Steve996

Appreciate your wisdom guys and sorry for the long post I’m learning what is / is not relevant to include…https://rennlist.com/forums/graemlins/bowdown.gif

Here are the symptoms…looking for educated guesses on root cause:

Problem area - transmission, viscous coupling, front differential on a manual 2002 996 Turbo. Details:

• When in neutral, very tough to push / role the car because of medium / high resistance in the drivetrain.
• On the lift with motor off and car in neutral. If I spin one rear wheel clockwise, the opposite rear wheel spins counterclockwise with typical resistance - e.g. very easy to spin the wheel and no unusual noises - e.g. chirping / grinding; no movement occurs in the front wheels
• Easy to also rotate the front wheels with no chirping / grinding, the rear wheels don’t move
• Trying to rotate both rear wheels by hand in the same direction is very difficult
• Still on the lift with motor on and in gear, all wheels spin in the forward direction. I can “grab” a front wheel and stop it while the others continue to spin. There is no chirping / grinding noise when I do this
• While driving the car there is always a chirping sound as long as the car is moving whether clutch in or out. The frequency of the chirping is directly tied to the speed of the car - fast at high speed and low at low speed. The noise is closer to the squealing made by cold race brake pads on steel rotors vs. a grinding noise like clutch less shifting without proper rev match.
• If the car is stopped, transmission in neutral, and clutch engaged (i.e. clutch pedal not depressed and the engine and transmission connected), the car stays put and no noises are made in the driveline

Transmission was rebuilt by Isaac Fritsche in 2013 (lost 2nd gear at Road America) - Fairly well known Porsche Motorsport mechanic spending most of his time in Europe now - I kept standard gear ratios, stuck with helical cut gears, but upgraded synchros to Cup parts. Nothing was touched in the driveline forward of the viscous coupling. I was happy with the work - the transmission was so smooth and positive in its movement, it was clear I had no idea what good was before that.

Motor is an EVOMS GT700, with a SACHS Stage 4 “puck clutch” - I frequently tracked the car before the transmission was rebuilt - to the tune of about 3,000 track miles. You may be wondering why I did not convert the car to rear wheel drive and install a limited slip. I decided it was better to buy a purpose built race car than keep modifying this one - so returned it to street form vs. hybrid track/street. Ultimately I am not wed to an AWD or RWD solution. I’ll probably go with the most cost effective.

Thanks for your help!!!

Carlo_Carrera

Have you checked the brake calipers and wheel bearings?

Steve996

I think the wheel bearing check is a good call out. Although I have not ruled it out, I pushed brake calipers further down the list since no noise present when I spun the wheels by hand and while on the rack. I have pagid yellows which will be "squeaky" especially when cold and under light pedal pressure. But, the noise is still there when the pads are hot and I apply heavy pedal pressure. I'll let you know what I find after examining wheel bearings...

mcbit

Tyre sizes?

Steve996

Stock tyre size is 225/40-18 fronts with 295/30-18 rears. I am running 235/40-18 fronts with 315/30-18 rears. (Michelin Pilot Sport PS2's)

"02996ttx50

the easiest way to determine a failed wheel bearing has ( for me ) been to "slalom" the car left right quickly and if ever the whine ( and thats what a bad bearing always sounds like..less so a "chirp!" ) ceases, the bearing is bad. e.g. turn left sharply and the noise stops? right bearing bad. the reverse would be true also ( or was for me..)

figuring out front vs rear is up to your ears, and these things are tough to ferret out sometimes! point is: if you slalom and the noise goes away? bearing(s).

Richard.

to test for a failing wheel bearing can't you just jack the wheel up off the ground and see if it rocks when you grab the tyre to check for play ? Thats what our local vehicle testing centre does

Carlo_Carrera

On my car both front bearings had gone bad yet there was no play or rocking in them. The only sign was the terrible noise they were making.

EVOMS

Porsche wheel bearings and balljoints like to squeak when they're bad, as opposed to clunking and grinding like other German makes. Wheel bearing failures are pretty rare from a service shop standpoint, but I've done a couple and they sounded like there was a stone or rock stuck between the brake rotor and aluminum dust shield. It's something to consider, as when you test the car on the lift there's no weight on the wheels. A good diagnostic option is to use a mechanic's stethoscope against the wheel carrier while the wheel is spinning. This will get you 'closer' to the sound, and can sometimes expose very subtle noises otherwise inaudible during that type of use due to shop noise/exhaust/no load on wheels/etc.

One other test you can do on the center diff is to hold the front wheels in place and spin the propshaft (with car in neutral/rear wheels free). There should be resistance, but it should rotate smoothly.

Sam