I have a stupid idea..

instead of spec'ing weights, displacement, compression, etc etc etc etc, why not just let the engines go and limit power/weight or torque/weight for each class- given the engine is from same MFR as the car. (Acura= honda, mercury=ford, infiniti = nissan, etc)

Say STO max is 5lb/hp, STU is 10lb/hp, and STL is 15lb/hp. (just grabbing numbers out of thin air. I'm sure those aren't applicable but you get the point.)

Then each competitor can build whatever engine they want and they can either weigh the car down and have tons of power, or they can run a light car with less power.

NASA does that with Performance Touring although looking at the rules it looks like they're having fun policing it w/ dynos, GPSs, and weight issues.

It's at least another alternative to think about...