OS P4 is great. I started with a P3 because I run it in every other motor I have (Speed, Ninja, Vspec, P5). The motor did not like the P3 and seemed flat, I thought it was just not broken in yet, or it was just the smoothness that everyone talks about. After about a gallon it fried a P3 so I tried a P4 and it has been solid ever since(about 2 more gallons). With the P4 it screams and holds a tune really well. I did try another turbo head in it with a 97t and it ran similar to the P4, so since I have OS in everything else and can get them locally in Japan, I switched back.