Building a 986 S for SCCA CS Autocross for National competition

[Javelin]

I came in 13th and my co-driver 15th out of 19. The car had no issues whatsoever. We are clearly rookie drivers but are very happy with how we did. We were the fastest of the 3 986's. My PAX Overall was 171 out of 281. For what is essentially a stock 986 on RE71's, that's pretty good.

We are going back this weekend for the National ProSolo where I hope my drag racing background helps me chop the tree down and do a little better. My codriver will be running L2 trying for a trophy (she would have been 1 spot out had NT used L2 instead of classes) and I will be running on the FP index in R2 to help friends fill out a field.

[Javelin]

ProSolo is in the books now, too. The car did great and my co-driver was the fastest CSL and got 9th in the L2 Index class. I bumped to R2 to help fill a class since the CS guys were all the ones faster than me last week anyway and had fun chopping the tree down. Our car does not like drag-style launches at all, so we were relegated to doing idle-starts and cutting 2.2-2.4 60's.

The car scaled at 2975 lbs with 1/4 tank of gas, so we aren't bad, but there's still lots to be lost in the exhaust and wheels, which are both stock.

Videos:

https://www.youtube.com/watch?v=xfpUz_VPaE0

https://www.youtube.com/watch?v=vzUX_f8lu1o

[thstone]

Originally Posted by Javelin

Our car does not like drag-style launches at all,...

Can you explain what you mean by a "drag style launch"?

[SCCA_AX]

Normal autocross typically has a turn right off the start to attempt to minimize launch advantage. You also can start at your leisure, because you have to break the start beam to start the clock.

Pro Solo has an actual drag tree and at minimum 100' before the first turn. Timer starts when the light turns green, reaction counts.



Pro Solo is kinda like autocross on cocaine.

[lawren]

Just bought a 1998 base Boxster for autocross (c street) and track fun. Planning on new tires, an alignment, and adjustable front sway bar and drop links. Came with 17"x 7" fronts and 17"x 8.5" rear stock wheels. What size (width) RE-71Rs do you recommend for C Street autocross on the 17" stock wheels?

[Anker]

Stock rears (255/40R17) and 225/45-17s on the front.

[thstone]

Originally Posted by SCCA_AX

Normal autocross typically has a turn right off the start to attempt to minimize launch advantage. You also can start at your leisure, because you have to break the start beam to start the clock.

Pro Solo has an actual drag tree and at minimum 100' before the first turn. Timer starts when the light turns green, reaction counts.

Pro Solo is kinda like autocross on cocaine.

That is awesome! I have done a couple of road course races with a standing start - when the whole field does a drag race its pretty exciting!

[lawren]

I have my first autocross (SCCA STR Class) in a few weeks. I was wondering if someone could give me some tire pressure recommendations for:

1998 Boxster Base Model (2.5L)
Stock 17"x 7" fronts and 17"x 8.5" rear wheels
Bridgestone RE71Rs (225/45-17 front & 255/40-17 rear)
Front LCA (this is my only suspension mod at this point)

[SCCA_AX]

Originally Posted by lawren

I have my first autocross (SCCA STR Class) in a few weeks. I was wondering if someone could give me some tire pressure recommendations for:

1998 Boxster Base Model (2.5L)
Stock 17"x 7" fronts and 17"x 8.5" rear wheels
Bridgestone RE71Rs (225/45-17 front & 255/40-17 rear)
Front LCA (this is my only suspension mod at this point)

I wouldn't sweat it too much, the RE-71R is pretty pressure agnostic. Start at 35-37 square. Too much pressure (40+) and they get a little too sharp / antsy. Too little pressure and they just burn off the shoulders quicker, but aren't really any faster or slower.

If you go further down the STR rabbit hole and start running lots of camber you can probably start dropping down to the low 30s, especially in the front, and see time gains. Gotta test though.

FYI, if the tires are brand new, they tend to take an event or two to really hit full grip. Street miles don't speed up this process. Then they're pretty consistent until the tread is almost gone, as long as you don't cook 'em at a track day.

Good luck.

[Anker]

35-37 is way too high pressure to start with. Start with 34, do a run or two and watch whether the wear mark comes down to the small triangles on the sidewalls just inside the treads. If not, drop the pressure by 1 PSI and repeat. As the tire heats up after each run you will have to let out air to keep it at your desired pressure. Same pressure on all 4 tires.

If the track is hot (sunny and over 70 degree air temperature) the RE71-R will get greasy after 3-4 runs. You will need to get a garden sprayer with water and spray the tires to keep the temperature down.

If you really want to do it properly get a tire pyrometer and measure the tread temperature at the middle and each side of the tread immediately after each run. You want to temperature to be as even as possible across the tire. If the center is warmer than the outside edge your pressure is too high. Because of the limited camber of the Boxster suspension you will not be able to get the inside and outside temperatures to be the same. The outside will always be warmer.

Since this is your first autocross you will not be driving the car nearly as hard as experienced autocrossers, so don't let the pressure get under 30 PSI, even if the wear mark doesn't get down to the triangles.

Turning performance will improve if you change toe-in to 0 in front and maximize front end camber, but not more than a couple of degrees. On a stock suspension you will not even be able to achieve 1 degree camber.

Don't forget to add pressure to the tires before you drive home!

Have fun!

[JayG]

Originally Posted by SCCA_AX

I wouldn't sweat it too much, the RE-71R is pretty pressure agnostic. Start at 35-37 square. Too much pressure (40+) and they get a little too sharp / antsy. Too little pressure and they just burn off the shoulders quicker, but aren't really any faster or slower.

If you go further down the STR rabbit hole and start running lots of camber you can probably start dropping down to the low 30s, especially in the front, and see time gains. Gotta test though.

FYI, if the tires are brand new, they tend to take an event or two to really hit full grip. Street miles don't speed up this process. Then they're pretty consistent until the tread is almost gone, as long as you don't cook 'em at a track day.

Good luck.

Originally Posted by Anker

35-37 is way too high pressure to start with. Start with 34, do a run or two and watch whether the wear mark comes down to the small triangles on the sidewalls just inside the treads. If not, drop the pressure by 1 PSI and repeat. As the tire heats up after each run you will have to let out air to keep it at your desired pressure. Same pressure on all 4 tires.

If the track is hot (sunny and over 70 degree air temperature) the RE71-R will get greasy after 3-4 runs. You will need to get a garden sprayer with water and spray the tires to keep the temperature down.

If you really want to do it properly get a tire pyrometer and measure the tread temperature at the middle and each side of the tread immediately after each run. You want to temperature to be as even as possible across the tire. If the center is warmer than the outside edge your pressure is too high. Because of the limited camber of the Boxster suspension you will not be able to get the inside and outside temperatures to be the same. The outside will always be warmer.

Since this is your first autocross you will not be driving the car nearly as hard as experienced autocrossers, so don't let the pressure get under 30 PSI, even if the wear mark doesn't get down to the triangles.

Turning performance will improve if you change toe-in to 0 in front and maximize front end camber, but not more than a couple of degrees. On a stock suspension you will not even be able to achieve 1 degree camber.

Don't forget to add pressure to the tires before you drive home!

Have fun!

34 May even be too high to start
I typically would run 30-32 hot. So cold around 28

[lawren]

I had my first autocross weekend in the Boxster. During the day (total of 6 runs) I kept lowering the pressures in the RE71Rs until i was rolling over to the tips of the triangles on all four tires. Not surprisingly, when i got there i was getting my fastest times - that occurred right at about 30psi. I think that will be the pressures i am shooting for from now on. Anybody surprised to hear that? I do have LCA's in the front with just under -3 degrees camber.

[Stroked & Blown]

I find the best grip on RE-71Rs at 27F and 30R hot.
Camber -2.3F, -2.0R
Tires 235/40-18F 265/35-18R

[SCCA_AX]

Originally Posted by lawren

that occurred right at about 30psi. I think that will be the pressures i am shooting for from now on. Anybody surprised to hear that? I do have LCA's in the front with just under -3 degrees camber.

With that kind of camber, I'm not surprised. At OEM camber levels, I would be surprised.

Be aware of the multiple variable effect. Multiple looks at a course, and you'd have gone faster with or without the pressure change. So don't necessarily attribute the lower times to the lower pressure.

The right way to do it is with a skidpad, a pad of paper, and a stopwatch. There's like maybe 20 autocrossers in the country that go to those lengths. The rest of us shoot from the hip.

[LAP1DOUG]

Originally Posted by Stroked & Blown

I find the best grip on RE-71Rs at 27F and 30R hot.
Camber -2.3F, -2.0R
Tires 235/40-18F 265/35-18R

My data matches pretty well with this guy. Only real difference is that I'm running 255 Front / 285 Rear, but similar 30 mm stagger front to rear

Looks like SCCA is still handicapping these cars with too little wheel and tire.