andrew66

well, after staring at my broken ryobi most of the winter, ive finally worked up the motivation to try and tune this thing for more power. when i had it, it was decent for power in the plane that i had it in, but i want enough for aerobatics.
so far i have removed the head gasket to increase compression and raised the exhaust port to a duration of 150 Degrees.
this is when i got the idea to modify the transfer ports........
i looked at my homelite 25cc jug and noticed that the transfer ports looked like this: [img][/img], and my ryobi transfer ports looked like this: [img][/img],
so i modified them to look like this: [img][/img],
do you guys think that this will work? or did i just ruin a perfectly good jug?
your input is greatly appreciated.
thx,
andrew

av8tor1977

My Feedback: (6)

A little hard to tell as your pics came out blurry. However, I'd be a bit worried the ring might pop, or bulge out into transfer ports so wide, and hang up on the top of the port....

Let us know how it works.

AV8TOR

andrew66

sorry 'bout the pics, cameras and me dont get along.
about the rings, the rings for this engine are .047thick so i dont think that they would bulge out. and the transfers arent a whole lot wider than the homelite...but iguess i will find out when i run this thing. right now i just need a case (in the process of taking care of that) piston rings, prop hub and to modify the muffler and im ready to go.
when the weather gets nicer (right now its still too cold to be outside playing with engines) i will run it.
my target RPM is 75oo-7800 with an 18-8 prop.
PS
does anyone know of any formulas that actually work to calculate speed given prop pitch, diameter and rpm?
thx,
andrew

av8tor1977

My Feedback: (6)

You can figure it out mathematically, but the easiest way is with a program called "ThrustHP". It's a free download, but you'll need to do a search to find it as I don't have the link.

Thrust HP is ok for what you want, pitch/speed/rpm comparisons, but don't trust the thrust numbers it gives as they are inaccurate.

Whoops, I think I just found the link. Try this: http://freespace.virgin.net/barry.hobson/

Have fun,
AV8TOR

av8tor1977

My Feedback: (6)

Nope, that link is dead.

Here's another one: http://www.bmaps.net/

Click on "Goodies" in the left hand panel, then download the ThrustHP latest version.

AV8TOR

andrew66

thanx alot. how much are the thrust numbers out by?
according to the speed that was given, this ryobi (if it will spin a 18-8 @ 7500 rpm) will fit a project i had in mind quite nicely.

captinjohn

Andy...0n the photos...try to get further away form subject and use a little zoom power. May help. Capt,n

av8tor1977

My Feedback: (6)

I don't know why, but those thrust numbers are all out of whack. Some hit rather close, and other combos are way off. But you should be able to trust the speed numbers... mostly. I say "mostly" because an engine will unload in flight and turn more rpms than static, and also because of the slippage factor which will vary with how draggy the plane is. But it's a good ballpark, and should get you close enough that a little prop pitch experimenting will finish it up.

Kudos to you on thinking of "pitch speed". A lot don't, and it messes them up when going to gasoline engines. Most glow engines like to run in the 10 to 12 grand range, and the gassers are more in the 6500 to 8500 range. Big difference in pitch speed.

Let us know how that Ryobi works out. I think the most gains with a Ryobi would actually be in making up some sort of a larger reed/inlet assembly, or a retrofit rotary valve. At any rate, prop it at or below 7500 rpms for it to live long.

AV8TOR

davewallace

My Feedback: (29)

Andrew,
The cylinder looks like a Ryobi, but I haven't seen one with dual transfer passages before. The reason for the wide bridge in the middle of the transfer area is to provide a support for the trust side of the piston. Now it appears that there is no support and it will wear very fast. Ryobi's have a tendency to wear in this area with a stock cylinder. If the drag isn't to great, it will produce more power. Are the two pictures on the right from a Ryobi?

Dave

andrew66

the two pics are a ryobi. the first one is what the cylinder looked like before the mod, and the second one is of the mod that i did.
there is still quite a bit of cylinder left to support the ring (more than a homelite) so i am pretty confident that this will work.
as soon as i get all the parts rounded up and the weather gets nicer, i will test this engine and post the results.

av8tor:
i saw a website that had a few different ways to mod. the reed valve for more power (i wish i remembered the website) but i lack the tooling and materials to do so in the way that they suggested.
i think that if i rigged up some sort of check valve inside the "intake port" it would help by having a straight unrestricted path into the case instead of having to flow around the reed.
if anyone has any ideas on how to rig this up, please feel free to share them

captinjohn

Andrew66: The reed valve is a check valve. It lets fuel & air in but not back out. The "mod" you done looks like just cutting the support out to make ports wider. Is that correct? Capt,n

andrew66

yes, i figured i would try to open up the transfers to allow more flow. the stock configuration doesnt allow much flow, which i think is one of the reasons that the ryobi isnt a power house.

av8tor1977

My Feedback: (6)

Hi,

Yes, I reviewed that site you spoke of, but I don't remember where I put the files. Interesting stuff, but I have too many other projects going to make a rotary valve for my Ryobi...

Let us know how yours works out, and have fun.
AV8TOR