Keep in mind, the Jett 120 engine is a physically smaller engine. The BSE is a "60" block..... the SJ-120 is only very slightly larger (it started as a modified FanJett-95 crankcase - eventually became its own animal).

The OS 1.08 is a "90" block engine.........has a big crankcase, HUGE transfer ports, a pretty big carb, and is pretty efficient with fuel transfer in the crankshaft (big hole there too). "Breaths" very well. That extra internal volume goes a long way to providing fuel/energy/power.

I seem to recall my old OS 108 turning somewhere between 10-11K rpm with a 15x8 (may have been a 15x6... hard to remember) .. this in the 1985-86 time frame. It had gobs of power.

Basically, the BSE-120L performs the same........ 15x8 at about 11K rpm ---- just is a whole lot smaller/lighter engine. However, due to its size and construction, it can not 'breath' enough to make it much of a higher rpm engine.