Tire Pressure November 22, 2005
The cheapest handling upgrade you can perform on a car is to find the “right” tire pressure. The recommended tire pressure for a car is a compromise based on factors that include ride comfort and noise. At an autocross event the only factor that matters is traction, on street tires increasing the tire pressure can shave a second or more off your time.
Unfortunately there is no magic number that’s right for every car and tire, you have to figure out what works best through trial-and-error. Start by splitting the difference between the car manufacturer’s recommended tire pressure and the maximum pressure listed on the tire itself. For my car the owners manual recommends 32PSI all-around and the Kumho MXs I use for autocross list 50PSI as the max, so 41PSI is where I would start from.
The goal is to increase the sidewall stiffness so that they don’t “roll” during hard cornering. Traction on the tread is good, on the sidewall not so much. Take some white chalk or shoe polish and make some large markings where the tread meets the sidewall at the 12, 3, 6, and 9 o’clock positions. After your first run check the markings. If the markings haven’t rubbed off the tread then the tires are over-inflated — let some air out. If the markings on the sidewall are gone, add a few more PSI. Re-mark the tires and repeat, keeping in mind that tire pressure will increase as the tires heat up.
If you aren’t hyper-competitive you can eventually settle on a cold tire pressure that provides good traction and won’t result in over-inflation as the tires warm up.
Once you’ve found the “right” tire pressure for maximum traction you can experiment further by using different pressures in the front or rear. Less pressure will generally mean less traction at the limits, so you can create a little bit of understeer by running lower in the front or oversteer by running lower in the rear. My car is AWD and tends to understeer at the limits so I run the rear tires around 5PSI lower than the front. The effect is not dramatic but at the limits every little bit helps.
When you are deflating the tires for the drive home you should aim for a few PSI above the recommended level, since tire pressure will be higher while they’re still hot, and check them again once they’ve had a few hours to cool down. Don’t drive home with the tires still inflated to autocross levels — the road noise will annoy you, every bump will jostle you, the car will try to follow every track in the road, and road hazards will pose a much greater risk of puncture. It would not be a pleasant drive.
Tools of the Trade
Having an air pump in your trunk is a good idea in general. I recommend buying the cheapest one that you can find because there is no such thing as a good lighter-socket powered air pump. You should not need to use your own pump at most autocross events, any sponsoring club in good enough financial shape to have a laptop for timing ought to have a proper 110v or gas-powered air compressor for everyone to use. In my area it’s the Porsche club that has no compressor and records times with pencil and paper, go figure.
A good tire gauge is, of course, absolutely essential. Never trust an air pump or those “pop-up stick” type gauges to provide an accurate reading, my pump reads about +5 at 40PSI. I received a digital gauge as a give-away item from a local car dealer — Rick Case Acura I think, the printing wore off — that feels like it cost about a quarter to manufacture but is accurate enough and easy to use. The disadvantage to digital is that most will display the highest pressure reading for several seconds — if you are deflating a tire or moving on to another then between readings you must wait until it resets itself. I’m an impatient man so I picked up an analog gauge this week similar to the one below for $12 at my neighborhood auto parts store. The bleeder valve makes deflating tires much easier and faster.
Autocross #10 November 21, 2005
Yesterday was the Gold Coast Porsche Club of America event at the Bank Atlantic Center. This was my second event with that club and at the site. One thing that I really love about this club is that they do a parade lap at the start of each run group’s first session. Becoming familiar with the course is a key part of autocrossing. The normal procedure is to walk the course once or twice in the morning and try not to forget everything before your run group comes up — which could be some time later if your group isn’t first. I usually try to keep my memory fresh by doing some spectating from the sidelines right before my group is called or paying close attention to certain cars during my work assignment.
This unfortunately does not work very well at the Bank Atlantic Center — elevation changes, medians, trees, and lightposts obscure portions of the course from every safe vantage point. Without the parade lap it would be nearly impossible for me to keep the course memorized. Knowing this time that we would have the parade lap I didn’t even bother walking the course — being in the last run group guaranteed I wouldn’t remember anything and I don’t enjoy walking around a massive asphalt lot in the hot South Florida sun.
What I don’t like about the GC PCA events is that it’s so hard to judge my performance. I like using Jorge as my benchmark because he is a good driver and his WRX is basically modified to the Street Touring X limits, but he was a no-show at this event. So I was left comparing my performance against another guy that I’d met my first time autocrossing, Drew in a WRX STi running in E Street Prepared. His STi has extensive suspension and handling upgrades but the powertrain is still stock. My best raw time for the day was less than 2 seconds behind him. The last time we were at the same event was in August with Drew over 7 seconds ahead of me, but keep in mind that was my second time autocrossing and the Saabaru was still unmodified at the time.
That does make it seem like I did pretty well yesterday but I can’t say that I was feeling it. The Bank Atlantic Center site itself is part of that. The constant elevation changes are an interesting feature, exciting even, but they put the car “off balance” coming into many of the turns and slaloms. That’s an added challenge that I’m just not used to and it takes away confidence in my performance.
I also know that I wasn’t getting some things right. Not keeping my eyes focused far enough ahead was probably my worst sin. Keeping your eyes locked on where you want to go, not where you’re going, is probably the most important thing in autocrossing after keeping your hands on the wheel and not hitting any courseworkers. It’s very easy for me to forget that when I feel challenged by a course. On my final run I kept silently repeating to myself “Look further ahead” and picked up about half a second, too bad I hadn’t thought of that earlier in the day.
My new Kumho MXs in the rear are still a bit slippery so I’m going to keep them on the car for the next week or two to break them in a bit. Need to give the fronts a good examination this week, they may only have a few events left in them. When it’s time for replacements I must remember to order them shaved.
The next event is in two weeks with Equipe Rapide at the same site. I’m hoping that time of year and the shift from Miami-Dade to Broward county will significantly reduce the turn-out. ER’s events tend not to be the best organized, too many cars and not enough staff to keep things running smoothly, and this site’s layout will pose some additional staging challenges if we have the typical 100+ entrants.
Hopefully with the upcoming four-day weekend I can find some time to edit the video from this event and the previous one and get them online.
Street Touring November 20, 2005
That last batch of upgrades for my car move it from D Stock to Street Touring X. The SCCA added the Street Touring category in 1997 as a way of reaching out to the Sport Compact / Import Racing crowd, whose cars could be forced into categories typically consisting of more prepared cars over parts that had negligible performance impact. “Bling” items such as body kits and spoilers being prime examples.
There are two major differences between the Street Touring and Stock categories:
- Dress-up parts and performance upgrades suitable for daily driven cars are generally allowed.
- Tires must be 140 treadwear or higher, with tire and wheel widths restricted on a per-class basis
Street Touring’s treadwear restriction can have a significant impact on the cost of autocrossing competitively. In the other SCCA Solo categories any DOT approved R-Compound “racing slick” that meets the size restrictions is allowed. Such tires can cost $800-$1,200 per set, should never be driven on public roads, are typically good for just one season (10-12 events, 40-60 runs), and will improve times by a whopping 2-4 seconds. At the local level few people will be running R-Comps in Stock classes, but if you have the misfortune of being in the same class as someone who uses them you stand little chance of winning on street tires. In my area this is an issue at larger events, especially in the GS and HS classes.
High Performance street tires, on the other hand, don’t really cost more than any other tire — $350-$1,000 depending on what brand you choose. There are many tires to choose from that perform great on the track and street, and unless you are smoking the tires regularly the added treadwear from autocrossing is negligible. I use Kumho MXs, a popular street tire choice amongst autocrossers. They cost around $500/set and have a higher treadwear rating than the significantly more expensive and less capable performance tires that are original equipment for my car.
Street Touring presently has two competition classes: Street Touring S and Street Touring X. STS allows engines up to 3.1L N/A and 1.8L forced induction, STX bumps it up to 5.1L N/A and 2.0L forced induction. There are also two supplemental Street Touring classes available at the local level: Street Touring U and Street Touring S2. STU allows unlimited displacement for N/A engines and 3.1L for forced induction, STS2 is essentially STS for 2-seaters with naturally aspirated engines up to 1.9L. Each class excludes certain cars, consult the SCCA Solo Rules for details.
In my local area the STX competition is mixed. For the Central Florida Region events at Indian River Community College, Jorge’s WRX is often the only entrant. My raw times were within a few tenths of Jorge before this last round of upgrades so I expect some 1st-place wins next season. Equipe Rapide’s larger turn-out translates into 5-15 STX entrants. Subarus dominate in numbers but the lighter FWD imports seem to take home all the trophies. At the last ER event my best raw time was 1.483 seconds ahead of the quickest WRX but that still left me 3.769 seconds behind the 1st-place driver. I’ll need to improve quite a bit to make up nearly 4 seconds, I expect it will be frustrating for a while.
My next couple of planned modifications are allowed in Street Touring — rear swaybar, endlinks, intake, springs, and maybe an FMIC. After that I’ll have to decide between turbo upgrades that would put me in Street Modified or buying another car…
Stage 2+ November 17, 2005
With a new downpipe, uppipe, and three factory catalytic converters replaced with a single high-flow cat, the Saabaru is officially at Stage 2. And then some. The car has taken on an entirely different personality. The overall exhaust note is louder, boost comes on quicker, 1st gear goes by in a heartbeat, and the turbo can be heard spooling up, wwwsssthththth! Every time I press down on the gas it’s hard to resist pushing further to hear that beautiful sound…
Murphy struck one more time with the double-adjustable short-shifter. I purchased it used, at a savings of around $50. When I received it the upper bolt and nut were stuck. I told the guys a TAV Racing not to put it in if they couldn’t pry the nut loose, but of course they installed it anyways. It took several doses of penetrating lubricant, a long wrench, pliers, a deep 19mm socket, and a drill with 150ft/lbs of torque to solve the problem. Several sections of thread are messed up and I’m not sure that it will be possible to fully remove the nut and bolt again, but I did finally get it adjusted to my liking — about as short as it will go with the factory shift knob.
I’d say the shifter is set for a Miata-like throw. Very short shifts, firm and precise. It doesn’t feel as great as the Miata shifter, or even a BMW’s, but it’s a heck of an improvement over an Impreza.
As for the front swaybar, I’m not ready to say much about that yet. Street driving is a poor way to judge handling upgrades. I need an autocross course.
For autocrossing this series of mods moves my car from D-Stock to Street Touring X. I’ll write a bit about what that means in my next posting.
Murphy Strikes Again November 15, 2005
So I finally got an appointment with TAV Racing to do my installs — front swaybar, rear swaybar, catless up-pipe, catted down-pipe, and a short-shifter. I dropped the car off this morning and they called me shortly after lunch. “You have two FSBs”. Uhhh, no, they should be part numbers BSF12Z (front) and BSR36XZ (rear). “You’ve got a BSF36XZ, an FSB for the STi.” Damn it.
I could have sworn that I checked the part numbers when they arrived, but… Seeing as how both the vendor and I missed the distinction between BSF and BSR I guess that it’s an easy mistake to make.
So I’ve e-mailed them about getting the right parts swapped. Once I’ve got the right part I may just put it on myself — from what I understand it’s a pretty easy job.
Update 3:27PM — The vendor called, apologized about a zillion times, and promised to ship out the correct part in the morning. UPS will collect the incorrect part when they deliver the new one. That’s great service. Thank you, JnJ Associates.
Update 11/22 — The BSR36XZ arrived yesterday but the UPS guy pulled a knock-drop-and-run. I was surprised to find a bottle of Shining Monkey D-Spec Carnauba Wax in the box. I was happy enough that someone from JnJ called me promptly when I reported the mistake, was genuinely apologetic, and shipped out the correct part immediately. A $10 make-up gift is an incredibly nice gesture from a vendor that must be operating on razor-thin margins. I returned the favor by spending another $180 on endlinks and dropped the BSF36XZ at the UPS store on my way home from work tonight.
| older posts »
