On many forums I have been called crazy, or clueless when it comes to rc engines. An engine is an air pump. Whether its two cycle or four cycle it has to adhere to the rules of flow dynamics. I don't know much when it comes to how to use flow dynamics in regards to engines, but I do know a bit about cylinder heads, cams, cranks, pistons, and displacement.
There are a number of ways to increase the displacement of an engine without having to bore oversized. There are also a number of ways to increase the compression of an engine. Cylinder head gasket thickness, stroke length and so on. In the case of an rc two cycle, compression can be bumped in two ways. Lengthening the stroke, and reducing the thickness of the cylinder gasket. this reduces the space that opens up between the piston and the combustion chamber at the bottom of the stroke, also making the space smaller between the piston and combustion chamber at TDC, increasing compression. Doing this makes a couple of things mandatory. The cdi hall sensor position must be altered. Not much, but some. as the spark timing must be changed to allow for the longer stroke.
The other things Performance is doing is a change to the piston dome profile...the change in stroke means the piston won't drop below the exhaust port as much..so it has to be changed so the piston itself doesn't restrict exhaust flow. How much that change is..no clue, but it depends on the change in stroke. I also know the ports are being port matched to stock zenoah gaskets, and the ports are being smoothed. Better flow, better throttle response, etc.
The new intake has a longer run from carberator to intake port..this will slow throttle response slightly but improve torque and power from idle to mid range..right where its going to need it with the varioprop. The slower throttle up won't overstress things...just have to be mindful of it in flight and plan maneuvers accordingly.
Now, I've been told by multiple people with engines modified by Performance what the effect was for them and their aircraft. May be long tales, but based on what I've been told I am on the right track. I'll detail what I've been told by 11 customers of theirs now.
1. prior to modification a toprc P51 with a stock zenoah engine using a 23x8 prop was running around 6500 rpm. After modification, same prop, engine was at 7600 rpm. He went to a 24x8 and is now running 6800. Airplane now flies very scale, with power to spare in the vertical.
2. a rc flight team in Katy Tx flies 4 mustangs and a corsair. The corsair uses a Moki 120 radial. the mustangs, all zenoahs. Prior to modification, the zenoah mustangs could not keep up with the corsair in vertical moves..even stalling at times. After modification, in the vertical, no problem, and the corsair struggles to keep up in the mustangs at WOT level flight. They have had to up their prop sizes to 25 inch diameter so the corsair can keep up. RPM on the 25x6 props..6100 unloaded.
Thats just two of the reports, 5 people..I could keep going but they are basically all the same, with similar reported rpm. Differences were in the choice of props..from Meizek to Xoar, to APC.
When selecting the 23.5 inch varioprop for this setup, it was a three way conversation between Christian, Performance Unlimited, and myself. I detailed the goal for the build, scale flight with a 4 blade propellor. A bit of back and forth later between Performance and Christian, it was decided my best option was the 23.5 inch on a 25F hub. Performance estimates a final pitch setting in the neighborhood of 12 to 14 degrees based on data Christian provided on the prop airfoil and blade shape. It was recommended that I start the engine with this prop at 18 degrees first time and adjust from there but to use a 23x8 two blade for tuning carb prior to mounting the varioprop.
So, in some pictures as the build progresses, you will see either a 22x10 or 23x8 xoar mounted to the engine some of the time. I don't have an engine test stand, room for one, or access to one, so the engine won't be run until the plane is mostly complete or, even complete all the way and in flying trim. Its just how it is. Sometimes we do what we have to. I'd love to be able to put it on a test stand first, run and tune before mounting but would just be added expense and would have to toss it after the build to save room.
So, in the meantime, enjoy the thread, cross your fingers for me this all works as I hope. With the new intake, the exhaust could get interesting. That will be the last hurdle. Funny how when you change one thing it affects four others..Reason why the reed valve conversion got nixed...it would cause too much havoc with the airframe and engine.
