MikeF

Thinking about "upgrading" from my 17" 996 wheels to a set of 3.6 Turbo 3pc wheels. I believe Porsche does not recommend 18" wheels on this car....why is this? I suppose I could stick with 17's to be safe but would like to understand the rationale. Also, how wide can I go in the rear (standard body 94 C2)? Running 9's now, wondering if I can go up to 10".

I know I've seen 964's with 18's but don't want to stress my suspension unnecessarily.

Thanks,

Eddie911

I have a 90 C4 narrow body with 18x10 in the rear and 18x8 up front with 225/40 and 265/35 S02. A couple of points. It REALLY fills the fenders expecially on my car which has been lowered. It stresses the suspension which in my case has been completely upgraded with Konis. The rear fender lips need to be rolled up to avoid rubbing. The camber and catser has to be set utside spec to accomodate the tires. Lastly, different tires has different widths even though the sizes are the same. I originally bought BFG KA TD's but they were too wide. If I were doing it again, I would probably cut back in the front to a 7.5 in wide rim instead of a 8. Other than that, the look is really aggressive and the thing drives like it is on rails. Email me if you want pics. BTW, my rims are turbo twist not the turbo Speedline you are talking about.

Bill Gregory

If memory serves correctly, there was some earlier discussion on this. I believe Porsche issued a bulletin saying that the C2/C4 shouldn't run 18" rims. In the discussion, I believe Adrian said that installation of a turbo front hub, the front hub being the weak point, would handle the additional stresses induced by 18" wheels. Note the turbo front hub is also set up for turbo calipers, which attach differently than the stock C2/C4 calipers.

Cupcar

Porsche added 18" rims to the 964 Carrera Cup cars in 1993. Many of the teams bolted the rims onto cars that had been raced with 17" rims the year before. It was after this that the recommendation from the factory was published stating that 18" rims were not recommended on cars not so fitted from the factory. Could it be that in the fierce Carrera Cup series with the cars pounding the alligator teeth corner curbing that suspension failures ocurred with 18" wheels? Hence the recommendation for no 18" wheels. The 964 Turbo with 18" rims has a reinforced front suspension upright, and I have heard that this upright put in service during the Carrera Cup series. I don't know of any other reinforcement changes to the chassis in the 964 Turbo or 18" Carrera Cup cars. This would mean that 18" wheels should be safe if you use the Turbo upright. Anybody else-Adrian?- have information on this.

Adrian

The tests carried out on the 964 using 18 inch wheels showed a structural problem in the whole front and rear wheel suspension system, especially the wheel carriers. To further compound this problem, the 1990 C2s had another wheel carrier problem which if not solved (TSB issued to replace the wheel carrier covers) it only makes the situation worse. Only the Turbo S and Turbo 3.6 were fitted with 18 inch wheels in the Turbo range and the RS 3.8 in the N/A range. The ROW and USA Turbo 3.6s had different rear ends.
If the need to install 18 inch wheels on a 964 is so great I can only recommend complete replacement of the wheel suspension system, replacement and "Turbo" Installation of the front struts and reinforcement of the rear axle assembly ala RS 3.8 which used basically a RS (NGT) rear axle as did the ROW Turbo 3.6. Bushing angles have to be changed as well. If you decide to lower as well, Turbo S style modifications may also be required to further strengthen the front end. If you want to read about what can go wrong with 18 inch wheels. Read about the RS4 from Audi. Production is now cancelled and new wheels are still in the design stage but five complete suspension failures have been reported in Germany alone.
Anyway back to the 964s. This is all a lot of work. Very expensive and the only 18 inch wheels that look any good after all of this are the "Speedline for Porsche" wheels, which as already mentioned many times are as rare as rocking horse pooh and very expensive. In the end the simple questions to be answered are Why? and For what benefit?.
Ciao,
Adrian
911C4

PS: With C4 owners do not forget the 6% differential in tyre sizes rule.