Sorry about starting another thread on this but I felt its on a slighty different topic.
A mate of mine owns a 2000-reg XJR and already has the lower pulley done by Powerhouse in the UK. He's taking it back there shortly to get the upper pulley and uprated raditor with pump.

So 2 questions, as it is at the moment standard car for arguement sake lets say it had the orginal 370BHP, with lower pulley only what will his peak BHP be currently.

After both pulleys and cooling upgrade what will his peak BHP be?

 

I have never seen any dyno slips from Powerhouse, that's very odd imho.

There's much written about it already, lower end you will feel it better power, but upper end it will diminish as the Eaton roots becomes so much more inefficient (by generating much more heat and also consuming much more power from the engine).

A thick core radiator (what is usually used) will not help imo (based on tests), it can actually work counterproductive as it reduces airflow, which is what you need for cooling. More effective would be a larger surface area, but that is slightly more difficult to make.

So you’ll have to do more to combat some of the inefficiencies (heat), probably best to also go for a water/meth kit. Most dyno info I have seen on the web with the upper/lower pulleys is about 50 to 60 rwhp (but these where with water/meth kit and ecu tunes), without a water kit/tune, probably 10 to 20 lower on the 1st run, and each consecutive power run you will generate less hp due to the heat soak. But even that can be optimistic, as when I had a lower pulley with the Eaton, top end was about the same as without.

Let him do a before and(or just) after run, then everyone will know.