Hard to say I think Dan. Depends on the type of engine, the performance you intend to get out of it and the fuel economy you want to get. My engine likes running about 170-175 degrees. Engine feels snappy and makes good power. At say 190, it doesn't run as well, less throttle response and a bit harder to start. I suspect underhood temps in general play a huge role in how these engines run. Hard to get the hot air out with a flat hood. When we dyno'd my engine at Tony Shaffer's, he keeps the water temp low