way to figure this one out.

 

Could any of those cause it to occur at a specific RPM range though? That's really the only thing that confuses me.

The guy that did the balancing is DEFINITELY an old school 350 guy. I'll talk to him again, but I need to understand it fully myself first. How should it have been balanced as far as the front/rear goes??

Also - the new flexplate was installed correctly, with the correct side facing out, and noting the pin location, even though NO pin was installed in the crank.

 

LT-1s came internally balanced on the front and external on the rear. Older 350s were all external. They had a big balancer with lots of weight on it. both use the same flexplate because both are balanced the same on the rear. And yes, it could, that just how harmonics works. If you pull the wheel weights off a tire it will vibrate from say 60 - 75 MPH and then it will be smooth as can be. Diveshafts can be the same way.

 

Okay, I just spoke to the builder who also did the balancing. He said the motor was externally balanced on the rear using the larger flexplate (a mistake by parts supplier), but internally balanced at the front. After I went to bolt up the starter, he verified with the parts supplier twice and they claimed it was correct each time. Finally they looked into it more and they eventually agreed the number of teeth were incorrect, but claim the balance is the same (definitely suspect).

The crank hub (new with keyway) and the harmonic balancer (original factory stock)were both zero balanced. Turns out I did not send him the blower pulley, so I will remove it tonight and run with just the factory balancer to rule out the blower pulley.

He recommended taking my stock LT1 flexplate to a local machine shop and having it compared to the incorrect larger one. If that checks out, he'd suspect the torque converter next. I'm guessing that torque converters should be zero balanced as well? Is that something I can have checked once fluid is in it??

Just a random though, but are LS1 converters any different as far as balance goes? I need to verify with the guy I bought it from, but I think it was in an LT1 car originally.

Does everything sound reasonable?? Thanks!

 

I just spoke to Precision Industries (Vigilante manufacturer) and they said if it was the converter it would continue to get worse with RPM, which makes sense. Any converter built/checked/rebuilt goes through their leak check station and are neutral balanced on an electronic balancer. They were more than willing to help by checking it out again if needed. I'm gonna focus on getting the flexplate checked and then try dropping the blower pulley, trans, and converter to see if the problem is still there.

 

yeah my bet is its the flexplate, good luck.

 

Did you ever figure out what was causing your vibration? I'm in the same boat right now...strange vibration between 1500-2500 RPMs, but I can feel it the worst in 3rd gear. Brand new 385, Scat cast crank, forged rods and pistons, balanced internal front, external rear, Vigilante 2800 converter, stock flexplate. I guess my next step will be unbolting the torque converter from the flexplate and seeing if the vibration goes away to determine if it's the engine or the converter/transmission.

A friend of mine had a 3600 stall LS1 Vigilante...said it did the same thing to him (vibrations down low, worst in 3rd). Sent it in to have it restalled to 4800 and said the vibrations went away after the restall. Maybe it is the converter?

EDIT: Forgot to mention, I also have poly motor mounts and a poly tranny mount.

 

Still in the process of figuring it out. I had the replacement flexplate balanced to equal the one that was sent with the new motor, and I have it back now. I'm waiting on the converter back from P.I., hopefully will have it by Friday. If so, we'll be installing it all this weekend. The builder said the flexplate was 22 grams off; I dunno if that's enough to cause it or not. I'll let you know how it goes...