NJ-GT

Porsche OEM 18" wheels won't clear the front 997 GT3 PCCB. Some aftermarket 18" wheels will clear the 997 GT3 PCCB. My CCW wheels clear 380mm rotors, as I measured them in an attempt to get a Brembo GTR brake kit for cheap. Any 18" wheel with a similar construction for the middle section between inner and outer lip, and spokes depth as the CCW 1-piece and BBS Racing fits the 380mm brakes.

Indeed, the 997 GT3 Cup uses 18" wheels and 380mm front rotors. Some people have used the Brembo GTR (380mm front rotors) with 18" aftermarket wheels.

By the way, I don't see a problem running 18" instead of 19" wheels on the 997 GT3. The 997 GT3 is delivered with 19" because Porsche increased the overall diameter of the rear tires with the 987/997, to increase rear grip. The taller diameter tire has a bigger contact patch.

Porsche MotorSport runs both 997 race cars (997 Cup and 997 RSR) on 18" wheels. Tire choices in 18" will determine whether the car will do better or worse than with the stock 19".

Lift one side of the car, with both rear wheel and front wheel on the air, put two pieces of letter size paper, and drop the car slowly. Once at rest, lift the car again and you will obtain the area of both foot prints. Any tire size/model that produces a bigger area, will provide more grip (assuming similar compound).

Personally, for track duties I would get 18" lightweight for the 997 GT3/GT3 RS, lower diameter rear tires, and lower diameter front tires, both of them wider (think 265+/335+), to compensate for the needed minimum load rating. Wider front wheels (at least 9.5") and wider rear wheels just for the RS (13").

MPSC are available in 265 and 285 for the front, and 315 for the rear (it's actually a 325), or 345.

grussell

NJ-GT,

Good info and nice write up! Thanks!

frayed

The 997 race cars don't have ABS, PASM, TC, correct? My concern is how all the electronics cope with a smaller diamater 18" tire. Also, do the teams change setup when they swap b/t the smaller diameter wheels and larger? Or do the slicks they use have the same rolling diameter 18 vs. 19? The most critical component of these cars is rubber selection, and the balance of the chassis is typically developed around a particular tire; I'd be surprised if the teams did not markedly change their setups when changing tire diamter.

On the 997S, stock 19" PS2s vs. 18" MPSCs in 996 sizing showed that the 19" tire worked better on the car.

I guess I'm on the conservative side preferring to leverage off the countless hours Porsche used in fine tuning the chassis setup around a given tire sizing, and am cautious about how much you take away from race cars, which are a far cry from road going counterparts.

nkhalidi

Are the 997GT3 PCCBs different from the regular 997 PCCBs? Bigger rotors? Huger caliper? Generally more badass?

Frayed: Nice to see you've embraced the pornographic affluence that apparently goes hand-in-hand with the 4.8is.

frayed

hey mang, the rotor is 380mm; thirty bigger than on the S. Probably the same calipers, but unsure.

mooty

thx bud.
next would be to find some nice strong lwt wheels for it (steel).

NJ-GT

There are plenty of 18" tires with the same diameter as the stock 19", and even though they have taller sidewalls, they're stiffer than stock.

You need to keep the front/rear diameter ratio within 5% from stock diameters not to mess with the electronics, I use 4% as my limit. Why is this? because the electronics need to consider the range of brand new front tires, and worn-out rear tires, and any combination in between. So, you can see a variance of 0.7" in diameter using stock tire sizes just because of the wear. With difference in tire pressure, such variance can get up to 1".

Again, choosing tires wisely (width, diameter, compound) will determine if the car will get better/worse.

Regardless of the engineering effort from Porsche, the timer doesn't lie.

Do a search on the racing forum and 997 forum for cgomez. He tracks a 997S at crazy fast speeds. He runs 18". Good friend, and even better driver.

eclou

I have no probs using 18" Hoosiers on the track in my 997TT. As NJ said, I kept the ratios within less than 1% of stock so as to not mutilate the center diff on the AWD and mess with the PSM, ABS, ETC. I also lost 5.6 lbs front and 11.4lbs rear per corner in rotating weight - a total of 34 lbs rotating weight, all for a very reasonable price of $1300 for new OEM Carrera 3's and $600 for new 245/305 Hoosiers off fleaBay. That is less than 1/4 the price of PCCB's and less than the cost of 1 BBS Magnesium wheel!

frayed

I pulled some of the possible 18's out there.

I like Toyos a lot, and they seem the obvious choice. Wear like iron, streetable, and tractable.

The Hooters don't match up well but have a similar 1" spread in rolling diameter.

Didn't realize there was almost an exact match for the rear rolling diameter as I was looking at Hoosiers and MPSCs.

Stock
235/35-19 25.5"...............................305/30-19 26.3"


RA1
245/40-18 25.6"...............................305/35-18 26.3
275/35-18 25.5"................................335/30-18 25.9


PSC
265/35-18" 25.2"...........................345/35-18 26.2
235/40-18" 25.4"...........................295/35-18 25.2"
....................................................315/35-18" 25.5"

Hooters
P245/35-18 24.7"...........................315/30-18 25.6"

Ed Newman

Call PMNA, LOL, that is funny!

On a serious note, I started racing with Enkei RC-5's, very similiar to the Volk and I learned after a visit to the local gravel trap that one down side to a RC-5 or Volk is that they are not repairable. Any damage to the rim and you are done as compared to 3 piece wheels like the BBS, Fikse, etc. which you can replace the inner and outer rims on for much less than a whole wheel.

eclou

That sounds great until you realized that the 3pc wheel may still cost more to r&r an inner or outer barrel and reseal, and downtime with shipping (to and from) and from the rebuild.

mooty

well, yes and no.
i run fikses. i had a light two wheel off. it was light b/c i got back on pavement and car felt just fine and set my fastest lap that day.

when i dismounted tires from wheel, saw inner half bent, so sent to fikse. they told me i need in ant out halves both replace and that the center is out of plane too.... so basically i needed new wheels.

mind you this is a very very light off during DE. in racing situation, you wouldn't even call this an OFF.

maybe i just got unlucky.

frayed

Good point. Order up a new wheel for next day shipment or deal with reubuilding a 3pc?

I think that one of the brands, either the BBS motorsport wheels or the Fikses, use a rim strip for sealing and not liquid sealant, making for at home repairs a breeze (supposedly). So with the right construction, and accessability of parts, a 3 pc wheel would be a good option.

grussell

Looks like 18" RA-1's are the way to go. Are they any good? How do they compare to the MPSC's?

frayed

Look some posts by or ask Seth Thomas. He races speedvision world challenge for bimmerworld and thinks the ra1s are the shiznit on the gt3. I think he said he can pull laptimes out of his 996 gt3 w/i 1 or 2 seconds of Hoosiers on a big course (Road Atlanta).

https://rennlist.com/forums/showthre...&highlight=ra1