Does anyone know of a conversion chart that would give the approximate comparason between 2 and 4 stroke engines in terms of performance? For example, I am told that a .61 2-stroke and a .91 4-stroke are about equal. I am thinking of changing my .46 to a 4-stroke but I don't know which size.
Thanks for the help.

ys 63, saito 72, os 70

it all depends on how fast you whant to go most 40 size planes are rated for a 52to a70 in a four stroke but a magnum engin gives you the most bang for you buck

Mike, the 30 to 40% gap that used to exist between 2 and 4 stroke engines of similar size has nearly disappeared as of late though some are still clinging to that thought. Brands were not part of your question so I'll not comment on that. As far as the chart goes, there may be too many variables to make one useful. There is plenty of info around RCU about what guys are using on what planes and how they fly.

may be this will help - below is a typical graph of a 4s and 2s engine, please note that this is not 100% but just an indication of how the power curve works.

That's a pretty cool graph of 4-stroke versus 2-stroke power curves.

Both the 2 stroke graph is a lot more peaky than shown on your graph. A two stroke with 2 Hp peak at 11,000 would most likely have one HP at 6,000 or so. A four stroke with over 1 1/2 HP would most likely do so above 10,000 RPM and would have less HP at speeds of 9,000 RPM.

50% is about what they are then and now. A .90 four stroke is about the same as a .60 two stroke, and a .70 four stroke is about the same as a .46 two stroke. Do the math .90 over .60 is 1.5 or 50% greater the .70 over .46 is 1.52 or 52%.