997gt3north

while it could be the diff, it sounds like turbo spooling, torque twist inducing 'slop'

i can tell you the difference on a gt3, never mind a gt2 with that kind of massive tq hit, is noticeable when you:
- change engine mounts
- replace rear rubber suspension components

if you are comparing your race cars to this street suspension turbo twisting widow maker than that is what you are feeling - all the rubber to make it comfortable on the street

i think to make a gt2 really fast on the track you would have to make the rear slop go away otherwise you will never have confidence in the car

996FLT6

^he's doing moderate acceleration. I can see it happening with turbo spool up at launch but seems to me it's not LSD behavior more like alignment issue. If its a fully functional LSD u shouldn't have this behavior. Hell on a straightline with a fried LSD don't see that happening too. Mike

Johnb55

Lot's of good input. Thanks guys. I'll do the "Diff check" and get an alignment, and see where we go from there.

GTgears

I don't think the diff is the root cause here.

Johnb55

I did the "Diff check" one wheel up one down. In gear the raised wheel doesnt spin, in neutral, it spins freely. Both sides are the same. So bottom line, the diff passes this simple test, no? Also, while raised I gave each wheel a good check for loose or worn bushings. There was no movement at all on either side, so the bushings seem ok.

I really don't think it's alignment because under a constant load the car runs true and doesn't pull to either side. Only when accelerating / decelerating does it yaw slightly left / right respectively. Nonetheless, I will have the alignment checked. Tire pressures are spot on spec, 29F 34R.

I'm going to have to get to the bottom of this. I really find the yaw intolerable in a car of this performance.

Jenner

One wheel on the ground in neutral is the way to test. If you think about it for a minute you'll realize why it should not spin in gear. This video is a great example:

KaiB

I begin to wonder about engine mounts.

Johnb55

Ahh! The raised wheel spins freely in neutral, so the diff is bad!

Guard time!!

Jenner

KaiB makes a great point. Certainly worth checking for a broken/cracked/loose engine mount before dropping $3-5k on a diff rebuild or replacement.

Nick Wong

Check to see if the subframe is attached correctly. If it is offset to one side your car could be crabbing and on acceleration/decel it can cause some instability.

TRAKCAR

I had the same. Triple checked allignment, LSD, subframe. Turned out it was the tires, even though the road force was not too bad..

997gt3north

NO

If the Porsche wasn't a near no load diff by design, then your diff would be fried. But, as has been written on here a few times by some knowledgeable folks, it has by design extremely low pre-load and thus the fact that it freely spins in neutral doesn't necessarily mean it is bad.


I like Peter's suggestion of trying new tires - or at least swapping on a set from another car and seeing if it helps.

Second would be engine mounts - it sounds like a twisting issue to me.

If this is going to be a tracked car, then you are going to want a better diff with more pre-load and much better internals so that is a different consideration.

The wevo or RSS engine mounts are so easy to do they are a no brainer anyways so I would just order them and remove the stock ones.

Another thing to check is if you may have a single bad rear shock.

KaiB

Should ya'll reinforce your engine carrier assy when the stiffer mounts are installed - as we do?

Igooz

I have an '08 GT2 and it does not pull at all to the right or to the left as you have described. I have around 6,000 miles on mine. (I will take the car out next week when/if the weather clears and test it for you..but no way, no pulls...).

I also had an '03 GT2 and it was the same from new to 22,400 miles and decent track time.

Dumb Q: are your tire pressures ok? ... Good luck.

Johnb55

It was a bad tire!
The car had been CPO'ed and received a brend new set of Michelin Pilot Sport Cups, but one of the rears was defective. This was identified by switching the wheels & tires from side to side & test driving the car. The yaw was then exactly reversed! Right under acceleration, left under deceleration. The Wheel / tires were also tested on a "Road Force" balance machine, one rear produced 50lbs of load while the other was 20 lbs. The acceptable limit is 20 lbs. They are being sent to Michelin with the Road Force info for a claim to cover the cost of Replacent.

As the Diff was also worn and had basically zero pre-load I took the opportunity to install a Guard GT2 Club diff as well.

The car now runs straight as an arrow, and I can now enjoy driving it to the fullest!