ORIGINAL: Darren40
A good rule of thumb is that a 4-stroke engine needs to have 1.5X the displacement of a 2-stroke for the same power.
That's an old rule from the days when 4 strokes weren't as powerful as they are now. Yup, most people still use this rule, but then most people seem to like to overpower their planes
ORIGINAL: Darren40
...compare the specs of a Saito .72 4-stroke to a O.S.Max .61 FX and you'll see that the .72 puts out 1.0 HP@10,200rpm and the .61 puts out 1.9HP@16,000rpm. Now, the 4-stroke will swing a bigger prop so it has a better grab on the airstream, but a .72 4-stroke is not a good substitute for a .60 size 2-stroke. The .72 4-stroke is an excellent alternative for a ~.45-.50 2-stroke. Again, this is a general rule since some engines (YS) are powerhouses for their size.
My Saito 72 will swing an APC 13x6 at around the speed you mentioned ie 10,2000rpm, thus generating its peak horsepower. My 65LA (allegedly more powerful than the 61FX) will swing an APC 12x6 at around 12,000rpm, nowhere near the 16,000rpm where the maximum HP is generated. Where we're really talking about here, is a 2,000rpm gain against a smaller disk area. Someone else can do the actual thrust calculations, but I'd say there's not a lot of difference.... Bear in mind also that the Saito 72 is about the weight of a 46FX and will be a fair bit lighter than a 61 2 stroke...
When I get my Tiger 60 together, I'll probably do a comparison between the 65LA and the OSFS70. Should be interesting...