SCCA STR Autocross Carrera Build
 

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. ;)

http://oi46.tinypic.com/vcqibc.jpg

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.

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

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

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.

all that for 0:50 worth of driving

<rolleyes>


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

http://lh3.googleusercontent.com/-qr...124_111111.jpg

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.

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!

So true! It's a sickness.

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.

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!

LOL

spending money - modding

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

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.

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.

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

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.)
You can do 140twr.

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

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.

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

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.)

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

http://lh4.googleusercontent.com/-cl...202_144931.jpg

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.

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.

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.

I'll be using Euromeister fake Fuchs, 17" variety.

http://i48.tinypic.com/1gkgte.jpg

Well, took the car out of the garage for a quick squeak check. The side skirts are off, it's dirty, and totally not aligned, but it drives again. Definitely stiffer, a little louder, and the 150k mile shocks cannot handle the torsion rates. Height "looks" pretty good in the rear, but I have to raise the front to get correct geometry back. It also seems that my car can get a lot more camber on the right rear than the left rear, is this common?

It's taken me 10 days so far to do the suspension. Good thing I pay myself by the hour. In beer.

Also agree that 23/30 is less than ideal for autocross. Neither is 21/31 or 22/33 IMO. 21/29 or 22/30 would be a better choice for a Carrera on an autox course. You need to get the car to rotate easily, and you also need some compliance. The right shocks are even more important than torsion bars, as you are usually dealing with parking lot surfaces and abrupt transitions. I would also go with the lowest aspect 15-inch sticky street tire you can find to help with your car's tall 2nd gear (2225-45/15 Hankook Ventus R-S3 wouid work).

Well, you guys have me doubting my choices so far. Perhaps I should have asked on this board first. Ah well, live and learn.

I did a string alignment last weekend. Decided to go a little aggressive with zero rear toe since the consensus is that I'll be pushy with my setup. I'm not going to touch front camber until I verify that the tires will fit in the fenders - it is now set with the camber plates maxed out (negative.)

http://lh4.googleusercontent.com/-Gh.../alignment.jpg

So far I'm impressed with all of the products. The car actually rides quieter, and my wife thinks it's more comfortable than before. Go figure. My old bushings must have been in much worse shape than they looked. But really, even though spring rate more than doubled, it's not nearly as stiff as I expected. The shocks are inadequate - the car does "bounce" (what's faster than a bounce but slower than a vibrate? A jiggle?) Can't wait to get them revalved.

The rear seems to have settled a bit, so when I corner balance I think I'll raise it up. Then just shocks and tires left to do before I unleash the car on the cones.

No massive update, but the car has been all back together for a while, going for a nice exercise a few times a week since the roads have been pretty dry here.

http://lh3.googleusercontent.com/-Kj...115_170145.jpg

And some nice Dunlop Z2 tires showed up today:

http://lh3.googleusercontent.com/-mV...121_165848.jpg

These are the 225/45-17 for the front. I'm still undecided on the rears, but am leaning towards the 245/40-17 for better balance and improved gearing over the 255s. Comments?

I did extensive autocrossing with my RSR, and going back to 15's (from 17's) made a HUGE improvement in my times. Consider doing some 7 & 9 Group 4 wheels, perhaps?

Tire selection. Nothing wider than 225s in 140tw+ in 15".

Matt, I'd listen to Michael...been in a car with him on the autocross and he is quick...even with the limited choice 15's.

The advantage of using 15 X 7, 8 is you get a lower car without changing the suspension...almost 1" lower and quicker acceleration. AutoX is about accelerating from tight turns. I was thinking of going to 16s but the tire size does not allow any smaller diameter for lowering the car. I was wondering if 15 X 8 and 9 would work with a non turbo car?

I'm not convinced. The M coupe, 350z, and Z4 are all also rans in the class. It's dominated by the Miata and S2000. Power doesn't mean anything if you can't put it down. Grip is everything.

I would gladly run 255s on 15" wheels if they existed, but the smallest diameter to get them in is 17". I'm not giving up that grip advantage, especially when the 911 has a torque advantage over the top two cars already.

Numbers are misleading until you race. I can beat Miatas but S2000 are hard to beat. Grip is nothing without minimizing body lean and fast times depend on a very responsive car that can break and accelerate. A good alignment can really tell you what your base is before you change costly parts. Also make sure you don't have any rust on the front end that weakens stiffness. I was not willing to spend big bucks until I had an idea of the handling. Tell me if the bigger tires work. You can also buy used tires for half price (SCCA Sports Car Mag).

Minor update -Avoiding the Porsche Tax

I'm not too keen on paying $200 for a strut tower brace when they are available on ebay for every other car for $20 or less. So I decided to adapt my own. I bought a Honda Accord brace for $8 including shipping. Took some scrap aluminum square tube, and made my own side brackets. Let's just say there is a reason I don't work in a machine shop:
http://lh5.googleusercontent.com/-D-...h373/photo.jpg

Two issues I ran into:
The Honda Accord brace was about 6" shorter than what would be perfect, so my brackets had to extend the extra distance.
The strut tops aren't flat. What a pain. My solution was to chamfer the ends of the plates and let them bend to shape as I tightened them. Worked like a charm, but the brackets ended up pointing down slightly. Ah well.

End result:
http://lh6.googleusercontent.com/-wl...h373/photo.jpg

Once I cut the carpet it'll hide my shoddy aluminum fabrication. ;) Tensioned it nice and tight with preload. Not perfect, but certainly not $192 worse than the off the shelf option.

I think it will flex under a load that woudl flex the chassis there. You may be adding weight. I would re design or ditch it. I do applaud your efforts - better looking than some of my fabbed pieces ;-)

Hey Ben!!! Nice to see you here, buddy! How's the car?

Well, looks like BraidUSA has listened to you all and is running 205 / 225 - 15 combo.

Me, I'm hard headed as all getup and just got my 225 / 255 - 17 combo installed.

We'll be competing at the same events, so looks like this will be a great experiment!

Just in case you are all right I have my OEM 15s in a nice stack in the corner of my garage...

http://4.bp.blogspot.com/-fXIDM5HXWv...329_153002.jpg

The racing season is upon us. Just a few things left to do before the first event - still need a corner balance and a catalytic install.

Stay tuned for the first results of all that hard work.

Hey Matt, is your front bumper shortened? I mean...recessed? I've been wanting to do that mod and it looks like yours is recessed.

I hope not, it's not an allowed modification. Looks normal to me, maybe the lighting and cell phone camera combined to make a weird effect?

https://lh6.googleusercontent.com/-T...329_153122.jpg

Ha! Hard headed is right....255 is WAY too much tire for an AutoX car. Hell, it's almost too much tire for a track car with a stock drivetrain (factory weight).

Come beat me and I'll listen to you. ;)

Was that the sound of a glove being thrown down?

I'd expect an autocross car to have more use of wider tires than a track car. I don't recall seeing too many Miatas on 255s or 275s on a track, but they sure do love them on an autocross course. I ran my Impreza autocross car (same weight as a Carrera 3.2) on 255s, and I was the odd one out when everyone else was running them on 275s.
-N