Porsche 996 Turbo Wheel and Tire Test - Why Change A Good Thing?

For countless years now we've featured cars with plus-sized wheels and tires. Wheels and tires are often the first things that get changed on a modified car. Funny thing is, no one's really thought about what changing them does. Sure there's endless speculation about rotational inertia, fuel economy and ride quality-blah blah blah. As far as we know, no one's even posted basic performance numbers for a car with new wheels and rubber. So we decided to find our favorite European car, test it, slap on some plus-one high-end wheels and tires and test it again. We thought it would be an open and shut case of added traction and possibly slower acceleration. What we found came as quite a surprise and still has me scratching my head.

As far as good things go, the Porsche 996 Turbo is about as good as it gets. For tires, we picked the best street tire we know of, the Michelin Pilot Sport 2. These were mounted on 19-inch Avus AF-M02 wheels by Shoreline Motoring of Huntington Beach, Calif. Nothing but the best for our European thoroughbreds.

In each configuration (stock and plus-one) we put the Carrera through a 700-foot slalom, 80 to zero mph braking and 200-foot skidpad tests. Each test helped us evaluate whether the difference was on account of the tires or wheels. We left out quarter mile acceleration tests just because replacing 996TT transmissions were out of our editorial budget, or our insurance coverage for that matter. Each test was performed at California Speedway's test facilities on the same day back to back, which meant a lot of tire changing. I now understand why no one else does this.

The Turbo came riding on stock chrome wheels and factory recommended N3 tires. N-Specification tires are fine-tuned tires designed by O.E. tire suppliers in conjunction with Porsche to meet its traction, noise, comfort and treadwear needs. Tires are rated from N0, which are mostly snow sport tires, to N5, the tires found on the GT2. The number on the N rating is the number of times a specific tire size from a particular tire manufacturer has gone through the homologation process. N-spec tires wear an N-spec badge on their sidewalls and are available through Porsche's dealer network. The N-Spec tires for the 996 include the Bridgestone Potenza S-02, Continental ContiSportContact 2, Michelin Pilot Sport and PS2 and Pirelli P Zero Asimmetrico, Rosso and 240 Winter Snow Sport. Although these tires share the same names as the tires available to the replacement aftermarket, they are not necessarily identical. The 19-inch Michelin PS2s we used weren't N-Spec units even though 19-inch N-Spec PS2s are available for the 997.

From our initial street driving impressions we knew the PS2s were a huge improvement over the tires mounted on the stock wheels. The stiffer wheel and tire combination added a more precise steering feel and more road feedback. Grip was dramatically increased while road noise was lessened. After putting roughly 200 break-in miles on the tires, we headed for the track.

We expected the new PS2s to be the better tire, so it was no surprise to see superior grip in both the braking and the skidpad tests. With the massive ABS brakes on the Turbo, braking distance is strictly a function of tire grip. Every pass with the PS2 was significantly shorter than the best braking distance on the N3 tires, regardless of tire temperature. The skidpad results, however, raised eyebrows. As predicted, the increased lateral grip did result in better numbers, but with the plus-sized wheels and tires the 996 could maintain higher average lateral g forces while in severe oversteer. Depending on the amount of throttle steering, the plus-sized 996 could run the skidpad in slight oversteer or extreme drift angles. Most cars obtain the best skidpad numbers when driven at a speed where a neutral attitude is maintained, which was exactly the case for the Turbo on the stock wheels and tires with the PSM turn on. This made no sense, since the new rear wheels are significantly wider than stock and offer added contact patch. If anything, the car should have understeered. Clearly there was more grip, but the stock 996's vehicle dynamics controls and suspension were not calibrated for it. I assume higher lateral g forces could easily be achieved if the car was set up for the larger wheels and tires. Results were not significantly different with PSM enabled or disabled.

The slalom yielded similar confusion. Where the stock Turbo blitzed through the cones with predictable precision, the larger wheel and tire package caused it to handle with dangerous unpredictability, sometimes in oversteer, sometimes in understeer. Even just after performing perfect slaloms on stock tires, we managed to continuously clip cones while attempting to get a better plus-one time. The car could be faster through the slalom, but we weren't about to wrap a $150,000 car around a light pole to find out how much faster.

I consulted Michelin and Porsche engineers along with some seasoned Porsche racers about this. Porsche's engineers did not find our problem surprising and said, "All the fuel and chassis control systems on the 996 Turbo are built and tested around the wheel and tire size and their corresponding rolling radius. Altering the standard size not only has adverse effects on the performance and handling but most likely could end in damaged drivetrain parts such as a broken front differential or viscous clutch damage (not a warranty matter). The weight of the wheels and tires is not a major factor." This just goes to show how dialed in Porsche's cars are out of the box.

Porsche racers experience similar problems when using R-compound tires on stock cars. The added grip and increased overall spring rate overworks the suspension and effectively makes the car roll too much, thus altering the weight transfer and tire load.

Without a doubt, the Turbo looks fantastic with the 19-inch Avus wheels and we know the Michelin PS2s are great tires. On the street, this combination has more grip than the average driver can sensibly overcome. You'd have to drive this car at nine-tenths to get it to even start behaving as we've described. But if you do, be prepared for it to potentially bite you back. If I were to do this to my own Carrera (not that I can afford one), I'd stick with the factory sizes and the N-Spec PS2s. Bigger wheels and tires are great, but dialing in suspension systems and vehicle dynamics electronics better than the Germans will be next to impossible.