The definition of the American muscle car centers on lots of performance for not much money. Big engines, driving the rear wheels, in swoopy 2-door bodies. And of all these fairly affordable muscle cars that once poured off the American industry's assembly lines, the General Motors F-body cousins--the Pontiac Firebird and Chevrolet Camaro--are the only two that have offered, year in and year out, an unbroken line of performance of this uniquely American recipe. Some diehard sentimentalists moan the passing of what they remember as truly fast cars, but they ought to open their eyes. The current Firebird and Camaro are far faster than their ancestors ever thought about being, and they're better in every other way, too. At the top you'll find the Firebird Formula and Trans Am, available with GM's familiar 5.7-liter V8 rated at 285 hp. With the optional WS6 Ram Air package, horsepower is upped to a remarkable 305, zero-60 mph will take less than six seconds and the top speed will nudge 160. But there's something even more surprising than that--and with more real-world significance, as well. The base Firebird--and its Camaro cousin--is fitted with GM's 3800 Series II, a 3.8-liter V6 that's been massaged and polished to 200 hp and 225 lb.-ft. of torque, and it will go to 6000 rpm. It doesn't require elephantine memory to remember when V8 Firebirds didn't have 200 horsepower. What you have with the V6 Firebird is V8-level performance, 6-cyl. economy, a total price under $20,000 and a car your insurance agent probably won't notice unless you get giddy and tell him.