i have a 400ex with a stage 1 hot cam and a fmf powerbomb header. I need to know what kind of slip on would give me the most top end.

 

Since you have an FMF header, I'd go with an FMF slip on.

 

FMF powercore 4 was a horrible exhaust on my friends ex. He first got it and we were like wow nice gains, that all ended when he got his e series. We did speed runs with the stock exhaust baffle removed, eseries all disks, fmf powercore 4.......the powercore 4 wasnt much ahead of the stock exhaust with the baffle pulled and the eseries took them both by about 4mph. This was all recorded using the gps.....we later took apart the fmf powercore 4 because we thought maybe the packing was blown what we found was a square baffle tube running down the entire muffler...not really a good way to make hp i was thinking since all the high hp pipes use round tubes not square.... anyways we then put it back together and took the spark arrestor out of the fmf powercore 4 made a tiny bit of difference but much more noise than anything. The FMF powercore 4 is just not a good exhaust for top end, sure it sounds nice and has good mid gains but the e series blows it away on top end power. Another exhaust that was tested by another friend on a 400ex also that weekend was a dg muffler the one with the disks not sure the name but it was low of power compared to the friends 400ex with e series and couldnt keep up. My conclusion is out of all these pipes the e series is the winner for top end power. The sparks exhaust might be a consideration i have never seen one of them on a ex but hear they are good.