bernocco930S

Great old thread and GreggT (the last post) really addressed my concern. Bottom line: "".... the general rule of thumb (per "AWD, PSM, ABS) is that the front is within 3% of the back. Some call it 'running diameter' but it's circumference.""

I am going with 997 Turbo wheels, 245/35 F and 305/30 R They will be at 1.85% difference. I think that with the +56 offset on the front wheels I will be good to go. I am also running X73 suspension so I should have better clearance.

Unless I hear differently, I will let you know how it goes when I order them next week. BTW, 2004 turbo.
eric

adam_

Nobody I've read in the last 3 years of posts here and on 6speed recommends those sizes. The 3% posted I have no idea where that was pulled from!

The stock 996TT sizes are 25.1" front and 25.0" rear; with 829 and 832 revolutions per mile/

here is the key: this means the differential needs to compensate for only 3 revs/mile (832-829=3)


The sizes you propose are 25.8 and 26.3 inches, and 807 and 788 rev/mile.

The delta is 19 revs per mile. 600% more than stock, the diff will be revolving 6 times the speed of the stock set up.

Oh, and 19/788 is 2.4%, well past the 3/829 or 0.36% the stock set up runs

997TT sizes on a 996TT are a no-no.

Report back in 10k miles....

A

PS My calcs are based on PS2s...sizes vary with specific brands, so it may be off

bernocco930S

Well, what kinda' dispels your theory on the thing is that the stock 997T uses F: 235/35-19 (816 R/Mi) and R: 305/30-19R (791 R/Mi) and I am proposing F: 245/35-19 (806 R/Mi)
My ratio is better than stock factory 997T ??

Please explain again - one of us is not getting it?
Thanks, eric

C5Driver951

Remember the 996TT and the 997TT use different differentials. The 997 is capable of handling a much larger difference in tire circumference front to rear. So, we can't compare what the 997 has vs the 996.

Most folks will tell you to maintain within 1% to avoid killing the viscous coupler. The Tire Rack personnel will tell you within 4%. I know I don't feel like finding out how far I can push it.

I'm running 235/40-19s and 305/30-19s in Bridgestone RE050As. They are 790 and 792 revs per mile respectively.

Joel

2005 996TTS
1993 968

adam_

The 997TT differential is designed to have different sizes front and rear- it is 'happy' with it's 'designed mismatch'.

Just like running 997 sizes on a 996 will stress the 996 diff, running the same size front and rear on a 997 will stress the 997 diff.

If tirerack puts in in writing and tells you they'll cover the diff failure, go with their recommendation....

bernocco930S

I like your thinking here and the 235/40 was exactly what I had in mine when I went looking but a very rare find in that size. The Hankooks I was looking for and many others do not offer that size - ergo - the compromise to 245/35.

That other guy with the 600% different than stock and 6 times faster spinning worried about the diff's and soforth just did not make any sense to me.

I am the first to admit that I am not very up on these cars but I have built/raced/won road racing championships throughout my life and after awhile you aquire some basic common senses about machines. I have always pushed the envelope with my cars and seemed to come out all right. In fact, there have been some that have even used my ideas.....

Maybe I just have a RedNeck mentality!

C5Driver951

Is that one of the ex-Ken Schrader Hendricks Chevies? I think all if us who work on cars have some red neck in us... nothing wrong with that.

Joel

adam_

Really? I guess I am 'that other guy".

It was pretty basic and straightforward, but I guess you need to understand how a differential works.


Note that Joel's sizes are within 2 revs per mile of perfect- a precise match of the 996TT stock application. Yours are way off.

Are you going to track this? Let us know how it works out. Surely replacing a diff is not going to be a big deal if you've been racing and winning so much.

pwengland

From personal experience, if you want 19s just stick with the correct size you will see everywhere. 235/35 and 315/25. I have been through all sorts of PSM ABS stuff running the wrong size 19s due to the previous owners wheel choice. Now run the above size PS2 on BBS CH-Rs 8.5 front 12 rear and couldn't be happier. Zero rubbing, electronics back to normal (no lasting damage to the diff) and couldn't be happier.. Why go with any other tyre size on 19s is the question...

bernocco930S

Yess 'er that there car is a Schrader car from the same shop as Gordons. The chassis was HMS23 which Kenny used in a Kodiak Lumina and then had Hendrick rebody to the Monte Carlo. That was 1995 and Kenny came off of a great 94 season and was picked to win the cup in 95. Of course we know that he had a dismal season (almost killed himself at Degga) and Jeff went on to win his first cup title in the 24 car - Oh Well. This car was one of Kennys favorite that he called Rooster. Now that there is horsepower boy.

As far as my tire deal...... I am taking PWENGLAND's lead and although I cannot find the 315/25's, I am changing the sizes to equal the same numerical differential he has with his combo (7)....... let's wait and see. I pick them up today. I do appreciate the feedback and ideas. Nuff Said.