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Rooy · Joined: 16 Jul 2005 Posts: 41 Location: Neenah, WI

Ok, I'm still chasing this vibration that occurs a little over 3000 rpm then goes away around 4000 rpm. Car is a '78 924 NA with trans and TT from an '82. It does not seem to be the bearings themselves, as there is no noise usually associated with bearing failure.

What I am looking for is some more info on this 'vibration damper' inside the tube. I haven't been able to find any info on what it is, how exactly it works, could it fail, etc. I appreciate any info given! Thanks.
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Andrew
Neenah, WI
'78 924 with dual Dellortos

crazyinkc · Joined: 10 Nov 2002 Posts: 226 Location: Olathe KS

Auto or manual? The manual has 3 or 4 rubber housings that hold the bearings. I wonder if you have a problem with your carbs since it seems to be consistant at a certain RPM range. Have you sync'd your carbs lately? The auto has the rubber damper mounted to the flywheel and it will fall apart after a while.

leadfoot · Posted: Mon Oct 24, 2005 6:36 pm Post subject:

This is the mysterious harmonic vibration that is suffered in alot of N/A's...
One thing that helps is to check your right side engine mount, my vibration was tinny sounding as the spring had rusted and broken, my car also did not have the vibration damper inbetween the starter and alternator as it was an auto conversion so I made my own using urathane bushings.
Both of these when in good condition will help take alot of the harmonics out of the engine.
Leadfoot
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

totally agree with leadfoot; the right-side engine mount is always the prime suspect. If you haven't already, do a search on "engine mount".
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

Rooy · Joined: 16 Jul 2005 Posts: 41 Location: Neenah, WI

Transmission is a 5 speed. The vibration is not the engine mounts or carbs because: with the car stationary, clutch not depressed, rev it and the vibration is present. Depress clutch, rev it, no vibration. So it's happening only when the driveshaft is spinning.

The vibration damper I'm talking about is shown in the Haynes manual as something inside the tube between the 2nd and 3rd bearings. This is what I'd like to know more about.
_________________
Andrew
Neenah, WI
'78 924 with dual Dellortos

leadfoot · Posted: Tue Oct 25, 2005 5:46 pm Post subject:

Could be a torque tube bearing starting to go, but the only way I know of checking this is to take it out and move the shaft up and down in the tube, if there's free play you need another torque tube or a bearing house that can replace the inside bearing.
If you start hearing grinding soon then thats the clutch end of the torque tube being ground out... don't ask me how I know this...
I'd also check your transmission mounts to see if they are cracked or bent... these can also have an adverse effect on vibration...
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

Rooy · Joined: 16 Jul 2005 Posts: 41 Location: Neenah, WI

It just doesn't seem to be a bearing. With the car in the air, in gear, clutch in, turning the rear wheels to spin just the driveshaft and clutch disc, and watching the clutch disc through the inspection cover showed no runout which I thought might point to either slop in the bearings or a bent shaft. No odd noises either.

Transmission mounts are fine. I still want to know about this mysterious vibration damper.
_________________
Andrew
Neenah, WI
'78 924 with dual Dellortos

bryanc · Joined: 27 Feb 2003 Posts: 233 Location: San Antonio, Texas

I know this sounds crazy but...
With the engine cold (exhuast cold) grab the tailpipe and lift up and down and see if it changes the noise. I had an exhuast leak that sounded sooo mechanical, like bearings grinding and it turned out to be an exhuast leak. Also does the 78 have a catalytic converter? Bang on this to see if there is a rattle. Loose pieces in the cat can cause a mechanical rattle.
Good luck!
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'84 944
I'd rather have a bottle in front a' me than a frontal lobotomy
Tom Waits

Rooy · Joined: 16 Jul 2005 Posts: 41 Location: Neenah, WI

It's not a noise; it's a vibration. As I previously stated, the vibration is only there when the driveshaft is spinning, so it's not related to the exhaust system.
_________________
Andrew
Neenah, WI
'78 924 with dual Dellortos

Min · Posted: Wed Oct 26, 2005 1:19 pm Post subject:

leadfoot · Posted: Fri Oct 28, 2005 9:03 am Post subject:

You will have to take the torque tube out to test it properly... the noise in my car was most noticable if I reved the car up to 5000 held it there for a couple of seconds and still in gear and clutch out let off the accelerator sharply.
Only after dismantling the torque tube from the car could I see what damaged had occured, the clutch end of the tube was jiggling quite severely in the end of the housing...
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...