I am thinking of purchasing a set of 19s from Tech1, but am hesitating because of certain issues with running "non-OE' tire sizes.

I have the Bilstein damptornics installed. Tech1 has recommended that I run with these wheels:

Front: 19x8.5 ET51
Rear: 19x12 ET47

And with these tires for maximum traction at the track:

Front: 235/35R19
Rear: 325/30R19

I have read in several past forums that folks here have messed up their differentials running 315s and 325s, but Tech1 is certain that these will work on the TT.

Has anyone run the setup above without burning up their differentials?

 

According to the tire manufacturer's website, the 325 is virtually identical in height to the 305 tires that come on the vehicle. There is .006% difference or about 10 tire revolutions per mile. Worn tires will vary more than this.

If you want the front/rear relationship to be identical to OEM, you can use the 245 front tire. It, too, is 10 revs per mile different.

Finally, you can run the OE 305 tires on 12 inch wide rims if you wish. The GT3s all do it.

We recommend the 325 because the Turbo is a traction-limited car.

Mike

 

you should be fine as long as you keep your front to rear ration within 5%.

 

I would be interested in what you end up using. Just like to add one comment regarding the tire width, since stock is 235/35-19 front, 305/30-19 rear:

If you increase rear tire width and keep front the same, you will increase understeer. This is probably not good thing with the 4WD Turbo, which tends to understeer (as opposed to 2WD GT3 for example).

I think a better solution might be to increase FRONT to 245 and actually keep rear the same at 305. I believe (not sure, anyone corrects me as needed) this is what Techart does to its car at a recent Tuner Grand Prix.
What do you think about this, Mike (Burroughs)? Would running 245 front/305 rear be feasible or cause a problem? TIA

 

I wanna know about this issue too! Subscribed.

 

Running a 245/305 combination will work. the 245 fronts are a scant two tenths of an inch taller than the 235s. this is well within tolerance for the AWD system.

 

^^^Thanks Mike.

Does anyone remember the person who damaged his Turbo's differential, what exactly did he do? What tire sizes did he change to that caused such damage?

 

It was Fred
315/25/19 in the back, stock sizes in front??

https://www.6speedonline.com/forums/...15s-rears.html

 

A lot of tuned Turbos have traction problems off the line with stock diameter tires. Stock diameter is for less than stock power as far as I can see.

 

The 315/25R19 rear tire is a disaster for the Turbo. It is 1.5 inches shorter than the original tire and will cause major problems with the differentials.

 

Ed and Mike, thanks for the info. I went to this web site http://www.1010tires.com/TireSizeCal...?action=submit to get the numbers. I used the calculator to get the circumference, as I assume that's what matters here. The number that's been thrown about is 4-5% difference is ok, although I don't know whether 4-5% with respect to front to rear, or 4-5% with respect to change from stock, either front or rear (I hope I am making sense here LOL.)

***FRONT:
Stock 235/35/19 circumference=80.0 inches
Mod 245/35/19 circ=80.9 inch (1.1% larger than stock)
***REAR:
Stock 305/30/19 circ=82.3
Mod 325/30/19 circ=83.8 (1.8% larger than stock)
Mod (Fred's, NOT ok!) 315/25/19 circ=79.1 (4% smaller than stock)

1. Note that the rear stock is already 2.8% larger than front.
2. Looking at the numbers, it would appear Mike/Tech1 is once again on the money (thanks for the professional help) as far as what's ok and what's not.
I would imagine as important as what you do to one axle, is what you do to one axle relative to the other. If you make circumference of front wheel larger, how soon you run into trouble depends on whether you make the rear larger or smaller (smaller would be much worse in this scenario); this is correct?
3. Mike, so to confirm: I could run 245/35/19 front (assuming there is such a tire) and keep rear stock 305/30/19 without problem? I think this would do wonders to increase front cornering force and reduce the understeer of the 4wd Turbo.

 

Can
The 4-5% variance refers to the tire you are replacing, not the difference between front and back. Inposed this same question in another thread but never got a definitive answer. I too would like to know how much of a difference between front and back the differential can tolerate.

 

You are spot-on, of course. The onboard computers on the Turbo constantly monitor the speed RELATIONSHIP between front and rear.
If the front turns slower, and the rear turns slower to a similar percentage, no problem.
However, if the rears turn too fast in relation to the front, the CPU thinks there is a loss of traction and splits the torque to the front wheels. That's fine, occasionally. But if there is constant torque split, the diff will overheat, the CPU will severely detune the engine (limp home mode) and all kinds of warning and overheat lights come on.
Therefore, the important thing in tire choice is to maintain the correct "static loaded radius" relationship between front and rear.
Then the computers stay happy and the smilie light on the dash comes on.

Regarding the compensation capability of the computers, or flexibility in the tire speed relationship, the car must be able to tolerate the approximate 2% wear factor of the tires from new to old. The front and rear tires do not wear at the same rate. I am told by many experts that as long as the difference in revolutions per mile doesn't exceed 2.5%, you are safe.
Of course, you could ask Porsche, but you might as well ask the tire.

 

The 245/35/19 and 325/30/19 combo would be what I would use as well....done it before, no issues. The 315/25/19 is WAAAYY shorter than stock, what a disaster

 

Very good thread as i was trying to decide on GT2RS tires sized for my tuned turbo. I cant wait for new corsas N spec to come out at 325/30/19 245/35/19.