Recruiter_Bruno

So I have a track day tomorrow and after swapping to my track wheels/tires this weekend I get a crazy hesitation when accelerating.
Like the car is misfiring or running out of gas.
It's really bad and happens consistently.

But when I hit the sport button it almost goes away.
I can still notice it but its much better.
A dealer mechanic that I know thinks it's the tire sizes throwing off the computer.

I bought the setup used last year (came off a 996 Turbo) and put one track day on them and had the same problem.
235-40-18 front and 315-30-18 rear. Toyo R888R

Thoughts?

Thx

roadie13

What's your car?

Recruiter_Bruno

I guess you can't see my signature?
2010 C4S

TheTorch

Your track wheels:
Front diameter: 25.4"
Rear: 25.4"

Stock Front: 25.5"
Stock Rear: 26.2"

Why did the tech think it was the wheels?

Recruiter_Bruno

Because the only variable that causes the hesitation is the wheels/tires.
The car is perfect on my daily wheels and this only happens after I swap.
The track wheels do not have TPMS sensors and I have the wear sensors zip tied out of the way but that should not make a difference.

Something about the car/computer just goes crazy when these wheels are installed.
And then in SPORT mode it's mainly gone.

CAVU

During a straight line acceleration (and it will do it power on in with steering inputs too), if the car's computer(s) sense ANY traction loss the computer(s) will direct a reduction in power to the wheels. When the tires get nice and warm, it goes away because the tires are more "sticky".

Not saying this is your case, but I was getting ready to replace my thread bare rear tires and had recently used launch control at the Porsche Experience Center and decided to try it on my C4S (yes I had never used mine). First time in my car it scared the $hit out of me. Thought i had broken my toy or did the launch wrong. Then I realized that my Michelin street tires were probably not warm enough and the car was doing exactly what it is supposed to do. So I did several hard braking processes to heat the tires up. Did a second launch and much better!. Did a few more hard braking actions. Did a third launch and ALL IS BETTER. Our cars will not permit laying rubber down via uncontrolled spinning of the tires. The programming wants maximum traction, even during a max power acceleration.

Recruiter_Bruno

So upon talking to a local tire shop that sells a lot of track tires (for Porsche) he told me my rears are 5.1% taller than OEM spec and he would not recommend me take these tires to the track again.
So I'm getting rid of them.
Oh well.
At least we know.

TheTorch

hmmmm. Your track tires are about 3% shorter, not taller. Someone here must know what's going on, whether there is a computer reset that might address this, etc.

CAVU

Really like the look of that 997.2 GT3 front bumper/Cup Lip!

Aclmin

Traction control is kicking in because your rear tire diameters are too small (compared to the fronts) making the ecu think you’re spinning the wheels.

David Berry

winner! 🎉🎁

kisik

I would agree that this has to do with rear tire too short. I have heard that AWD cars are much more sensitive to tire differences. I have C2S and and had no issues with 235/40-18 and 295/30-18 set up. I do prefer 285-305/35 for rear though You can always just put your street set up back to see if problem goes away.

Recruiter_Bruno

Thanks everyone.
Just ordered new Toyo R888R tires.
245-40-18 fronts and 305-35-18 rears and that should keep me at a close enough overall diameter.
Was also able to move my track day by 2 weeks so all good.

Petza914

Definitely a PSM intervening issue as stated. With the new sizes, you should be OK. The standard size Front / Rear ratio is 1.03 (26.2/25.5). With your new setup, you'll be 26.4 / 25.72 = 1.026 so right on it.

The reason it's less noticeable in sport mode is because the intervention point is relaxed a little bit so what's being interpreted as wheel spin or slippage is being allowed to occur for a little longer or at a higher threshold before intervention.

On an AWD car like yours, aside from the PSM intervening you can actually damage the transfer case for the AWD system with significant rolling diameter differences front to rear.

I'm on 19s, but when I replace my tires this time around, I'm going with 255-40/19 F and 325/30-19 Rear, but also have 12" wide rear wheels. That will put my ratio at 1.01. Your new ratio is better, but that's the closest I can come in 19" with the tires I prefer to run in their available sizes, which are Nitto Invos.

mujeriega

This might be a stupid question but what happens when you turn PSM off?