Since the fuel inside the rails is still liquid, cooling it does not make it denser (ok, maybe minutely) since liquids are not very compressible.
That said, if the fuel is cooler, it will tend to make the air that it mixes with (after being injected) cooler as well. If there is a noticable difference (I'm guessing it will be negligable), that is why. Calculations would have to be done ....
Then you've got the question, does the fuel vaporize/atomize in the f/a mixtures easier or faster if it is warm since there is less energy to overcome regarding the heat of vaporization? And would this be advantageous?
I question your implementation. If heat from the engine is the culprit, why not just wrap the fuel line in insulating tape, then wrap that with a foil type material?
I think cooling the incoming air would have a better effect. My 2 cents.
Seth
