Potential 911 GTS entry

[Jas0n]

Thinking about prepping my 911 for GTS2/3. I've been running with PCA, but my car doesn't fit competitively into their club racing classes ...

 

My car is a 78 911 with an RSR fiberglass wide body; 3.4L motor with 46mm PMO carbs and crank fire ignition, single plug; big torsion bars, bilstein sport struts, sway bars, adjustable spring plate, polybronze bearings; rebuilt stock brakes with pagid pads, driller rotors and stainless lines; 245/315mm NT01'S on 17" BBS Motorsports wheels; ducktail.

 

I'm going to have a custom cage fabricated to replace the Safety Devices bolt-in cage I'm currently running. Beyond that, I had a few questions that you 911 running guys might be able to help with:

 

- Torsion Bars. My car is very flat and seems stiff enough, but many race 911's are running coil overs. I'm not skilled enough to be able to take advantage of the adjustability they offer - but if I stick with torsion bars, will I be limiting my competitiveness?

 

- Aero. I can already feel the need to reduce front-end lift, so I've order an RSR splitter and an under tray. Would adding something like a GT racing GT3 style wing be sufficient? https://www.gt-racing.com/products-page/911-parts/911-gt3-style-tail-base-and-wing/

 

- Brakes. Braking power seems good enough (I can lock them easily, but there is some fade during longer sessions. Improved cooling seems like a good solution - do I have to step up to a turbo style brake or will cooling suffice?

 

I haven't dyno'd the motor yet - it currently weighs in at 2450 pounds without driver (full interior, two seats, stereo). I assume that I'm better off detuning the motor to run GT2 (180ish HP) v. running against E36 m3's and RS America's in GTS3 (230ish HP) @ around 2650 pounds. That way, it seems I could afford to add a little more weight via a more robust cage and even keep a passenger seat - sound mostly right? Anything else I'm missing? Thanks!

[JSG1901]

Hi Jason, and welcome.

 

Hopefully we can get a few of the 911 experts on here to give you some solid advice on most of the specifics of your car. My two-cents' worth is that, all things being equal, lighter is faster. You haven't mentioned how much you (and your gear) weigh, but as you probably know, our power-to-weight ratios include the driver, so you'll need to factor that in if you haven't.

 

To that end, you probably ought to get it dynoed sooner rather than later. There's a post over on Rennlist (http://forums.pelicanparts.com/porsche-911-technical-forum/653310-3-2l-carrera-pmo-carbs-bosch-motronic-msd.html) where one of the respondents claims 287 hp from a 3.4 single-plug with Motronic. I don't know if that's at the wheels (like we use) or at the flywheel, nor how yours might compare, but if your starting point is anything like that you may want/need to think about a faster class.

 

Once you know that, you'll know a lot more about how you do or don't fit into whichever class it is you're shooting for. If you have the choice (because of how the class ratios compare), I'd be awfully tempted to try to get as much of the easy weight out as possible as lower weight will make it both handle and stop better. Of course, depending on power levels, that may not be possible/feasible.

 

Finally, a few thoughts on aero. Aero is, as much as anything, a balancing act. The more you have in the rear, the more you'll need in the front to balance it out. Smaller wings seem like they would make less drag but very often you have to run them at higher angles than larger ones which, in the long run, can end up making them actually MORE drag rather than less. So, take a little time to ask questions (as you're doing) and learn before you jump.

 

Good luck!

[cstreit911]

Hi Jason,

 

Welcome to GTS! Happy to have some more 911's in the fold.

 

Regarding Torsion bars ultimately this may limit you simply because you aren't able to get enough spring rate out of them. The earlier 911's seem to run a range from about 400/600# springs up to close to 1000#'ers for heavier cars. I think the max spring rate you can get out of t-bars is about 400#... How dramatic a difference? Not really sure.

 

Regarding lift a lot of people mischaracterize the front-end lightness and wander on high speed straights as lift when it is often just the toe-out alignment. That said, a splitter is a good idea and combined with a good rear wing certainly won't hurt. GT3 wing is good and should balance nicely with the RSR splitter. Our National Director makes some really nice wings that are popular in GTS.

 

If you can lock on the brakes at any time, you have enough... Though I would say that once you get to the pointy end of the grid in GTS, stock brakes probably won't have the heat capacity. There are a lot of options out there from 964 to 993 brakes that will fit your car when needed.

 

I'm guessing a 3.4L is going to make between 275 to 325 at the wheels depending on how it's been built. (That's 80-90HP per liter.). Assuming you are 175# that's 11.3:1 to 9.5:1 range. This puts you in GTS4 range. I don't think you'll get the car into GTS2 with a 3.4L. Even if you weight 200# you;d have to get the car to 180HP total to make GTS2. A stock 3.0L with webers and an exhaust is going to make close to that.

 

Feel free to message me and we can talk. I ran a 1970 911 competetively for 6 years in GTS4.

[Jas0n]

Thanks for the replies, guys

 

I don't think GTS2 would be likely, but there's always hoping!! My car currently weights 2450# without driver - so add my 170 pounds and I'm looking at 2630 racing weight, which gives me 230rwhp to work with (I think that's entirely possible, figuring that's roughly 275 crank hp). I think GTS4 would be too much of a jump for this car, too much work to make it competitive.

 

I've heard/read people say torsion bars don't offer enough spring rate, but the car sees plenty flat right now - I suppose its one of those things I can work towards and don't need to count as a must to start racing (though R6 v. NT01's might make that need more apparent).

[cstreit911]

Thanks for the replies, guys

 

I don't think GTS2 would be likely, but there's always hoping!! My car currently weights 2450# without driver - so add my 170 pounds and I'm looking at 2630 racing weight, which gives me 230rwhp to work with (I think that's entirely possible, figuring that's roughly 275 crank hp). I think GTS4 would be too much of a jump for this car, too much work to make it competitive.

 

I've heard/read people say torsion bars don't offer enough spring rate, but the car sees plenty flat right now - I suppose its one of those things I can work towards and don't need to count as a must to start racing (though R6 v. NT01's might make that need more apparent).

 

You're in the right range. Definitely get a Dyno early so you know what you are building towards. Plenty of 911's are racing with Torsion Bars and there is certainly a lot of data there, particularly in the vintage groups. Adding coilovers is a relatively simple upgrade when you are ready, but I recommend just running at first, get to know GTS, then look at upgrades later. Have fun first, THEN spend money.

 

R6's are definitely the way to go to be at the sharper end of the grid. What region are you in?

[Jas0n]

Northeast - just north of NYC in Westchester county.

 

Hoping I'll get my logbook and provisional license this year, but its going to be tight. Looks like AJ Hartmann will be building my cage, just need to find the right span of time so I don't miss any track time.