i wouldnt say 400 horse is unreasonable , but on an original flatty well it would be a skittish tempermental time bomb waiting to go off . the new mark kirby unit i thinks makes 375 horse with a single 4 on it , but honestly can't recall . it's an all new all aluminum flatty with lot's of improvements . and on h&h's site they had a dyno run on one equipped with ardun heads that made nice horsepower . there was one on you tube in a 34 {?} sedan they were using as a race only deal that was making something along those lines and i believe running in the 10's but like i said . skittish !!!!!!!!!!!!!!! the most i've seen on one of oe flatties is like 250 or 300 and be driveable , but i honestly don't pay that much attention . later !!!!!!!!!!