The OS 91 Surpass II pumped/nonpumped is almost the standard engine of the Senior Pattern Association.

A sport type airplane like a Four Star loves a four stroke engine.

Theres not much difference in the way the OS 91 Surpass I & II runs. The Surpass I came out in the early 90's. The Surpass II came out in late 2001. Before that there was the 90 four stroke and it didn't have as much power but still enough for a Four Star.

The Four Star flies great with any two stroke from 60 up through 91. It doesn't make much difference. They also work fine with the four strokes from 65 up trough 110

I think the Tiger was intended for a 60/61 two stroke. The current OS91FX two stroke has the same external dimensions and is a fine engine.

Some planes originally designed for 60/61 two strokes do not have enough room up front nor will they balance easily with a four stroke engine.