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Gearbox issue ideas.

 
Old 07-30-2012, 05:02 PM
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wanna911
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Default Gearbox issue ideas.

Just started happening increasingly recently, to the point I stopped driving it. Relatively slow acceleration in 3rd (compared to other gears) gear, also sounds like the flywheel is still moving loosely like it's not engaged, increasingly loud buzzing/humming noise under WOT.

Car is 03' Turbo (track only) with 5.5 Tilton Cup Clutch and LWFW. Third gear (minus synchros) was replaced early last year. Problem only in 3rd gear.

TIA
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Old 07-30-2012, 05:11 PM
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mobonic
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take it back to the shop that did the work and have it inspected.
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Old 07-30-2012, 05:17 PM
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wanna911
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Therein lies the problem. The 3rd gear was done by one shop and the clutch by another. Would like some ideas on which one it could be. The clutch is even newer than the 3rd gear so just looking for ideas.
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Old 07-30-2012, 07:08 PM
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hi. I doubt its the clutch if you only hear the noise when third gear is selected. sounds like something to do with third gear...

Todd
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Old 07-30-2012, 07:56 PM
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Originally Posted by wanna911 View Post
Therein lies the problem. The 3rd gear was done by one shop and the clutch by another. Would like some ideas on which one it could be. The clutch is even newer than the 3rd gear so just looking for ideas.
Clutch problem should manifest itself in all gears not just 3rd.

Peter
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Old 07-30-2012, 09:49 PM
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wanna911
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I figured as much, was just hoping not to have to open the box back up.
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Old 07-31-2012, 12:24 PM
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stownsen914
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Try draining the tranny fluid for starters. If there's a problem with 3rd that's causing what you describe (noises, down on power), you might expect to see stuff come out in the tranny fluid.

Scott
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Old 07-31-2012, 12:51 PM
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GTgears
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Sounds to me like you might be missing a few teeth from your 3rd gear...
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Old 07-31-2012, 02:00 PM
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Originally Posted by stownsen914 View Post
Try draining the tranny fluid for starters. If there's a problem with 3rd that's causing what you describe (noises, down on power), you might expect to see stuff come out in the tranny fluid.

Scott
+1
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Old 08-05-2012, 01:37 AM
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wanna911
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Originally Posted by GTgears View Post
Sounds to me like you might be missing a few teeth from your 3rd gear...
I broke 3rd gear at the end of 2010 and it sounded like a loud crunch and a bunch of loose metal slinging around. This sounds a lot different, any chance it could be synchros and not the gear? Seems hard to believe I broke it that fast with probably 1500 miles on it tops since the last replacement.

Probably have 15 events, half of which are time trial events that are maybe 15 laps for the weekend.
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Old 08-05-2012, 08:22 AM
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coryf
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A syncro is not going to make noise when the gear is engaged. It is essentially connected to the engagement teeth on the gear once the slider is fully engaged. If it is making a higher pitch, or other wise different tone when only in 3rd then there is more than likely a damaged section of the gear. You can pull the transmission cooler pump off the side of the case and look at the 3-4 gears to see if anything is wrong without having to pull the tranny out of the car.
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Old 08-05-2012, 05:02 PM
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GTgears
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Originally Posted by wanna911 View Post
I broke 3rd gear at the end of 2010 and it sounded like a loud crunch and a bunch of loose metal slinging around. This sounds a lot different, any chance it could be synchros and not the gear? Seems hard to believe I broke it that fast with probably 1500 miles on it tops since the last replacement.

Probably have 15 events, half of which are time trial events that are maybe 15 laps for the weekend.
Depends on the ratio. I give our short ratio 3rd gears a 25-30 hour track life. The gear doesn't care if you are running Grand Am or NASA TTs. The TT is maybe 80% of the same stress that a pro racer would put on it, but it is still significant stress. The miles are irrelevant. It is how many hours at temp and under "race" load that count.

I recently saw a 3rd gear from a gearbox in a 996TT that failed after 2 events. It failed at Watkins Glen and the car owner was running the stock mainshaft with 1st and 2nd. 3rd gear was a fair bit shorter than the stock 1.41 ratio. This car ripped the teeth off down to the root. The twisting torque on the gear teeth was insane. The engagement teeth were still attached and the gear itself was still "whole". It was just missing 3 teeth that had been plucked right off the gear from the force. This particular car, I have been told, makes over 600whp.

The right way to gear a turbo car is the way that Porsche did with the GT2 RS. 3.154 1st gear, 1.889 2nd gear. Leave 3rd gear alone at 1.41. The 996 TT got a 3.818 1st and 2.05 2nd gear. Even very reputible race shops fall into the trap of throwing a short 3rd in the turbo boxes to try to improve the rpm drops from 2-3. All it really does is put a weaker gear in there and makes you use 3rd gear a lot more on a track like RA. A taller 2nd and sharing the work load between 2nd and 3rd is the right way to build a durable and fast car, IMO. If you are going to run a short 3rd gear in there, replace it annually with the number of hours you put on the car. Just factor the service into your operating costs budget.
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Old 08-06-2012, 02:28 PM
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Hmmm, that sucks, guess I will have to put it into the budget. I guess it's a good thing I haven't had to buy any tires all year then.

My gear was stock gear. Do you have any beefier ones? Will straight cut be stronger?
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Old 08-06-2012, 02:29 PM
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Originally Posted by coryf View Post
A syncro is not going to make noise when the gear is engaged. It is essentially connected to the engagement teeth on the gear once the slider is fully engaged. If it is making a higher pitch, or other wise different tone when only in 3rd then there is more than likely a damaged section of the gear. You can pull the transmission cooler pump off the side of the case and look at the 3-4 gears to see if anything is wrong without having to pull the tranny out of the car.
There is no tranny cooler on the 996 TT. Can it still be evaluated?
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Old 08-06-2012, 07:36 PM
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Originally Posted by wanna911 View Post
There is no tranny cooler on the 996 TT. Can it still be evaluated?
Ah, forgot it doesn't have the pump. I was thinking GT2/3. It might be possible to put a borescope through the fill plug and see something. Other than that, without a pump there really isn't a good way.
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