so the extra 100bhp is made from the bigger exhaust housing on the t4? if everythign else is the same? if you get more waste out of the chamber then you have to put more fresh air in.

also compressor efficiency has got to come into it hasnt it - to get a smaller turbo supplying the same boost as a bigger one the effiecincy is going to be poorer therefore temperature higher therefore less MASS of air for same volume = less power and less fuel required.

unless you say bigger intercooler with the smaller turbo?

hmmm actually i think im with same cylinder pressure theory, but with waste gases in there so leaves less power - but doubt thats 100bhp worth for the exhaust housing only.