Perhaps I should clarify my position a bit...
I agree that you cannot use a "hard and fast" method to classify cars, though I should note I never used that term or anything like it. There are simply too many varieties of cars out there to not have some additions or subtractions to the base formula. Those adders should be consistent within a given class however - ie FWD should always be a 100 lb subtraction for ITS, 50 lb for ITA, 0 for ITB or ITC - actual numbers for example only!!!
That said however, I believe that all cars should start out with the same basic formula, then you add/subtract for differences in vehicle architecture. A single percentage for HP increase in IT trim should be used. If Car A and Car B both have the same stock HP, torque, drive configuration, suspension setup, etc then they should weigh exactly the same. If Car A happens to get 1 or 2% better HP increase than the bogey, while Car B gets exactly the bogey then so be it. Car A will be a (slightly) better car.
Using "real" dyno data is dangerous, IMHO. Unless a single control dyno is used and the engines are confirmed to be maxed, legal and properly tuned, I don't believe that it is proof one car responds better to IT mods than another.
To Andy's examples...
Assuming these cars are in ITS, the base formula would be 170*1.25*12.9, which equals 2741.25. If both cars were RWD, strut suspension, etc, the ITS weight should be 2750.
For purposes of discussion lets use some simple weight add/subtracts:
Double wishbone: +50 lbs per axle
50-50 weight dist: +50 lbs
FWD: -100 lbs
Trans ratios are tough to quantify. I think in ITS, most of the cars should have good to excellent ratios, they're mostly performance based vehicles after all. I think I would consider this a wash in ITS, unless a given car has economy based gears, then I would make it a subtractor of 25-50 lbs? An older 4-speed ITS car should be given a break as well. Personally I think gearboxes play a larger role in the classifications in the lower classes where you are likely to see some REALLY bad ratios.
Car #1 should be 2900 lbs.
Car #2 should be 2650 lbs.
Based on the specs of each vehicle as listed, I think that those are good weights. We can quibble on the values of the individual add/subtracts but we are left with a FWD, 60-40, strut car that is 250 lbs lighter than a identical HP, torque RWD, 50-50, double wishbone car. That seems about right to me, give a take a few lbs one way or the other.
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