I made the mistake of buying a T4 off E-bay, only to find out it wasn't a genuine RS500 one, but a T4 of something totally different..... Never Did find out what though.

In the end, I ended up exchanging it against my T38 (best thing I ever did). For some reason (Specification) the T4 just never spooled up as soon as it should have done. Karl was totally lost as to why it wouldn't spool up as quick as all the other T4 conversions, but there are quite a few versions of the T4 available, and don't just assume that all T4'S are the same... They're not, as I found out to my expence. Impellors can be different, Shaft Size Can be Different, Get it checked out by one of the Turbo Guys before you spend all your hard earned money on it.