As far as I am aware petrol engines run better when cooler and diesels run better when over 55oc.

Engine in my '32 Hot Rod is a 4.2 from an LSE Range Rover 200bhp standard, but now runs a Piper cam and Edelbrock 4 barrel carb. Runs cool as you like as ive had the rad fitted with more cooling channels, and piped up the inlet manifold correctly to let water to circulate and reduce hotspots (look on Cobra forums as they use Rovers alot!). The 4.2 uses a dizzy where as the 4.0 and 4.6 dont so its a lot of work to run a dizzy on these engines if your car uses one. Car goes like stink, but 3 speed auto dont like it!!!