No, being a manual has little to do with it. The Mustang 427 has 427 hp at the crank and probally around 450LBS of tq. At the wheel the mustang is putting down roughly 375 rwhp and 396 rwtq. On the other hand the Challenger is putting down a little less hp and tq. Another factor is the Mustang weighs less and has a lower final gear. The challenger has a 3.06 and the mustang as a 3.31, 3.55 or a 3.73