You're right AV8TOR when I used 87 Octane gasoline on my fuel mix for the Ryobi 31 it would pre-ignite and made a metal like sound, then I changed to 91 octane as you suggested and the ignition got better that is why we recommend using 91 octane gasoline.
On my weedeater 25 engine the fuel workrd just fine at 87 octane so I think that each engine brand has it's own requirements as the fuel mix is concerned.

Hi guy!

How's things in Tijuana? Weather is beautiful here and the wind finally quit blowing. (It blew all summer this year.) My last project, the big Nitroplanes Tiger Moth biplane with a Ryobi on Gas/Glow turned out well, though slightly underpowered for me. It flies just fine and will loop, roll, etc..... it's just that I'm used to "rockets"!!

Take care,
AV8TOR

Hey Capt, i have been working on the same thing. I am building a muffer that you can dial in the back pressure. I would like to put a small pressure gage in it for a record of the best settings. Still in the design stage but working on it. I can tell you this the muffer will be a very smooth flowing muffer.
Ken

box car: I am thinking of taking the butterfly out of a junk Walbro or other brand carb and machining a spacer that would fit directly to exhaust port. The spacer would have a hole drilled through for the butterfly shaft. A arm could be made with linkage to work with carb openings. Hope you get my Idea. I did Mechanical drawing in High school. I may get back to drawing things out. Come on people....share you thoughts. Thanks, Capt,n

Yep AV8TOR.
I know that feeling, I myself built a Bellanca Decathlon 80" wing span (ARF from Hobby People) that was given to me by a friend, put in a Ryobi on it but there was no thrill for me I like same as you do "Bullet/Rocket" type of plane, the Bellanca would fly fast and it would do the basic aerobatics but still I like something that will "SNAP and DAZZLE" at your command.
Weather up here is very nice it changes very often sometimes cool and sometimes hot, it's the Santana weather condition that we have up here but in general it's nice.

Well I wish all of you "CONVERTS" a happy THANKSGIVING and don't eat to much turkey or else be grounded for the weekend (to heavy to walk or fly around).

"Happy Flying and Landings" to you all from your Buddy from down BAJA.
Bill Pomplun A.K.A. Aero Nut.

P.S. Keep those GAS/GLOW conversions coming, I know that the word GAS/GLOW is now becomming a very familiar word and option in our hobby thanks to everybodie's contribiutions.

I am getting allot vibration out of My Stihl 27cc. I have it set up for gas glow. It vibrates real bad. Any one know if the fly wheel acts as a counter balance?

camcclellan: At what RPM is the vibes worst? Is it around 1/8 to 1/4 throttle? Will it smooth down at a real low idle? Try to lean the low end and see if that helps. Good luck Capt,n

Cap't It vibrates bad at all throttle settings. I balanced my prop. It is 17/6 prop. I will try to work on it in morning. I read somewhere on this thread (I think) that you can reduce the vibration with the prop position on the crank shaft.[sm=spinnyeyes.gif]

I do not think the position of the prop will help much in your case. Unless the prop was out of balance and you got it in a position to sort of counter balance. I would just try a old prop you may have damaged (like one bad tip) and place it in diffeent positions to see if any less vibes will occur. You could even, take more weight of one blade to see what that does. If it is high on compression...mayby that would cause it. Maybe less nitro? Ask av8tor about that. he has quite a lot of experience with gas/glow set-ups. let us know of your progress. Capt,n UPDATE more on balaning http://www.rcuniverse.com/forum/m_17...tm.htm#1734528

I've read as much of this thread as my poor old eyes can muster so I may have missed something but has anyone tried an "oxygenated" fuel in this application?

As in this product,

http://www.permatex.com/products/aut...ng_Formula.htm

that raises octane by 6 points and, of all things, contains nitromethane.

Haven't tried it yet, we're kind of snowbound for the moment.

In glow engines...high compression means less nitro. Like the Moki engines that run on NO Nitro fuel or at most 5%. Capt,n[:-]

Capt, I see what you are doing, may work out ok. On mine i want to put the butterfly in the lower end of the muffler and link it to the throttle. When i get the muffler done i will post some pictures.
Ken

box car: Sounds good....I wish I had a pane ready and some warm weather to try my Idea. Right now I got to go work on getting some carpet laid and some yard work done. SHOOT! Capt,n

This is an excellent idea. If any of you have dabbled with 1/2A, you may know that they are notorious for being difficult to throttle. I've worked on the problem for years and have had much luck in getting good throttling out of the tiny engines. An exhaust baffle coupled to the throttle works very well. The question is why.

Here's what's happening. When you throttle at the intake, this tends to drive the fuel mixture rich. When you throttle at the exhaust, (as in Cox .049) this tends to drive the mixture, (at idle) lean. When coupled together, the two effects cancel each other out.

I've done several articles in Flying Models covering the subject of throttling 1/2A engines. There's no reason why the principle wouldn't apply to any other size of engine.

Right now I'm converting a Tanaka 23cc to diesel. Some of that can be seen here.

http://www.rcuniverse.com/forum/m_45...page_24/tm.htm

And more about throttling small engines here.

http://www.youtube.com/profile_videos?user=hopeso

The Tanaka conversion will be done up for diesel, glow and gas and all fuels will be experimented with including such things as WD40, Varsol and 100% Biodiesel. The issue for glow on low oil mixes has always been the plain bushed rod. The Tanaka ought to eliminate the problem with its needle bearinged rod on both ends.

I have encountered two engines which vibrate badly on the "stock" glow/gas fuel mix . One is a Stihl 35cc, the other is a Shindawa 23cc. None of my other conversions have shown this tendency. It definitely is not a balance problem, but ignition timing with the fuel mix.
I have run both engines on EI syncro spark and the vibration is not present. On glow/gas mix it was sever enough that the glow plug element failed within three minutes.
For now, I am playing with the Stihl. I modified the "standard" glow gas mix by increasing the methanol content to 30%. That helped some, but the vibration is still significant. I had originally cut down the base of the cylinder to increase the compression for operation on EI, so I tried adding a gasket under the cylinder to lower the compression. Some improvement, but vibration level still unacceptable. I added a second gasket and the vibration level reduced some more, but still now what it was running EI.

I don't want to reduce the compression any more, because with each reduction I lost about 400 rpm on the top end. On EI the Stihl would turn a Top Flight 18-8 @ 8200. On the "standard" glow/gas mix, the rpm was essentially the same, but with unacceptable vibration.

After Thanksgiving I am going to work on the theory that increasing the octane of the fuel mix will help in taming the advanced ignition on glow/gas. I am going to stay with the 30% methanol concentration, but will try some of the typical octane boosters and see what happens. The three that I will try are ethanol (R octane 124), Toluene (R octane 117) and MMT which is a organo metallic octane booster and has anti knock properties similar to tetra ethyl lead. In addition I am going to experiment with increasing the exhaust back pressure as suggested by Captjohn and see what effect it has.

I really don't know why these particular engines exhibit the sever vibration when my conversions of Echo, Zenoah and Kiroitz engines have been flawless.

Will,

I haven't owned many large engines and I don't remeber much about that Quadra so many years ago but what of the piston? I note that the Tanaka has a domed piston. What of the other engines. Might this have something to do with it? What value a domed piston?

Andy,

I can not speak for all of the engine designs, but every one that I have worked on has a domed piston. That includes Echo, Homelite, Kioritz, Zenoah, Poulan, Stihl,Shindawa and McCulloch. I don't believe that it has any thing to do with the vibration level of the Stihl and Shindawa.
Why they have domed pistons I can only speculate that it has to do with combustion chamber shape and perhaps structual strength. A domed piston would seem to be stronger and resist the compression loads better than a flat top piston. I would have to defer to someone who is more knowledgable about the design features than I.

Will

Will,

Many thanks, I was just wondering if that may have had any bearing. Appears not and if domed pistons had any advantage, I'd bet we'd see a lot more of them in the glow world. I'm speculating it's just a way of upping the compression in a limited space.

I think you may be on the right track looking at octane. Would not detonation account for the vibration?

I wonder if a turbo plug might make a difference?

Hello Will,

Here is an interesting article detailing the "Squish Action". It details many aspects of ignition variables that maybe of useful to you.

http://home.earthlink.net/~scloughn/id21.html

It did not copy and paste well, do to some nice graphics. I tried to post the entire article but it did not paste the drawings required for complete understanding. Give it a look over and tell us if this contains helpful information to resolve your shaking problems.

A second article:

Two stroke compression and head design

http://home.earthlink.net/~scloughn/id16.html

I hope this is helpful.

Kelly

I have a 32cc Mac that I would like to convert to glow. Where can I buy the adapter to replace the spark plug?

Horizon Hobbies

RingWinger,

Thanks for the links. The two articles gave me a much better understanding of why you could get different performance from engines which appear to be very similar. It would appear that for engines running fixed ignition timing such as one running on glow/gas, the corrected compression ration would be a good indicator of what kind of fuel mix would be required. Higher CCRs would require higher fuel octane number. Aeronut's post about needing to go to 91 octane gas on one of his engines would seem to give this some credence. I'm convinced that ignition timing is the root cause of the vibration problem since the engine does not vibrate on EI syncrospark. Since an engine running on glow is essentially operating on "fixed" timing for a given fuel, it would seem to follow that modifying the fuel mix is about the only variable which could be changed.

I'm going to determine the CCR for the Stihl and an Echo 38cc which runs very well on the standard glow/87 octane gas. I am also going to run the Stihl on an EI unit without the syncrospark unit to see at what point advancing the timing starts to cause vibration.

Kelly,

Thanks for the links. That Tanaka I'm working on will be getting a glow head as well as the diesel head. I was going to duplicate what we see in Norvels for the combustion chamber but it looks like a hemi head may be a good starting point. The volume required will be the big question.

Another thought. In very small engines, as in .049s, they use a glow head and this delivers good performance. These can be expensive so some have drilled and tapped the head to take a stock plug. A good idea except that you typically lose 1500 RPM ( out of 18,000). Throttling improves for some reason but no amount of increasing the compression brings back the power. As shown by the use of a Nelson or turbo plug, the problem appears to be the exposure of the plug's threads to combustion. As larger engines have combustion volumes much larger in proportion, the effect is not so noticeable. Having said that, I've found some nice gains by installing turbo plugs even in larger Norvels.

In a small, gasoline, two stroke, we have much larger plug threads and in fact, proportionally, they look to be equivalent to the 1/2A plug problem. So replacing the head entirely and installing a turbo plug or even a stock glow plug may reveal inefficiencies not thought of. This will be explored with the Tanaka project.

The other issue might be the angled plug versus one that sits parallel to the cylinder. I can visualize that this may make combustion characteristics vary from engine to engine.

You guys are all on the right track, and the more experiments the better. The problem with these industrial "conversion" engines is that with the one piece head/cylinder config., it sure makes compression experiments tough!! All I do is lower the cylinder until I get minimum deck clearance, and go with it. (After removing the squish band.) No problems so far with my Echo/Kioritz, Homelite, Ryobi, and Poulan engines, though the Poulan shows signs of needing more compression to work well with the present Gas/Glow mix. Either that or more nitro. Can't go to a hotter plug as I'm already running a four stroke plug....

The basic rule of thumb for glow engines is that if the rpms drop when you remove the glow heat (battery), that the engine needs either more compression, more nitro, or a hotter plug. If it detonates, it needs the reverse. Having gasoline in the mix is a new twist, and complicates things a bit. So far we haven't been having detonation problems when the mix uses high test gasoline.

The vibration you guys encountered is an interesting phenomenon, and I'll be interested to see what you find out.

Something else I haven't seen anybody mention, is the fact that the exhaust timing on a two stroke engine determines the "effective" compression ratio, which throws another variable into the equation. AAhh, that's what I love about engines.... always a challenge!

AV8TOR

this thread sure is getting interesting again !!

[&:]