j-rho

Hello,
First post here, at least since I had my 944S and posted on the email list many years ago. I'm now planning a build of a 964 911 C4 to compete (and win) in the SCCA's ProSolo and Solo2 Street Modified 2 (SM2) class. For those not familiar with this ruleset, here's a link to the rulebook:
http://www.scca.com/_FileLibrary/Fil...solo_rules.pdf

Our National Championships recently moved from a very grippy concrete surface to a rather slippery asphalt one. Because of this, and the power/acceleration needed to be competitive in SM2, I think the C4's all wheel drive will be the hot ticket. The car will be race-only, with the only nonrace miles being from inside the trailer to starting line. No compromises for streetability, emissions legality, or resale value.

Some preliminary parameters:
Starting with '91 964 C4 coupe
Rules dictate 2900lb. minimum weight
Planning to run 285/30-18 Hoosier A6 on 18x11" up front, 315/30-18 on 18x12" rear

Questions by area-

Bodywork:
I see GT-Racing has some good fenders and parts to fit the bigger wheels/tires. Any other places I should be looking? Is all that stuff generally usable on any year 911, at least up through the 993?

Suspension:
Is there a good one-stop shop for quality suspension components for these cars? I am limited to stock suspension mounting points, but everything else is free, including control arms and uprights. I'd like to end up with spherical bearings at all the pivot points (at least where they don't cause bind) and would also like to have a good range of adjustability (toe, camber, caster). A friend is a shock engineer at Bilstein and is anxious to put together some shocks/struts for the car but I'd like to see what other good (at least double-adjustable, can handle high spring rate) options there are out there before deciding. In searching I've seen Motons mentioned a lot??? I've run JRZs and Koni 28's in the past, just looking for a shock/spring that is already relatively well sorted for this sort of application.

Motor:
Need 400+hp to the wheels to be competitive. Ideal is a broad and flat-ish torque curve (3000-7000rpm); can't afford to wait for the car to come on cam or for boost to build out of a slow corner. Was thinking TPC supercharger with custom icewater intercooler, race gas, and maybe some mild headwork with cams. No cats, minimal muffling. Would that get me close, or would I need to lower compression and up boost? Remember auto-x is short bursts of 50-70 seconds, with lots of time to cool in between each run. I want it to be very reliable within this context, but it need not be as sturdy as a track day or endurance race type build. Would probably do upgraded oil cooler(s) and fans as a minimum.

Drivetrain:
Figure upgraded flywheel/clutch/pp at a minimum. Otherwise, how reliable are the C4's components? ProSolo autocross features drag-race starts, which I know are abusive and very un-Porsche-like, especially with the big sticky tires, but a good hard launch is essential to victory, and the car should launch great if I can keep the drivetrain from exploding. Are there upgraded components (diffs, half-axles) available? Maybe some of the AWD 911 turbo parts? My friend (known for his bad luck) broke the passenger halfshaft in his new (and stock) GT3 at just his second ProSolo. Is the tranny reliable at this power level, and if not are there stronger options (930?) compatible with the C4's layout?

Brakes:
I'm guessing the stock stuff will be okay for the low speeds of autocross, but should I need to upgrade seems like there's lots of good factory stuff (big reds)...

Well, that's all I have for now... I appreciate any insight those more knowledgeable can share on these subjects. If anybody has built a motor similar to the one I've described above, I would love to see a dyno from it. Many thanks in advance!!!

--Jason Rhoades

j-rho

Okay, that may have been a bit much for my first post, let me try something simpler and more concrete-

I have a set of 28-series Koni shocks/struts on my current racecar, and I'm trying to figure out if I might be able to re-use them on the 911. For you guys with really low cars, generally how much do you shorten the front struts? Basically I'm looking for representative fully compressed and fully extended lengths for a lowered application. I have the values for the stock parts, but it's unclear to me how much they need to be shortened in a racecar situation (1", 2", 4")????

Many thanks!

NJ-GT

That sounds like a very expensive not competitive project under the current SM2 rules. SM2 allows lighter fenders (front only for a 911), and either a lighter trunk lid or rear deck lid, but not both. At 2,900 lbs and 400rwhp, the car won't be competitive. SM2 crushed FP by 2.1 secs and XP by 4.5 secs.

I choosed to run FP with almost 300 lbs over minimum weight and no slicks. We ran at 40 degrees weather. My car was unable to put the power down on that weather, it lost its strongest advantage. I still came in second place at Nationals by 0.7 secs after two days, regardless of a cone on my fastest run.

I thought about running SM2, but the current rules give a huge edge to the RX7. When GT won SM2 on 2005 with a Z06, he was running on concrete, that car could not win anymore on the new slippery surface.

I think it would be much better to prepare the car for FP. You get to run it lighter (2,500 lbs), you can get 16" Hoosier slicks (not GoodYear for Heartland Park), and you can supercharge it. The maximum weight for a car in the class is 2,500 lbs plus penalty. Stick to 16"x10" wheels at the four wheels, strip the car, and then add weight to the bottom of the trunk. All the 3.6 911 are allowed in FP.

Shocks for a 964? Look for answers in the 964/964T forum.

R.

j-rho

A goal would be to have a bit of ballast in the car in SM2, which could come out to let somebody else (James Gunn-Wilkinson has already been bugging me) run it in XP. XP minimum weight would be 2610; I don't think I could get it that light in an unballasted SM trim, but maybe 27-something.

I've been wanting to build an SM2 car for a while, at least since co-driving with Thomason in 2005, where I ended up third in SM2. I waited to see and experience the new HPT surface before choosing a car. I'd been thinking about a turbocharged S2000, or a Saturn Sky with an LS7. You're right about GT's old car not being competitive any more - after seeing and experiencing the low grip (and watching BSP run!), I decided an AWD car would be the best way to go, especially if the course is at all cold, dirty, or wet. I think AWD would be great at the Pros too, which for me is the more fun aspect of the sport.

Andy and Erik both really struggled to put power down in their cars on the surface, even with the higher temps and cleaner track of heat 4 Thurs/Fri. Andy's car isn't making much over 400 to the wheels, though his high redline and short gearing combined with light weight (~2570lbs., Erik's car with the 3-rotor is about 2770) make his thrust graph look pretty good (I have speed vs. thrust graphs for most all of the top SM/SM2 cars). But again, if he and Erik can only put down 300 most of the time, and a C4 can put down all of its 400 while not getting all out of shape (I saw Erik throw away at least 1-2 runs due to almost losing the car when the boost came in) and/or not waiting for the boost to build, it will have the edge. I think... :-)

I figure vs. the SS GT3, this car would be 100 pounds lighter, have AWD vs. RWD, better weight distribution, way more torque in the 30-50mph speed range, 2" more front wheel/tire and 1.5" more rear wheel/tire, while being lower, narrower, and stiffer. It would have inferior rear suspension geometry, and its acceleration vs. an "on the cam" GT3 would only be about equal. Don't know if it'd be enough to go 2 seconds faster over the two days, but it's worth a try. The other thing I like about the platform is that it should respond well to more and more power, as the budget allows, maybe eventually a small TT setup or something.

Here's the SM allowance for body part replacement:
The way I interpret this for a 911 is that the rear lid thing is the engine cover and may be replaced, but that the front "hood" is in our case a trunklid, and may not be replaced.

You must be Radomin? My compliments on your FPL codriver...hopefully her presence helps get the car in some magazine photos...

NJ-GT

Jay,

You can run the same car in FP at 2,500 lbs, or XP at 2,610 lbs. The 964 C4 is a bit heavy, so it's difficult to get it down to that weigth and still keep it SM2 legal.

XP had a Z06 that lost the Championship due to cones on day 1, but they were about 1 second faster than the rest of the fast competitors. Eventually XP is going to be faster than all the other Prepared classes.

Definitely the AWD cars have the edge at Heartland Park, but so does the 911. Now an AWD 911 is an even better choice, as long as you can tune the AWD. Technically, a narrow 964 C4, with 400rwp, 2,900 lbs, big wheels and tires looks competitive, but I doubt it can be 1 second faster than a stock GT3, or stock Elise, and that's what you need to compete against Erik and Andy.

You could research on this Carrera 4 RS Lighweight to work on your project: