I hope this hasn,t been covered before . What are the deciding factors in an engines ability to swing a prop ? Is it HP , torque , or a combination of the two ? I,m thinking of going gas in a BUSA N 28 . A G 26 will fly it , but with a 16 to 18 inch prop . I really would like to use a 20 -22 inch . One of the advertisers here lists G26 with different HP ratings . My knowledge of engine mods leads me to belive that when a engine is modified to produce more HP , it useually ends up making more RPM , and I think thats not what I want . So , what do I need to turn a 20 - 22 inch prop in a N28 ?

Unless you gear the 26 drive you'll want something in 40cc's and up for a 22 inch prop. Not what you are looking for though. You could spin a 20 inch with the 26 but over heating the engine becomes a clear factor. perhaps a 35cc engine for a 20" prop, but now the engine is getting heavier. Think some more about using the smaller engine and prop. Perhaps you could cut down a 20 to 19" and have things happy, happy, but only experimenting would prove that.

Thanks for the reply . It seems that getting exactlly what I want is going to be problematical . More research fixes everything , I hope ........

It is torque that makes the engine turn, and it is horsepower that results from the combination of torque and rpm.
We need high torque engines if we want to operate them at ~7000 rpm. More power can be had at higher rpm, but then torque will be lower. My 26cc MVVS will swing a 20x6, the 35cc swings a 20x8, the 45 needs a 22x10, but will have the 20x10 for breakfast. In that diameter, a 20x10 three blade prop loads the engine down to it's best torque range. Power output will be slightly less, so it is up to you to define what you like best in your plane.
On a ST2300, the power output could be adjusted with the prop. An 16x6 would spin the engine right up to it's best power at 12000, but the engine would happily pull a 18x10 for glider tow.
For best engine performance, it is required to pit the engine against the needs of the airframe. An high strung and RPM hungry engine belongs in a speed plane, and a torque engine will be better suited for the slower planes. If you want to use the high rpm engine in slow planes, a reduction is needed to match the propeller to the plane. As an example of the latter, Zenoah engines with reductions have been long used for WW1 scale planes. In these applications, the added weight was welcome.

Pe Reivers , that was the kind of info I needed . Now I know what questions to ask the engine builders and what I want from an engine .

Thank you .

I learned a lot from that explanation as well....thanks.