Yes but if I understand pwdrhounds response having the fronts spin slightly faster keeps the car RWD mostly until the rears spin faster and then the VC is actually engaged. In this scenario I would think there is actually less wear and tear on the front differential and vc as opposed to having the car use the awd system more frequently.
I kinda like having the system RWD until I really need the fronts compared to a more full-time awd system.
I was referring to going with 245/35 over 245/40 .. I think for looks and performance 245/35 is better.. Its been established that 235/35 with a 305/30 is not a good move already....

 

I agree with you about what powerhound said. Having the car setup to be mainly rwd and only getting power to the front when the rears spin sounds like a good idea, but do we really know if that doesn't cause damage to anything?

My point is simply that if you want to match the factory setup you need a 245/40 front with a 305/30 rear. That we know for a fact is safe.

FWIW I've put about 10k miles on my "wrong" setup because I didn't think to check sizes till they were worn out. No idea how many miles the PO has done on this rubber.

 

Well that's fine too and may well be what he meant but they car is never technically speaking "rwd'. As i remember it it is always at least 5% to the front. But hey I can't remember really what set up is best for 19:s as I'm 18with a bullet and full time rwd. So go with what the experts say and pdrhounds one of em!

 

Could the 5% bias be a function of the rears spinning faster than the fronts?

 

as far i as i know, it's merely a function of how our particular system is set to work. minimum 5% to the front at all times, to a max of 40/60 IIRC?

 

guys
when youre cruising on the highway, the front drive is not getting any torque whatsoever.
you can put your car on the lift and stop the front wheels from spinning with your pinky toe. im not exaggerating. im dead serious. i have done it with Tim on SEVERAL cars.



a 245/40/19 will not even fit under the fenders.

jack the rears of your car up and give gas. see what happens...



a 235/305 set up is fine. a 245 is better for handling (less understeer).

 

Unless Porsche made a typo in all their technical manuals, both the front and rear final drive ration is 3.44 on the 996TT. This means that the transfer of torque is purely a function of the difference in rotation between the front and rear wheels. If the rears are rotating slightly faster than the fronts, a certain amount of torque goes to the front wheels via the thickened fluid in the VC. This may be 5% at some low speed and progressively gets higher as the speed of the vehicle increases to something like 40% at the vehicles top speed. This is due to the fact that as the speed of the vehicle increases, so does the difference in the rotation between the front and the rear).

By the same token, since both front and rear final drive is 3.44, if your front and rear wheel diameters are exactly the same, ZERO torque is being transferred to the front at any speed (until the rear wheels loose traction and begin to rotate faster than the fronts).

Finally, if your front wheels are at all a smaller diameter than the fronts as in a 305/30/19 and 245/35/19, they will be rotating faster than the rears and will be effectively providing driveline drag to the gearbox output shaft via the VC. Again, it is not until the rears loose traction and begin to rotate faster than the fronts that any torque can begins to transfer to the front wheels. Many people run this combo without issues because the speed difference is small enough that it won't damage the VC. Additionally, the amount of driveline drag caused by this is small enough that most people can't tell a difference anyway..

The whole Mickey Mouse 996TT AWD system is very primitive and relies solely on the speed difference between the front and real wheel. Nothing more, nothing less.. Finally, if the fluid in the VC gets cooked from abuse and looses it's ability to thicken with heat, your front wheel drive has effectively become useless. This is generally what has happened if you jack up the whole car and you get no rotation of the front wheels when the rears are turning. If you want to test this, make sure you do this after you have driven the car a few miles as this will ensure that the viscous fluid has warmed up and thickened. With the front diff cold, you will get very little rotation if any.

 

it's purely academic for me, but that's how i've always understood it: 5% to the front? up to a max of 40%. i've never fully understood the hows and whys but have read of the many different types of the awd system eg audi/vag etc.

but the foregoing only serves to illustrate my upthread assertion that: you're the expert and it sure isn't me lol

 

has been lots of AWD debate the last few days... lol
http://rennlist.com/forums/996-turbo...s-and-awd.html

 

So it sounds like it's impossible to get the same ratio and speed difference as oem without going to the 315 PS2 setup which is ~$600/tire and almost always backordered.

 

Honestly, just due to the tire headaches, I'd rip out the marginal front drive, put in a good LSD, and call it a day. You'll be able to run whatever tires you want and your car will have more grip and performance to boot. Really a win win all around. It'll cost about $4k give or take... Cheaper if you go with TBD instead of LSD. If you're a track guy, go with LSD. For street use TBD is fine...

 

once again, pwdrhounds advice is sage.

but re the ps2 setup? that's the "best" i have found and it is possibly the ONE place i will never again cut corners ( sorry lol ).

i pay roughly 475 ps2 315/30 and now run the PSS 235/40 which i've found for 188. so...

1325 per set on my 550 rwhp car is the price i feel i must pay and i believe it is worth it. not that the ps2's last any longer? as i'm still at two sets of rears annually, no matter what. it's basically a fixed maintenance cost, the way i see it.

 

Much thanks to pwdrhound for the info...but I don't have an extra $3-4k kicking around to blow on the car

The OEM setup is:
14 revs per mile faster in the rear
0.4 inch taller in the front

In 19s the closest I've been able to come so far by specs not going with a 305+ in the rear or 245/40 in front (this is for the Pilot Super Sport):
295/30 -- 12 revs per mile faster in the rear
235/35 -- 0.1 inch taller in the front

Any problems with running a 295/30 and 235/35?

Are revs per mile and diameter affected by wear, or is tread depth wear not considered in that calculation?

 

tread wear will definitely affect the activity of the psm e.g. when mixing new rears against old(er) fronts. though the ecu will adapt as the new tires get scrubbed. unless possibly the rolling diameter difference(s) are too great. but that *usually* will result in the dreaded wear to the vc that everyone talks about and no warning(s) will be given.

 

Why not try the following? Notice that a 305 section Michelin PSS is the same width as the S02A (our Bridgestones run wide). The only challenge might be bumping to a 245 section PSS (0.8" wider), but that's likely something one can address with an alignment/camber adjustment. As far as diameters, however, they're almost spot on to stock (0.1% off from stock diameter difference between the front and rear)! Clearly, this setup will cause your wheel/tire setup to have a greater "overall" diameter versus stock, causing your speedometer to be off by ~5-6%.