# Vibration at high rpm



## 66tripower (Mar 28, 2011)

Have a 66 GTO factory Tripower 4spd and it vibrates at high rpm under load when moving (seems worse in 3rd) through the shifter and car vibrates in the rear more at 60+. Went through the following: Checked the engine and tranny mounts, put in new u joints, has new wires, new plugs, new distributor cap (no crack), rotor and points- set with dwell to specks, new coil and set engine timing to 6 degrees off top dead ctr --all which helped any pinging/ knock under load. Compression test shows cyl 6 and 8 are slightly low (170-175 compared to 185 in others). Have not checked tires/rims as it seems to vibrate in shifter based on high rpms even at low speed. Works well -no vibration at 40-50 mph with no heavy acceleration. Seems slightly sluggish though. Plugs have a mild brown/cocoa color on cyl 6 and 8 after some use. My mechanic wants to do a valve job but the compression seems good. My other buddy where I buy parts says engine needs carb to be cleaned and is a fuel distribution problem and could cause vibration if engine is being starved for gas.....Before I enter into a 1200-1400 $ valve job any thoughts here.


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## BearGFR (Aug 25, 2008)

66tripower said:


> Have a 66 GTO factory Tripower 4spd and it vibrates at high rpm under load when moving (seems worse in 3rd) through the shifter and car vibrates in the rear more at 60+. Went through the following: Checked the engine and tranny mounts, put in new u joints, has new wires, new plugs, new distributor cap (no crack), rotor and points- set with dwell to specks, new coil and set engine timing to 6 degrees off top dead ctr --all which helped any pinging/ knock under load. Compression test shows cyl 6 and 8 are slightly low (170-175 compared to 185 in others). Have not checked tires/rims as it seems to vibrate in shifter based on high rpms even at low speed. Works well -no vibration at 40-50 mph with no heavy acceleration. Seems slightly sluggish though. Plugs have a mild brown/cocoa color on cyl 6 and 8 after some use. My mechanic wants to do a valve job but the compression seems good. My other buddy where I buy parts says engine needs carb to be cleaned and is a fuel distribution problem and could cause vibration if engine is being starved for gas.....Before I enter into a 1200-1400 $ valve job any thoughts here.


Check your driveline angles, especially if you've changed anything in the suspension (trans mounts, engine mounts, springs, rear trailing arms, etc.)
Here's a pretty good writeup on the why's and wherefore's:
Driveline 101

Short version: Beacuse of how cross-type Ujoints work and actually move, if the angle between the drive shaft and the transmission isn't exactly the same as the angle between the drive shaft and the rear axle it will cause a vibration.

Bear


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## Rukee (Feb 8, 2007)

What rims do you have on it?? I always chased a vibration at highway speeds with my Cragars until I installed some eccentric rings to perfectly center them on the hubs.


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## ppurfield001 (Jan 21, 2008)

66tripower said:


> Have a 66 GTO factory Tripower 4spd and it vibrates at high rpm under load when moving (seems worse in 3rd) through the shifter and car vibrates in the rear more at 60+. Went through the following: Checked the engine and tranny mounts, put in new u joints, has new wires, new plugs, new distributor cap (no crack), rotor and points- set with dwell to specks, new coil and set engine timing to 6 degrees off top dead ctr --all which helped any pinging/ knock under load. Compression test shows cyl 6 and 8 are slightly low (170-175 compared to 185 in others). Have not checked tires/rims as it seems to vibrate in shifter based on high rpms even at low speed. Works well -no vibration at 40-50 mph with no heavy acceleration. Seems slightly sluggish though. Plugs have a mild brown/cocoa color on cyl 6 and 8 after some use. My mechanic wants to do a valve job but the compression seems good. My other buddy where I buy parts says engine needs carb to be cleaned and is a fuel distribution problem and could cause vibration if engine is being starved for gas.....Before I enter into a 1200-1400 $ valve job any thoughts here.


My 1967 was vibrating between 60-65mph. Turns out the drive shaft was out of balance. Once balanced, no more issues. Good luck.


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## pontiac (Mar 6, 2011)

run the car in lower gear at same rpm range as when vibrated at 60+ in high gear, if vibration, then check driveshaft, joints, trans mount, problem is drivetrain; no vibration, then look at wheels, rims, wheel balance, wheel bearings, brakes, problem is suspension related.


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## 66tripower (Mar 28, 2011)

*vibration*

Thanks guys for the responses!! OK- so nobody seems to think it is engine related ...as high rpms make it worse....in any gear. Can a bent driveshaft become worse at high rpms??...Seems like that the driveshaft speed would only be related to actual speed, not engine speed.


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## geeteeohguy (Feb 2, 2008)

Possible bad pilot bearing/bushing, possible out of balance clutch or clutch cover assy, possible out of balance flywheel, possible harmonic balancer slipped or damaged, possible combination of a lot of things. I had a vibration in my '65 for a long time, finally got sick of it. Had a bent driveshaft. Fixed that, and now all is quiet. Tune-up related vibration (plugs/wires/etc) generally is masked by higher rpm. I think you have a mechanical issue with balance: either drivetrain or possibly engine/trans. Good luck, and keep us posted.


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