I've been going nuts with this on some various engine selections that I am considering for a 33% Super Decathlon. I have the Hobson calculator to determine thrust based on RPM and prop size, but this doesn't consider the comparison of single or twin cylinder engines. I know there has to be a difference in thrust and HP when when using a 26 x 10-12 prop on a ZDZ80 single versus the ZDZ80 twin. If these engines swing the same size prop, then why would RCshowcase rate the single at a higher thrust than the twin (50 vs. 41 (6500RPM))? The DA-100, 3W-85 and the BME95 are all considered as well, but without knowing what to expect for thrust it looks like the less expensive is the way to go. Can anyone help explain this? Thanks in advance.[]

thrust has NOTHING to do with the engine used - the reading on the Hobson chart is a reading taken with the powerplant on a stand.-
To get close to real world on that "thrust chart" --you must know rpm of specific propeller and the corrected altitude density -expressed in Hg (inches of mercury).
Still-this will tell you nothing about how the prop unloads in the air -which is simply an incalculable number unless every specific about airframe / engine/ prop / altitude / temp/etc... is known --even then you will likely miss it a mile.
The best setup?
get a good fish scale - calibrate it - get various props - run pull tests and record ALL info from each test.
After you are thoroughly tired of this exercize --with luck, you will see a pattern emerging as to what props and engines make what kind of thrust on which kinds of days .

Do not believe the thrust numbers you see on the RCShowcase website! They use this little program to calculate that... and this little program doesn't even take prop pitch into account when calculating thrust. So it shows the same thrust for (example) a 20x8 at 8000 and a 20x12 at 8000... Go figure

Two different engines turning the same prop at the same rpm will generate the same thrust.

The ZDZ 80 twin and single engines will be close... I would be very surprised to see that much of a difference between these two engines on the same prop.

Motor don"t make thrust, props make thrust. As "Rudeboy" stated, a given prop will create a certain thrust at a certain rpm. What is important, is what rpm will a given engine turn, say a 26x10 "Acme" prop as compared to another engine with the same prop. Now if motor "A" gets it up to 7000, and motor "B" takes it 7500, thats what you want to know. Now a calculator can be used to determine "static" thrust. But , thrust changes as speed increases, & I don't know what the effects of that are.