You need backpressure for low-end grunt ("off the line.") Take two identical cars, make one straight pipe exhaust (no backpressure) and leave the other one stock. Now race them. The stocker will pull ahead off the line, but the straight pipe vehicle will pull ahead when you start building RPMs. Depending on the characteristics of the car and what your goals are, either one can be desirable. If your engine revs out to 7500rpm or higher, and you are running a long, high-speed race, you'd probably want something with less restriction than the stock system. If your engine never sees high-rpm situations, and you are running a short track or racing light-to-light in the city, you might actually benefit from stock restriction. Paul Nimz left the restrictive stock exhaust tips on his exhaust (they taper to like 1-7/8" at one point) and he runs one of the fastest 1/4miles of any n/a V8SHO.