According to Donny Peterson in his book "DONNY PETERSEN'S TECHNICAL GUIDE to Harley Davidson 1936-2008 Volume 1 - the Twin Cam" . . .

The Twin Cam AKA - Fathead has 60% more cylinder fins. "The extra finning giving the engine a fatter appearance hence the engine nickname The Fathead remove heat from the engine and dissipates it into the air. Predecessor H-D engines contained heat inside the engine. The Twin Cam moves it to the outside explaining why they can be very hot to ride in warmer weather.

If you want to learn more than you will ever need to know about the Twin Cam motor, I highly recommend this book. I eagerly await volume 2 on performance upgrades for the TC.

http://www.amazon.com/Donnys-Unautho.../dp/0595439020

Steve