Johnny, according to the MVVS factory site the 15ccm is 552 grams or about 19 and 1/2 oz, almost the same exact specs as the OS 91FX without muffler.

I own an MVVS 91, and it's a real stump puller, especially with the mini-tuned pipe I picked up with the engine at Morris Hobbies. Don't have a OS 91FX to do a direct comparison, but a good flying buddy does and they seem equal in power pulling our 60 sized aerobats around. Difference is I'm running FAI in my MVVS and he's burning 20% nitro in his OS. I've put an APC 15x6 on the engine which it spun without issue. I know some 91 2C have problems spinning large diameter props...

As mentioned before the MVVS is an ABC engine unlike the ringed OS 91FX. They break in differently. Mine was good to fly after three 8 oz tanks run through it on the ground, while my friend needed to put almost half a gallon before he put his FX in the air...