[TonyS9]

You should be able to get 15psi boost with the standard stock exhaust and cat.

I did this in my 1995 car, although 15psi is a bit close to the limit for the max air flow reading allowed by the ECU. You will be running a bit lean then. I would recommend staying under 13psi without other fuel mods and a wideband AFR meter. I just used a bleed valve.

I'd say almost certainly you have a blocked cat.

I also run the similar 97 B230FT engine without a cat in my 360 and don't find alot more power, in fact its less, possibly because of a slightly defective HG. I have replaced it now, but havn't track tested yet. My 940 has a full stainless system using a cheap aftermarket sports cat. Both cars have the same emission results at the MOT, which is only checked at idle in N.Ireland. Mileage is similar 150-200K but the 360 engine did have valves ground in and is just a bit more spoilt, bigger bore exhaust etc.