TTI did a test of h-pipes vs. x-pipes, and the test showed that, in cars that develop around 400horsepower, a gain of 12 to 15 horsepower is realized at the rear wheels when using the x-pipe system. Cost-wise, the x-pipes for a Challenger are $ 54.00 more than for an h-pipe exhaust system, whole set-up is $ 581.00 for an h-pipe system, $ 635.00 for an x-pipe ( I know, I just talked to TTI yesterday about a system for my Challenger .) Anyway, $ 54.00 seems a small price to pay for the extra horsepower. These systems are well worth the price, though. Very simple and easy to install, at least it was when I put a TTI exhaust on my '71 Charger RT.