So back in the day, a variance of something like 5% was the maximum acceptable. OE tires are ~1.5% different front to rear. I also recall seeing front and rear axle circumference settings while tooling around with coding, so it may be moot as long as I code in the right numbers. Or that could just be for display purposes, and not AWD characteristics.

Is there a published spec for acceptable front to rear differences on the 991.2 C4? A quick look around and I'm only seeing anecdotal "I ran X and it's fine" and the like.

 

I remember seeing 2% at some point when I had my 993 C4S, but I don't recall if it was a published and Porsche-validated source. Possibly, in theory, the smaller the better as the difference will cause the system responsible for torque redistribution to have to 'work' harder relative to the degree of difference. If I understand it correctly, a smaller rear diameter, for example, will cause power to be shifted to the front axle as the relatively higher rear rotational rate will be interpreted by the system as losing traction? So, if you are looking to bias the system you could play with the diameter differences to meet your objectives. I'm not sure about the wear and tear implications though. I look forward to seeing what others say.

 

I'm not looking to bias the system. Keeping it under 2% would be the ideal here, but the current option I have in front of me is like a 3.5% variance front to rear.

 

Which is the smaller diameter?

 

The fronts, just like OE, but I just actually checked and axle to axle it's 4% smaller instead of 1.5% smaller. Pretty sure that will make the AWD system pitch a fit.

 

You should get more rear power bias though. Would this be less strain on the front diff??

 

At the rate of normal tire wear you would think if it really made a difference the car would pitch a fit early on.

This last set of tires I actually only changed rears and double stinted the fronts with no ill handling or mechanical issues.

I believe just so each axle has the correct rolling diameter Left/Right ...... so it's more of a concern if you have to replace one tire vs replacing both.

 

Power bias has absolutely no relevance here. I'm trying to figure out how much slower/faster the fronts can rotate compared to the rear before the car starts intervening. The car intervening involves engaging the front axle as a whole, and applying brakes at each corner individually.

Left/right isn't the concern here, it's front/back. The front differential is engaged via magnetic clutch at the differential, and the brakes are pulsed at each corner individually based on wheel speed differences (and steering input and a few other things).

Tire wear is a good point. OEM tires are 1.8% smaller in the front than the rear when brand new. At 3/32" tread up front and a brand new 8/32" tread tire in the rear, this difference increases to 3.4%. So we at least know that the expected range of difference is at least up to 3.4%.

It would make sense that it's 3.4% plus whatever differences the current steering angle introduces plus a small margin like 2% to accommodate tire manufacturing differences before TCS/DSC starts kicking in. It's just figuring out what that small margin is.

 

To calculate rolling radius deltas, fron to rear. 3% or less is what I look for...some say up to 5%, I am sticking with 3%.