Over rev - 3.2 Engine Vibration @ 2,000-3,400rpm. Missed Shift...

[Oldskl60]

Hi Guys
I'm looking for help re next steps to diagnose a vibration that my 3.2 (930/20) engine has developed after a recent track day. It drove to the track and ran flawlessly all day and then driving home I noticed a vibration under load around 3,000rpm. In the first lap of my last sprint, I grabbed second shifting to fourth from third. I felt the change wasn't right and didn't left the clutch all the way out. Looking at the in car footage the tach went to circa 7,200rpm. Still having good oil pressure etc and still sounding great I kept going for the remaining 4 laps. It idles and drives great with no knocking or odd noises aside this vibration.

When driving and stationery (more noticeable, with and without clutch engaged) I can notice a slight (moderate if very fussy - maybe I am!) but definite vibration from approximately 1,800rpm and it starts to taper off after 3,400rpm. It is worst from 2,600 to 3,200.

Things checked to date or observations from taking the motor out and splitting the gearbox:
- engine mounts good but changed to try difference(including cracking and re tightening the 4 cradle bolts). No improvement.
- compression test was good with all cylinders within 7% of each other.
- Leak down was good, all cylinders 2% - 5% loss.
- bores look great and no material marks on the piston tops.
- clutch and pressure plate look fine.
- gearbox bell housing was quite wet with oil (rear main seal now done)
- disconnected fan belt, vibration remains.

I am 7 days away from driving 2,200km for a 2 day circuit sprint event and hoping to get some more ideas to rule out any terminal issues ie stretched rod bolts etc.

Any help appreciated!

Kind regards Tom

[Alan L]

I guess the Q is - how bad is it? Is it something that has always been there and you just noticed it after the bad shift. Would any one else spot it? Is it really obvious? All engines have a harmonic pattern.
If it really did not go over 7200 everything should still be intact.
I am impressed your synchros did not give up.
It possibly could be in the clutch since that probably took the brunt. Maybe something bent.
Alan

[Oldskl60]

I know!! The synchros aren't normally that good on the street that's for sure, haha.

The thought that it's always been there has crossed my mind, but I don't think so. It is definitely noticeable if looking for it and a trained person would pick it but it is not 'bad' no.

[icarp]

Check for an ignition problem. Blown off plug lead
All your test results show the engine is in good condition
all 911's are good for 7200-7300 rpm , yes the 3.2 has the less desireable rodbolts , but you did not throw a rod .
It just sounds like a miss to me
Ian

[Oldskl60]

Originally Posted by icarp

Check for an ignition problem. Blown off plug lead
All your test results show the engine is in good condition
all 911's are good for 7200-7300 rpm , yes the 3.2 has the less desireable rodbolts , but you did not throw a rod .
It just sounds like a miss to me
Ian

Okay thanks, good call. I am waiting on a new dizzy cap and rotor. I noticed when doing the leak down that rhe rotor has a burnt out hole in the black resin part... Perhaps!?

[stownsen914]

A Porsche machinist once told me that overrevving a 911 can cause the clutch pressure plate to go out of balance, in his experience.

[scarceller]

Originally Posted by stownsen914

A Porsche machinist once told me that overrevving a 911 can cause the clutch pressure plate to go out of balance, in his experience.

My thoughts were the same, maybe clutch related?

[Alan L]

Yes - I wondered clutch. But he says makes no difference - clutch engaged or not. So that throws you off the scent a bit. But I would still be suspicious in that area unless another cause is found. Possibly a bent input shaft or similar, or something bent in the clutch. It would have been severely stressed.
Alan

[PeteKz]

I'll echo Ian's suggestion to check the ignition system. A mechanical vibration around 3000 ROM should also have a 2x frequency vibration around 6000.

[icarp]

Spark plugs , wires , cap, rotor, ignition grounds, sensors, micro switches , Barn door unit, injectors,
check and clean everything, go nuts , make a list , This process takes many hours !!!!!!!

Good luck

Ian

[Jonny042]

Does/did it change at all if the trans was in gear while stationary and revving? Could rule out input shaft/friction disc.

I'd change the friction disc it's easy to do at this point.

[sithot]

Leak Down

[stownsen914]

Originally Posted by Alan L

Yes - I wondered clutch. But he says makes no difference - clutch engaged or not. So that throws you off the scent a bit. But I would still be suspicious in that area unless another cause is found. Possibly a bent input shaft or similar, or something bent in the clutch. It would have been severely stressed.
Alan

If the pressure plate goes out of balance due to an overrev and causes a perceptible imbalance, you'll feel that all the time regardless of clutch engagement.

[Oldskl60]

Thanks guys, I appreciate the input.

The motor and a fresh gearbox (LSD, HD side plate etc) have gone back in. New clutch installed but I didn't do the pressure plate (wish I had now!). Replacing rotor button and cap. Will let you know how it goes.

Kind regards Tom

[Duc Hunter]

My 3.6 powered car has a similar issue. I discovered it when mounting an old exhaust, and rediscovered it with my current exhaust. If my muffler is attached to the rear cross brace in the body, I get the same kind of vibration. My vibration is more significant on deceleration than it is on acceleration, but it’s there regardless. It’s also their standing still in neutral, revving, or holding the engine at RPM. If I simply disconnect the muffler from the cross brace and the body, no vibration whatsoever. Might be an experiment for you to run.

My engine is a 964 3.6 with no fluid damper, or harmonic balancer, with full headers, an exhaust, and a lightweight flywheel. I’m running Roth sport racing pulleys, hence the no harmonic balancer. I had literally no vibration until I attach the muffler to the cross brace and then it’s there. The card exhaust is Roth sport headers and a GT three muffler so it’s not like I’m using cheap parts lol