SCCA STR Autocross Carrera Build

[Driven97]

EDIT: See build blog at strcarrera.blogspot.com

Ok, BRAIDusa (Paul) talked me into putting up a thread here. We're in the same area, have nearly identical cars, and are doing somewhat similar projects. I've got a two month head start on him though.



Here is my car, an 84 Carrera 3.2. I'm sure like most of you, it's a car I've always wanted, and when the opportunity arrived to get one, I jumped on it. I'm a long time autocrosser, and have decided to build my car for SCCA's STR class.

Oversimplification: Street cars, with simple bolt ons only, on street tires. Currently, the class is dominated by the S2000 and NC Miata. I may be bringing a knife to a gunfight, but I try to rationalize it as the car is lighter than the S2000 and narrower than the NC, with the shortest wheelbase, and a good bit more torque down low than either of them. I'm not doing a no-holds-barred build, just looking to get a competent "sportsman" level car out of the deal I can use locally for a long, long time, and not compromise the ability to use the car as a weekend pleasure cruiser.

I ran the car mostly stock in STR twice before the end of the season, and my PAX percentage was in the 85% range. Not great, my lofty but attainable goal is to get up to the 95% range, which is about how I did in my previous car.

An example list of things a I am allowed to modify:
Tires up to 255 width, 140tw minimum
Wheels up to 9" wide
Torsion bars
Camber plates
2-pt strut brace
Bushings & mounts - as long as they are not higher metal content than OEM.
Swaybars - as long as they mount without having to drill the chassis
ECU / Chip / piggyback
Exhaust post cat
Battery size and location
Intake before the MAF or Throttle
Seats, as long as they weight at least 25lb including hardware

I currently have the car apart getting a suspension refresh, I'll detail what I guessed would work well in a later post.

[zippy_gg]

I'd like to know what you will be going with on torsion bars, sway bars, and shocks...
Interesting project for sure!

[Elombard]

Bummer that you have to keep the cat. What are your plans there - keep the stock?

[Josh D]

How many times do you have to shift in autoX? I've always been curious about that because it seems some time could easily be shaved with some shift improvements as that seems to be the slowest part of our cars.

[ivangene]

all that for 0:50 worth of driving




LOL - kidding!
(kind of)

I ran a "real" atuox last year (I run a lot of un-timed ones) and posted numbers that surprised a few of the regular guys - they have been asking me to get involved but it really is NOT my thing

plus all the rules - if you do any of those, you have to do ALL of those to be competitive - I have a few too many things done, but not enough to really take on the big dogs (like Randy W on here - recent excellence article winner at autoX)

I think I will just keep doing it for fun

good luck on your build - post ppics

OH - have you done ANY autoX with this car?
if so - post your standing changes POST build

[Driven97]

I went with 23/30 bars, for two reasons:
1. It's very close to the stock balance, supposedly ~250% stiffer both front and rear.
2. It's the biggest you can get in the more economical Sway-A-Way solids. Price jumps up significantly when you go to hollows.

I can't use a through-body front swaybar, so I bought the Addco #827 bar. It's the biggest off the shelf option being slighty bigger than the biggest 930 bar (7/8" is about 22.25mm,) plus I picked it up brand new for $150. For the rear, I have an idea on how to avoid the Porsche Tax but it'll take some measuring and a little luck. I'll leave it at that for now, and if I can make something work I'll be sure to post it up.

For struts I got a little lucky, as one of the two options ordered on my car are the Bilsteins. I just happen to have a nationally competitive autocrosser who is also a professional engineer and has a shock dyno and everything needed to rebuild / revalve the shocks. I've sent him my data to get a baseline, and he asked for some time to research the car a little. Bad news is they will not be externally adjustable, but the good news is that I can have them tweaked within a few days if needed.

The cat rule just got reworded, and I don't know how solid it is. I'm thinking staying OEM for now is best course. The big bummer is that SSI's are out of the question unless I modify them to a 1-in style somehow. No matter, as those are out of the budget for now anyways.

If my research is correct, a 915 box Carrera will hit 73mph in 2nd (is this right?) So, 99.9% of the courses can probably be covered in a single 1-2 shift. My 'box isn't the freshest, but I've taken 15 runs with the car so far and haven't blown one. I wouldn't mind it being quicker, but it currently isn't slow enough for me to bother with.

[ivangene]

I dont the the SSI's would be a big game changer and cost a lot - the loss of weight is the only real benefit IMO - you can loose weight in other areas to make up that

nice shopping list!

[Driven97]

Originally Posted by ivangene

all that for 0:50 worth of driving

So true! It's a sickness.

Originally Posted by ivangene

OH - have you done ANY autoX with this car?
if so - post your standing changes POST build

I've had the car out twice, as purchased, just to get a feel for it.
Video
It's going to take some adjustment to my driving style - the 911 is definitely unique!

My metric for performance is going to be my PAX score. It's SCCA's theoretical index for comparing cars across different classes. It's far from perfect, but relatively repeatable. In my last two cars, I would score around 93-95% as fast as the top index time. My goal is to get the 911 there.

My stock benchmark is 84%. I have to wait until spring to see what all this suspension work gets me.

[Elombard]

Yah I was talking about the weight, if you go to a cat bypass and a light weight muffler I think you gain a few horse power and lose 30 or 40 lbs off the back end. From there I dont think the gains for SSIs are real significant...says the man who just spent $500 on headers.....just cant leave well enough alone!

[ivangene]

LOL

spending money - modding

that is NOT a sickness
(so I told my wife )

[DG624]

Reducing weight is a real help. I cut about 200lbs by eliminating the stock rubber mats under the rear seats and the rear seats, the rear mat in the engine compartment, 17 lb battery, Recarro seats, eliminate the rear heat shields for the CAT and had the CAT ceramic coated, and eliminated sunroof.

I am running SP but you can benefit from most of the changes. I replaced the stock sound deadening with Quiet Car a waterbased sound proofer that weighs almost nothing. I also used new carpet with a very light pad backing.

[Patrice911]

What size tire are you planning to used?
I got an 89 3.2 (stock chip) and an 84-85 915 in my car, so its like yours, and in 2nd with 225/50/15 the max is 64mph.
With 16" tire 2nd gear is more like 67mph. With the course they do around here I usually need to go in 3rd once per course.
If you put a chip with 6800 limit and 16" tire you will get to 70mph in 2nd.
Your torsion bar size will work good, lower the car and put as much neg. camber that you can.

[kjchristopher]

Originally Posted by Elombard

Bummer that you have to keep the cat.

He has to keep a cat. Doesn't have to be the stock one.

Originally Posted by Josh D

How many times do you have to shift in autoX? I've always been curious about that because it seems some time could easily be shaved with some shift improvements as that seems to be the slowest part of our cars.

Usually once. A good designer, unless they are dealing with a postage stamp for a lot, will keep things flowing. Most of the time, even when it is a slow, pinch turn, it is faster not to shift. (Even for easy shifting cars.)

Originally Posted by Driven97

Tires up to 255 width, 180tw minimum

You can do 140twr.

[McLaren-TAG]

Shifting depends...a friend sometimes finds himself needing to shift into 3rd on his 912. I've never had to.

[Driven97]

Originally Posted by Patrice911

What size tire are you planning to used?

There's not much (if any) selection in 16" tires in the correct sizes, so I'm going 17". I used the handy-dandy calculator spreadsheet from here. My numbers are 255/40-17 and the Steve Wong 6760 limit. Perhaps dropping down to a 245/40 might be better.

Originally Posted by kjchristopher

You can do 140twr.

Good catch, I fixed it. I know what I meant.

Originally Posted by DG624

Reducing weight is a real help. I cut about 200lbs by eliminating the stock rubber mats under the rear seats and the rear seats, the rear mat in the engine compartment, 17 lb battery, Recarro seats, eliminate the rear heat shields for the CAT and had the CAT ceramic coated, and eliminated sunroof.

If I'm reading the rules correctly, all I can do for weight reduction is:
-Incidental to exhaust replacement
-Incidental to wheel/tire change
-Incidental to legal suspension changes
-Battery change
-Fog light removal
-Brakes, though ligher rotors or calipers, with certain restrictions
-Seat swap
-Driver mod (I could probably lose about 30lb right there.)

[Driven97]

I may not have measured that spring plate angle correctly on the first try.

[winders]

For an autocross car, I think a 23/30 torsion bar setup is too stiff at the front. Personally, I would go 21/31 or 22/33. Otherwise, you will be fighting understeer on all but the fastest courses. I would get the same size anti-roll bars front and rear as well...

Scott

[Driven97]

Originally Posted by winders

For an autocross car, I think a 23/30 torsion bar setup is too stiff at the front. Personally, I would go 21/31 or 22/33. Otherwise, you will be fighting understeer on all but the fastest courses. I would get the same size anti-roll bars front and rear as well...

Scott

Is the relationship known between under-body and through-body FSBs? I suspected I might be a little front stiff, but I was planning on making up for it with a little extra rear bar.

[5:04]

Originally Posted by winders

For an autocross car, I think a 23/30 torsion bar setup is too stiff at the front. Personally, I would go 21/31 or 22/33. Otherwise, you will be fighting understeer on all but the fastest courses. I would get the same size anti-roll bars front and rear as well...

Scott

I agree 100%. Even on the track I think a 23/30 has too much push for my liking. I had 23/30's on my car and when I auto-x'ed it pushed like a son of *****. If I were to set the car up for Auto-X I'd either go stiffer in the rear or softer in the front and then balance it with sways.

[DG624]

My numbers are 255/40-17 and the Steve Wong 6760 limit. Perhaps dropping down to a 245/40 might be better.

The lower aspect ration may help reduce diameter. I was using 225/50/15 and 205/50/15 that reduce my tire diameter by ~ 1" It also reduce the ride height. the 7,8 X 15 Fuchs were around $800 so it was cheaper than aftermarket wheels.

Did you say what wheels you were using? Most of the 17" variety wheels are over $250 Ea. and they also are heavier than Fuchs.