Typically 4 strokes need to be larger than 2 strokes to produce the same level of power. Static RPM on a given prop doesn't necessarily translate to the same power output on two different engines. Although I have never run a 4 stroke, my understanding is that 90's are comparable to 60 2 strokes. The YS 63 apparently works well on 40 size models. I have a YS 53 (never run) which I thought I'd put into a 32 size scale model - just haven't decided which one.

I'll be interested to know how it performs on the Pulsar though!

David.