A bit of idle curiosity on this topic for local races. I was talking with a local race shop yesterday. It looks like there may be a bigger showing in my local track's NASA ST4 class than GTS2, and I was wondering about running the car in ST4 if I found myself the only GTS2 entrant on a race day. ST4 allows a 12:1 weight to power ratio, and a SPB is close to the GTS2 level of 14.5:1.

I could always just throw an ST4 sticker on the car and get killed. Even running light at 2600 lbs, 220RWHP over the last 1000-1500 RPMs is allowed, and the strongest SPBs peak at ~190whp. But lots of SPB restrictions disappear in ST4. So what could be expected from a quick and easy ST class change? Thinking of something simple and cheap like eBay headers plus a flashed ECU - would that be enough to expect 210 whp peak? That wouldn't be very competitive but might still be enough to keep up and have a good time.

And maybe I'm forgetting about some boat anchor we have hiding in these cars that could be easily removed for NASA ST/GTS events. At ~2500 lbs with Hoosiers and ~210whp it might actually be competitive...

I checked that out in more detail... they've made some updates to the ST5 rules in the last few months and the car looks a lot more competitive now than it did in December. Very interesting. I wonder how full this class will be locally.

Super Touring 5 (ST5) = “Adjusted Wt/HP Ratio” equal to, or greater than, 14.00:1
-Average horsepower is the average horsepower in the top 1750 RPMs of the dyno chart (for a SPB), and 180-185 seems to be a reasonable or slightly strong number for a SPB
-Tires are any DOT tire in up to 245 width if the car is at 2750+ lbs and 225 for 2400-2750 lbs (though this uses a physical NASA width measurement to compensate for tires that have a narrow stated width compared to real width)
-Up to 250 lbs of ballast is permitted

Base power to weight for a typical SPB at minimum weight will probably be 2650 lbs and ~183NASAwhp
2650/183=14.5
**This means a modification factor of 0.1 is worth 18.3 lbs of weight (reduction or ballast)

I suspect the modification factors for most everyone would be:
+0.5 = -92 lbs : Adjust 14.5:1 ratio to allowed 14.0:1
-0.5 = +92 lbs : Replace, modify, or remove control arms, camber arms/links, toe arms/links
-0.7 = +128 lbs : Non-OEM shocks/struts with shaft diameter 40mm or greater
-0.4 = +73 lbs : Rear-Mid or Rear engine layout (not Front-Mid)
__________
Total ballast or car weight would be +201 lbs from the spec minimum, so 2850 lbs which makes the -0.1 weight modification factor not apply but only barely - fuel level would have to be monitored

Also a bit more that could apply to some cars and/or regions:
-0.1 = +18 lbs : weight less than 2850 but more than 2650 lbs
-0.3 = +55 lbs : (Many SPBs) One or more cage bars that penetrate the front bulkhead/firewall
+0.4 = -73 lbs : (Spoiler down and negotiate with scruts) Base Trim Model Aero - this is only for cars with no factory spoiler or wing but exceptions exist for when the spoiler is removed

So basically, a SPB that is about 200 lbs overweight and running on 225 or 245 Hoosiers (I know they run wide so their 245 may be too wide) could compete in ST5 and be right at the class limit. The spec RRs could possibly work but it's more likely they will exceed the maximum NASA tread width and be illegal. I'll have to watch this class as well as GTS2. Some are suggesting ST and GTS be merged, but I think most of those folks making the suggestion don't run German cars...