I'm nearly certain my XJR feels quicker in cooler weather 6c or below, is it all in my head or does the 4.2 S/C actually benefit from the cold and run a higher psi etc?

 

Cooler air is more dense so holds more oxygen runs even better when it's foggy

 

It's true. Try driving around when it's sub -40C out. Even when the engine is warmed up (and we know these SC engines produce a lot of heat) the intake air temps are still well below 0C. It does give you an extra kick in the pants when you hit the gas in that kind of weather and you never have to worry about heat soak. Why else do you think I live in a place where it can get so cold you will actually die if you get stuck outside too long. So both my 4.2 SC Jags get a free HP boost! Who needs tuning and a pulley, just move up here.