It increases turbulence at the SMALL layer of air close to the wall. This wall actually lubricates the laminar air above it and puts up resistance to fuel hitting the wall. That would be my guess anyway.
A 4.0L LS2 would get more than 20mpg. A 4.0L LS2 that had the same power as the 6.0L LS2 would not have pushrods. How much would it cost GM to make the 4.0L like that? Far too much.
When you think that GM try and save $1 per car because it works out to millions of dollars at the end of the...
The pushrod engines like the LS1 and Viper motors get their power and reliability from capacity because gas is cheap in the USA. If you doubled, or tripled the price of gas in the USA then the car makers would start to look for ways of lowering capacity and but still maintaining power.
I was thinking an option might be to pass the water through the inside of the core and the charge air on the outside. I don't know if there would be an advantage though.
For terminal speed on a car I don't think weight is too much of a factor. It will certainly have an effect on how fast you get up to speed but not on top speed....in theory. The extra power that you'd get from a turbo would far outweigh the weight.