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LT1-LT4 Modifications 1993-97 Gen II Small Block V8

Serious Vibration at 4500k

 
Old 07-11-2019, 03:24 PM
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Default Serious Vibration at 4500k

I just bought a 1995 Pontiac Trans Am LT1. 103,000 miles with motor rebuilt at 70,000. Edelbrock headers, no cats and a hypertech tune. A McLeod clutch was installed last year with the flywheel being resurfaced by a regular Chevy dealership. When the car hits 4500k the whole car shakes like its on rumble strips. Does not matter if its in gear, not in gear clutch in or clutch out the car shakes terribly. Starting with spark to try and figure it out but I feel something is crazy off balance. Thoughts?
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Old 07-11-2019, 04:11 PM
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Many "rebuilt" motors are "internal" balance vs the external (rear) balance of a stock LT1. If the FW was not "neutral", aka zero, balanced than the motor will vibrate like a dog shiating razor blades

what is odd if the dealership just "resurfaced the FW and assuming it was neutral balanced it should be good. With that said dealerships typically (read never) resurface anything. They just bolt in new parts. If they put a stock FW in on a neutral balanced motor it will vibrate significantly. This will destroy bearings and cause RMS leak over time

Given you get this vibration with clutch in or out and if I understand correctly when just revving the motor...sounds like a balance conflict with motor & FW

A machine shop can neutral balance FW for around $40-$50 or you can buy a neutral balance FW but I would have it checked at a machine shop...my exploding wallet billet FW sold as neutral was 26 grams off

If it only happens at speed than your DL angle can be off or your drive shaft needs balancing, or is bent, or u joints are bad

If none of the above than "grounding" can cause vibrations throughout the car (like when exhaust is contacting frame, x member, etc)
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Old 07-11-2019, 04:40 PM
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Originally Posted by ****** View Post
Many "rebuilt" motors are "internal" balance vs the external (rear) balance of a stock LT1. If the FW was not "neutral", aka zero, balanced than the motor will vibrate like a dog shiating razor blades

what is odd if the dealership just "resurfaced the FW and assuming it was neutral balanced it should be good. With that said dealerships typically (read never) resurface anything. They just bolt in new parts. If they put a stock FW in on a neutral balanced motor it will vibrate significantly. This will destroy bearings and cause RMS leak over time

Given you get this vibration with clutch in or out and if I understand correctly when just revving the motor...sounds like a balance conflict with motor & FW

A machine shop can neutral balance FW for around $40-$50 or you can buy a neutral balance FW but I would have it checked at a machine shop...my exploding wallet billet FW sold as neutral was 26 grams off

If it only happens at speed than your DL angle can be off or your drive shaft needs balancing, or is bent, or u joints are bad

If none of the above than "grounding" can cause vibrations throughout the car (like when exhaust is contacting frame, x member, etc)
I spoke to the guy who did the motor and did not know to ask that question. He does do everything stock and two other shops told me he is OCD about his builds and they trust him.

You are correct, simply revving the motor causes this god awful noise and I suspect it is something out of balance and nothing in the DL. My mechanic wants to double check spark first then we are going to pull the transmission to check the FW.

Luckily this car was owned by an OCD clean freak who rarely drove it so I hope there isnít irreversible damage done to it already. Thank you for your input.
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Old 07-12-2019, 11:00 AM
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ask the builder if he did a internal or external engine balance. If the FW is a stock one there would be about a 3" cast in raised weight on the back of the FW. Also usually there are a few drill holes partially in the FW in random places. You could just pull the dust cover from bell housing and rotate engine around until you could see if it is there

your note the Dealership resurfaced the FW. You sure they did not replace it?

If they put a stock FW on a internal balanced motor it would vibrate but generally you feel that vibration at any rpm, even idle

you sure its a vibration and not misfire?
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Old 07-12-2019, 11:07 AM
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Originally Posted by ****** View Post
ask the builder if he did a internal or external engine balance. If the FW is a stock one there would be about a 3" cast in raised weight on the back of the FW. Also usually there are a few drill holes partially in the FW in random places. You could just pull the dust cover from bell housing and rotate engine around until you could see if it is there

your note the Dealership resurfaced the FW. You sure they did not replace it?

If they put a stock FW on a internal balanced motor it would vibrate but generally you feel that vibration at any rpm, even idle

you sure its a vibration and not misfire?
I will go back and ask him. We are going to dig into it on Sunday. The dealership said specifically resurfaced and the only new part was the clutch.

The best way to describe it is hitting rumble strips at about 50 mph when at 4,500 rpm. It does vibrate a bit at idle but not knowing these cars that well I assumed it was a 24 year old muscle car noise. My 84 Z28 did not vibrate like this at idle but different motor too. The idle vibration feels like a misfire or an air fuel mix issue.

I have done three plugs on the driverís side so far. Cylinder #4 is weeping a little oil from the plug hole. We are going to work on this Sunday, but I asked here for all those seasoned LT1 ownerís input in case it was something obvious. And I did hours of searching but only found a few threads like this.
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Old 07-12-2019, 12:39 PM
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A key answer is if the engine was rebuilt as "internal" balance...which is the most common way of rebuilding as the cars FW or flexplate is not used. Just the "rotating assembly" (crank, rods, pistons) is balanced. Those motors require a neutral FW.

If the PO had the stock FW re-installed with a internal balance motor he certainly should have the vibration also. The resurface should not have killed the external balance part of the stock FW

I would confirm with builder if it is internal or external balance before digging in any further.

If this is not a balance issue than ignition (misfire) is something to check

On the "stock clutch replacement"....the PP are balanced and there would be a paint dab on it to line up with the dowel pin on crank. MANY aftermarket clutches are not balanced at all as the Valeo OEM ones are long gone. There have been more than a few threads where people replace the clutch to find there is a vibration issue and it was the China clone PP that was not balanced.

You can tell if it is as the 3 "ears" on the PP face that engages disc will have holes in them of various depths and quantity (pic)
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