I have a fully forged internal engine with the ross forged pistons that I just rebuilt. When I start it up, I here some sort of knock or slapping in the engine especially underneath the car(I might possibly be able to hear it more underneath the car because I can't here the valvetrain as much) and after about 5 minutes it ALMOST goes completely away to the point where I have to concentrate to be able to hear it. Would you say this is piston slap? I have a 400rwhp motor and it was set up for an additional 150 shot of nitrous to go on there someday. Thanks for any help. Oh yeah also since there is eight pistons how come it sounds like I can hear only one slap, shouldn't it sound like a bunch of slapping?

 

probably piston slap. do you happen to know your piston to valve clearance? the old trw's and ross's are noisy.