Cold Weather More HP?

[Topless]

Yes,
Cold dry air is more dense so more O2 per compression = more HP. Somewhere on the internet there is a temp relative hp curve. It is not a huge difference but you can surely feel it. An airplane needs a longer runway for takeoff on a hot day for the opposite reason. Less HP and less dense air to provide lift to the wing.

[roadracer311]

It's true. Back when I was racing motorcycles, on hot days we'd keep the fuel on ice until it was time to grid up. We estimated a 3% gain in power. It only lasted about 1.5 laps, but that was enough to get a good start in the race.