ltd slip chatter-- normal?

[Steve B in SD]

my '04 with 5800 miles has a noticeable vibration during low speed acceleration in turns, such as turning out of a driveway, etc. Let the clutch out under straight acceleration, no noise. Accelerate from a red light making a turn, and there's a definite feeling like the limited slip is chattering.

Is this normal? If so, no problem, I just didn't want to ignore something that should be looked at. Maybe a different gear lube makes a difference?

[clord]

Hi Steve.... What you describe sounds totally normal in Mk II GT3s. Mine does the same thing. Noticable when reversing while turning too. Our cars have a mechanical, asymmetric, clutch type limited slip differential. It has significant lock-up percentages (40% under acceleration and 60% under overrun). This is typical of factory racing 911s. The LSD clutch pak has a preload plus pairs of angled ramps that come into play (and add LSD clutch pressure) under acceleration (one pair of ramps) or overrun (the other pair of ramps with a different angle). The angle determines the percentage of additional clutch clamping. In the tight turns you describe the slight driveline pressure of acceleration or reversing will cause the clutch pak to tighten and slip making the chattering sound you hear and feel. Try this experiment: For instance on a sloping driveway, back down while turning with the car in reverse gear and the engine slowly driving the wheels. Then try the same thing with the car coasting out of gear and listen/feel the difference. Without the engine driving (and loading the clutch pak further by engaging the respective pair of ramps) the LSD chatter will be less.

[Steve B in SD]

Thanks, I was fairly certain I knew what the noise/chatter is, I just wanted to be sure it's considered normal. I've had other cars with limited slip diffs, and chatter isn't normal, and there's an oil additive that changes the coefficient of friction of the oil just enough to prevent it. I didn't want to keep driving it assuming it was normal, then have some sort of failure, and be asked by Porsche---"why did you let it go so long without bringing it in"..

(you know how they are...)

I had a 996 C2 with limited slip/abd diff before, it didn't make any noise. I know the GT3 is completely different, and expected the noises to be different.

Is there any point to changing the gear lube in the transaxle to a high quality synthetic?

I just want to make sure I know the normal sounds/quirks so I know if something isn't right and tend to it.

There's a lot of rattling noise from the exhaust when it's cold, that goes away as soon as it warms up, figure that's normal too.

Awesome car! No comparison to the C2, even though mine had 030, H&R's, 18's, etc.. The GT3 is much more enjoyable and involving to drive.

[clord]

Hi Steve.... You already have a high quality synthetic gear lube in the transaxle from the factory....and it's already the one that Weissach would have us use. The chatter is normal and the lube is cool. Personally, I think that Weissach figured out the right lube to go with the LSD design they installed.

As for the "rattling noise from the exhaust when cold"....that's completely normal as well. What's happening is that, to reduce emissions when cold, the GT3, and some other 996's as well, have an electric auxilliary air injector pump that runs when the motor is dead cold until some sensor detects and signals "enough". The pump is electric and pumps fresh air into the exhaust just after the exhaust ports (if you ever remove the exhaust manifolds, you can see the channels milled into the heads next to the exhaust ports that channel the fresh air in). The fresh air on cold start mixes with the cold, unburnt gases exiting the exhaust ports and burns/detonates in the exhaust system, and, incidently, helps the cats to heat up to operating temp quickly. That's the popping you hear on cold start. Notice that when the sensor decides it's warm enough to run the cats normally, the revs drop and the popping stops. You have a very special 911 there and it sounds like it's performing like it should. Enjoy!!!

[Steve B in SD]

Hi Steve.... You already have a high quality synthetic gear lube in the transaxle from the factory....and it's already the one that Weissach would have us use. The chatter is normal and the lube is cool. Personally, I think that Weissach figured out the right lube to go with the LSD design they installed.

As for the "rattling noise from the exhaust when cold"....that's completely normal as well. What's happening is that, to reduce emissions when cold, the GT3, and some other 996's as well, have an electric auxilliary air injector pump that runs when the motor is dead cold until some sensor detects and signals "enough". The pump is electric and pumps fresh air into the exhaust just after the exhaust ports (if you ever remove the exhaust manifolds, you can see the channels milled into the heads next to the exhaust ports that channel the fresh air in). The fresh air on cold start mixes with the cold, unburnt gases exiting the exhaust ports and burns/detonates in the exhaust system, and, incidently, helps the cats to heat up to operating temp quickly. That's the popping you hear on cold start. Notice that when the sensor decides it's warm enough to run the cats normally, the revs drop and the popping stops. You have a very special 911 there and it sounds like it's performing like it should. Enjoy!!!

Thanks!

That's good to know. I figured it was normal, just don't want to ignore anything. It is a great car!

Now I have to see what I can modify--- or, just leave it alone, tough choice !