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| 996 FAQ |
There are a number of suppliers of wheel spacers for the 996, including TechART, Fabspeed, Gemballa, etc. There are several PAG options: 5 mms, 17 mms, 21 mms, 25mms and 31 mms. These are reported to not only improve the look of the car but help with understeer.
The general consensus is to go for 18" wheels. They are however more likely to get damaged where roads are poor (Central Ohio seems to have more potholes than the whole of Southern Burma) and tires are more expensive. Ride quality really shouldn't be a significant issue and resale value is probably going to be higher. (Note that the tire type will have an impact on ride comfort/performance probably more than wheel size.) Some have proposed getting 17" on their order, keeping them for resale and buying or exchanging them later for some aftermarket 18" like Fikse, Kenesis, BBS, etc. (There are a multitude of wheel manufacturers. Wheel Enhancements seems to be a good source of information and advice on this subject.)
One downside to 18" is that currently there is great difficulty in finding race tires like BFG R1's, Kumho's, the P-Zero C, and the Yokohama A032R for this size. (They may not even exist in some cases.) Hoosier seems to be the main exception to this.
Track drivers favor 18" for their larger contact patches and thinner sidewalls which deform less under hard cornering. Auto-X drivers favor smaller wheels for less weight and sharper turn-in. Both go for the significantly lighter and stronger aftermarket wheels. Those into car looks favor 18" or even 19" wheels, either OEM, since weight isn't a big deal, or aftermarket or a particular "look". Take yer pick!
Todd Serota has given us some excellent pointers on tires and tire pressures. The following are snippets from Todd and others on these tricky questions.
Consensus favors: Bridgestone S-02 Pole Positions, Pirelli P-Zeros and BFG G-Force KD's, in no particular order. The Conti's are fine but are not considered "real" performance tires by some. They will however generally last longer, are quieter and smoother.
Todd says: "If replacing only the rears in a C2, the decision is whether to choose 265's or go to 285's. Just be aware that if you go to 285's, the car will understeer a bit more, since traction is increased in the rear relative to the front. If you're replacing all four tires, you can go bigger in the front - like 245's, to go with the 285's in the rear. Wheel size will dictate tire sizing. 8" width up front, rather than the 7.5"of the regular 996 18" wheels, allow 245's (which strictly speaking are recommended for 8.5" rim widths but seem to work fine)."
A good supplier will give you advice on sizing and it is worth noting that tire sizing varies. In other words, the width of a 245 in one tire can be different in another. Also important to note is that without lowering the car the stock 996 is stuck with almost no ability to change camber. Without dialing in some negative camber it is possible that larger tires will rub the spring perch.
Bill (Chicago) Economos reports: "I have run Hoosiers 245/35/18 on both 8.5 and 9.5 inch width rims on the front with no tire rub. I have H&R lowering springs on my car."
Tire pressures have generated considerable debate in the past. Some favor only factory recommended tire pressures. However, anyone who has ever been to or seen a race track knows that one experiments with tire pressures to find the best for performance driving. It is certainly true that under- or over-inflated tires can be dangerous.
Most favor dropping cold pressures on their street tires somewhat to give greater stickiness and handling characteristics. Personally in my 996 C2 with 17"'ers, S-02's, I ran 34/36 on the track recently and noted appropriate wear on the tires. I could have dropped them a little more to get it just right (probably around 32/34 front/rear). It is worth mentioning here that had I stuck to factory recommendations on cold tire pressure, 36/36, I might have exceeded the maximum pressure for my tires once they had heated up under those extreme conditions. At cold pressures of only 32/34, pressures will increase to about 42-44 hot. This is an ideal pressure for many.
Please also see Mike B.'s comments on this subject under Drivers' Ed below.
For track tires, specifically Hoosiers, the following remarks have been made by Lynn and Bill (Chicago):
"I have run Hoosiers for three years on my race car. It is difficult to give you cold pressures to start because the increase in pressure will depend on the track, and how aggressively the car is driven (and the car). The key is to have the tires at 40 lbs when hot (this is only my opinion of course). At a track like Lime Rock for example, since all of the turns are right hand except for one, I start with the right front 1 psi above the right rear which is 1 psi above both left side tires. When I come in hot, the pressures are equal at 40 psi. Colder than this, or tires not all equal, or higher pressure, will lead to inferior lap times by several tenths of a second. Lynn"
"Andrew -
I run the exact same tires on the track. I have found that the tire presure increases closer to 8 - 10 psi after the first session, and will continue to rise throughout the day. I need to let air out after each session. I have also found that the left side of the car on most tracks will heat up more. According to Hoosier, for a car that weighs between 2500 - 3000 pounds, you should be running between 42 - 46 hot (see Tire Rack web site). I start at 32 front 33 rear, and run at 40 - 42 hot. I measure them after a cool down lap, so they are actually a little higher. For autocrossing you need to start much higher, since your tires will not heat up as much.
Most people run at around 40 hot like Lynn suggested. No matter what weight car you have, so you may want to experiment between the 40 and a little higher. If you talk to a Hoosier tech ask them what you should run given the weight of your car. I would be interested to know.
Bill.
BTW I always have about two pounds more in the back tires, since with my lowered car and the Hoosier tires my car will oversteer at high speeds. So I put a litte more in the back to even it out. I you have adjustable sway bar you will not need to do this."
Here is a more expanded discussion of tire pressures and the use and benefits of pyrometers from Todd:
"Tire Pressures
"I have the stock P-Zeros on 18" rims on my 996. I'm guessing you've got 17" rims on your Boxster. While pressures can be a bit lower on 18" tires because the profiles are a bit lower, my comments here apply to 17" tires as well.
Porsche's factory "recommended" pressures for the 996 are 36/44 front rear [Ed: My handbook says 36/36 for the C2 with 17" wheels], which are way too high, IMNSHO. I lowered mine to 31/33 cold, and the car works great that way. I'm not getting much rollover at all (just a smidgen - certainly nothing to be concerned about), and I can't imagine many people driving harder at a track event than I do. The Boxster Board members who went for rides with me can report on how well the car worked at those pressures. Also, a friend of mine at the event finally lowered the pressures on his C4S from the factory recommended 36/44 (I've been telling him to do it for some time now), and he watched his times drop by about a second, I believe.
Now, for the Boxster, I was very surprised at the factory recommended pressures of 29/36 front/rear, and said so to several of my students on Saturday. 29 is getting pretty low for a street tire, and 36 is on the high side. Even stranger, the Boxster has near 50/50 weight distribution, so I was very surprised to see a 7 pound spread recommended between front and rear. On a 911, there's normally a 2-4 pound spread (the factory's 8 pound spread is nuts, and is likely to assure understeer - see below), but that's because there's significantly more weight in the rear (about 59/41 weight distribution front/rear). Re the Boxster, I'd guess that Porsche is trying to assure the car has some understeer, for "safety" reasons. Generally, an understeering car is safer for 99% of the driving population. Unfortunately, it's hardly the fastest way around a race track if it's anything more than just a little.
Anyway, I told my Boxster students to try 32/34 front rear and see how that worked. I don't know if anyone tried it, or what the results are - perhaps they'll post in this thread. I can tell you, though, that the 5 or so students' Boxsters I drove on Saturday didn't all handle the same. I didn't have the chance to discuss how all of them were set up, but some certainly understeered more than others. Varying tire pressures may well have had something to do with this.
Finally, what tire pressures you use will vary somewhat, depending on the speeds you will hit at a given track. In a tight autocross, you'll have to start with higher relative pressures, because you won't be going very fast, and the tires won't heat up as much, meaning pressures won't increase all that much. On a super fast race track like Willow Springs, with very high cornering speeds (turn 8 in my car is 130+ mph), you would start lower, because the tires will heat up a lot, and thus pressures will go up a lot more. Of course, you never want to go so low that you're getting significant roll over.
One quick final note - on alignment. The factory alignment, particularly in the front, is geared toward adding understeer, and assuring maximum even tire wear. My 996 came with essentially 0 negative camber in the front. This is not optimal from a performance standpoint. If the Boxster is similar to the 996, then the range of camber adjustment in the front is severely limited, but I'd suggest you max it out at something just shy of 1 degree negative. This will make your front end stick better whatever pressures you have. Moreover, factory toe-in is substantial, and while it keeps the car tracking nice and straight, it works against crisp turn in. I had my 996 set to 1/16" toe-in - still enough that the car doesn't wander, but less than the original factory setting, so the car turns in better. 0 toe, or even 1/32" toe-out, would really make the car turn in great, but makes the car a pain to drive on the street."
"You don't compare pyrometer readings between tires - each tire is analyzed separately - it's the relationship between the 3 temps for each tire that is critical. I'll give you some easy examples, and I think you'll get the idea. Generally, the middle reading should be the average of the other two, and the spread should be as small as possible. The values I'll list below will be outside, middle, inside, for one tire.
170, 190, 170 - the tire is overinflated, causing the middle to be hotter than the sides.
190, 170, 190 - the tire is underinflated, causing the middle to be lower than the sides.
190, 185, 160 - not enough negative camber, and the tire is overinflated for the suspension setup. How do you know there's not enough negative camber? Because what caused the outside to be hotter than the inside is that the tire rolled over, enough that the inside of the tire wasn't making full contact under max g-load, and the outside was bearing too much of the load. It's overinflated because the middle is much higher than the average of the other two.
160, 165, 190 - too much negative camber, tire underinflated for the suspension setup. The logic is the same as above. The inside is hotter than the outside by so much because, with all of that negative camber, even under max g-load, the outside of the tire wasn't making full contact with the pavement, and the inside portion was bearing too much of the load. It's underinflated because the middle is much lower than the average of the other two.
170, 175, 180 - about perfect, likely as good as you're going to get with that suspension setup.
Does that help? Needless to say, since you're treating each tire as its own sub-system, you adjust each tire separately. That means that after adjusting, when the car cools, you may find different values in your left tires as compared with your right, in addition to the difference you'll see from front to rear - that's perfectly fine.
Re chalking, believe me, if you're underinflated, even with the stiff sidewall of a Kumho, you'll see it.
Finally, re hot pressures, generally speaking, you're pretty close. I would say you're just a tad high - 38/40 hot front/rear would be about optimal in your situation.
http://home1.gte.net/fastco/temp.htm "