They run cooler because the combustion process is complete before the exhaust port opens, a glow engine run at night with no muffler will have a blue flame standing at the exhaust port because the combustion process is incomplete. The Diesel exhaust on the other hand will have sparks exiting the exhaust because for the most part the carbon build up on the piston is migratory. A layer will form on the piston head but little build up. Over compressing a Diesel conversion will overheat them though.