Why so smooth in the rain ?

[632 Regal]

I used to run water injection to ported vacume on a couple cars, made the cam sound like a dragster and got a few more MPG.

2 reasons why.

the humidity slows down the combustion rate while simultaneously increasing the amount of fully combusted exhaust.

also it does help lower the pre compression intake temp, which lets more air into the cylinder.

there are probably other reasons, but those 2 are big.