I have 2 of them. The previous answer is correct, is is an enlarged version of the older .91. The newer .91XLS is in a .61 crankcase.

The Magnum 1.08 is a fine running engine if you run it right. The factory instructions are totally incorrect-it won't turn a small prop very well. It is more of a torque engine meant for big props. Consider the 1.08XL a tractor engine and the .91XLS more of a sports car engine. A 15-8 will give you good speed and a 16-6 is good for pull. I used a 16-6 on a heavy Joss Stick. On a scale Northrop A-17, pre-WW II dive bomber, and on a Beechcraft Staggerwing, I use a 15-8.

I run 10% Omega, castor blend, fuel. Like most Magnums, it should start in one back flip after a good wet prime.