Not a GTM but could equally apply. Went to see a young pal today who looks after a couple of Chevrons amongst other things (b8's i think, anyway mid-engine/front radiator). He had to fit a new multi-core alloy radiator to one because the car always ran hot, the new rad did'nt help, after much head scratching it turned out that at sometime the waterpipes had been replaced with larger ones, (the other one still has the original 1" bore pipes) He fitted a 1" restricter in one of the flexible pipes and hey presto, normal water temps. The water was flowing through the radiator too fast to lose enough heat. Incidently the BMW m10 engine in that car makes 260bhp, 1" bore does'nt sound much does it

Not surprised have seen this before myself, the idea is that you want the water to travel fast to the radiator, then slow down in the radiator, easiest way is by expansion thus allowing the radiator to work effectively to cool, then nice and fast back. same idea in terms of radiator ducts, if you look at a f1 car for example the rad duct opening is quite small, then expands in a diffuser to slow down the fast cool air so that it can make its way past the radiator then if you duct the air away from the rad you want to speed it back up again to the correct outlet speed.

a very general rule of thumb is a rad duct opening 1/3 the size of the radiator core. speeds etc all effect the exact size/effectiveness, but thats a good starting point.

thats one reason why I still use 1 inch pipes, plus being 1 inch compared to 1 1/2 they hold less water and therfore weight . next cooling mods to mine is to eventually fit ducting to the radiator to make more effective use of the air

WEST

Who uses 1 1/2inch pipes then?