# Cheating?



## Kludge (Jul 23, 2008)

I would like a ruling on something here. 

Another project (on the hook with everything else) is to try to "hotrod" some lawn trimmer engines, both two and four stroke. The idea is to be able to get as much power from them as possible at a constant speed (as opposed to blipped as done when in trimmer use) without breaking anything. The engine loads will be propellors (which are also going to be changed now and then to test different diameters & pitches) and I hope to have a test bed on which I can not only measure the thrust but also torque, even if it's an indirect method. The engines will be tested at full throttle where things can get real silent real fast after a rather unpleasant set of loud noises, and at 70-80% full throttle where they'd be spending most of the operating time. Oh, and the 2-stroke engines, being reed valve engines, will be tested in each direction with attendent ignition changes to manage that trick.

So anyway, the question is if it's cheating to start with engines that are already built and all that's being done to them is modifying them to a particular purpose outside the original design intent.

Best regards,

Kludge


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## Twinsquirrel (Jul 23, 2008)

Hi Kludge and welcome to the board BTW,

Nah not cheating as long as you post LOTS of pictures, just check out some of Rangersteamtoys posts on this forum, really interesting stuff from a young whippersnapper who I am sure is going to solve the energy crisis single handed some day.

I think you will get the best increase in performance from tuning and straightening the intake and exhuast path on these compact little engines as they are all squashed into a small package to make them easy to handle. A proper exhaust with expansion chamber and properly designed pulse reflector will really do wonders especially for a narrow power band. Personally I would push the RPM's to far though

What application are you going to use these engines for....It sounds interesting?

David


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## Brass_Machine (Jul 23, 2008)

I have done A LOT of 4 stroke tuning on smaller engines. I have eeked HP out of motorcycle engines etc... I have some ideas that have been proven on tuning the intak etc... Lemme know if you some pointers.

Eric


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## Bogstandard (Jul 23, 2008)

Are you talking about air or water that you will be running the propellors in?

John


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## Kludge (Jul 23, 2008)

Twinsquirrel  said:
			
		

> Hi Kludge and welcome to the board BTW,



Why, thank you, sir. It's a pleasure to be here.



> Nah not cheating as long as you post LOTS of pictures, just check out some of Rangersteamtoys posts on this forum, really interesting stuff from a young whippersnapper who I am sure is going to solve the energy crisis single handed some day.



But of course! I have a Canon 20D (which I used to take the sunset & moonset pics elsewhere) so photos aren't a problem.

Yeah, gotta watch those kids. They have a habit of outdoing the previous generation in one way or another. 



> What application are you going to use these engines for....It sounds interesting?



You'll find it here: http://www.ziroliplans.com/b25-sm-doss.html The B-25 has always been a favored airplane of mine though the version I'm making is a civilized one - no guns etc. Cameras will go in the bomb bay plus it'll be used to try out various electronics packages I've seen and want to play with.



> I think you will get the best increase in performance from tuning and straightening the intake and exhuast path on these compact little engines as they are all squashed into a small package to make them easy to handle. A proper exhaust with expansion chamber and properly designed pulse reflector will really do wonders especially for a narrow power band.



The engines I've got are Ryobis, one each 2- and 4-stroke. Thery're in the 25-26cc range although they also make a 31cc 2-stroke that has more bang per pound. One thing I love about these engines is that they're cheap and I don't have to go any further than my local lawn & garden store (or equivalent area of other stores) to get replacements.

Most modellers would just leave the intake alone and run the exhaust down the engine side of the firewall using two tubes from a box over the port. The scoops on top of the nacelles are too small to be used effectively for the intake much as I'd love to but I would like to find a way to get some ram air to the carbs. I may wind up cheating a bit and building NACA-type scoop inside the nacelles that curves up into an airbox. This I _think_ should be enough to bump the intake pressure up a bit. The more pressure there, the more in the cylinders where it counts.

As I mentioned, the usual path is to route the exhaust down through a couple undersized tubes from a box over the exhaust port which doesn't seem all that efficient. All it does is give it somewhere to expand ever so slightly before becoming part of the atmosphere. I've been looking at what Convair did on the 204 where they routed the exhaust the length of the nacelles and out through some fairly healthy sized pipes. (Of course, it had R-2600s which are ever so slightly larger.) Aside from being slightly quieter, it also apparently acted like a tuned exhaust system since it boosted the available power as well.



> Personally I would push the RPM's to far though



I assume you mean "wouldn't". ;D

Both engines are designed to turn around 8000 RPM which is plenty fast, especially since propellors lose efficiency if swung too quickly. (Turn them way too quickly and they lose interest in staying in one piece too. That's not considered a Good Thing.) I figure that if I plan for whatever the rated max is as the absolute redline, I can cruise comfortably at 3/4 of that without any major wear and tear on the engines. This isn't far off from normal piston pounder practice (Two or my ex's are pilot & aircraft mechanic.) and that has worked for many much years. 

I should add that this is _NOT_ a front burner project and that it includes a lot of different experiments all going on at the same time. Some are in electronics, some in the airframe, some in the engines etc. My concerns here are the engines. madmodder will likely get the rest ... poor folks. 

BEst regards,

Kludge


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## Kludge (Jul 23, 2008)

Hi, Eric,



			
				Brass_Machine  said:
			
		

> I have done A LOT of 4 stroke tuning on smaller engines. I have eeked HP out of motorcycle engines etc...



Flashback: muscle car era and being a mechanic with a golden [tuning] ear. 



> I have some ideas that have been proven on tuning the intak etc... Lemme know if you some pointers.



I shall do so. Most of the info on the project is in my reply to Dave so you can see what I've sketched out so far. Trust me when I say that neither North American nor Nick Zirolli had anything like this in mind. 

Best regards,

Kludge

Eric
[/quote]


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## Kludge (Jul 23, 2008)

Hi, John ... Bogs?



			
				Bogstandard  said:
			
		

> Are you talking about air or water that you will be running the propellors in?



Air. If they wind up in water, I've done something dreadfully wrong. On the other hand, it probably wouldn't hurt to waterproof all the equipment aboard just in case. 

BEst regards,

Kludge


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## zeusrekning (Jul 23, 2008)

Kludge, 
Check this site out. http://www.marinespecialties-rc.com/tunedpipes.htm


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## Kludge (Jul 23, 2008)

zeusrekning  said:
			
		

> Kludge,
> Check this site out. http://www.marinespecialties-rc.com/tunedpipes.htm



Oh, boy! Toyz! Thank you for the url. They have way cool stuff!

Best regards,

Kludge


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## Twinsquirrel (Jul 24, 2008)

Looks like a super project, from your first post I thought you might be making one of these    







It's called a "cri cri" and runs on 2*9hp chainsaw motors. And yes, they used to take off AND land on the top of the car!! I remember seeing one of these displayed at the Paris airshow in the late seventies... frightening!!!


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## Kludge (Jul 24, 2008)

Twinsquirrel  said:
			
		

> Looks like a super project, from your first post I thought you might be making one of these



Oh, dear! I think they call it a cri cri because crikey crikey takes too long. 

I may have been a good pilot and a sometimes crazy one but outright insanity was never part of the program. 

Well, not all _that_ often. 

BEst regards,

Kludge


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## Bogstandard (Jul 24, 2008)

Kludge,

The reason I asked about water, was that I envisioned one of those far eastern shallow water boats, engine at one end of a long shaft, prop at the other.

Bogs


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## DICKEYBIRD (Jul 25, 2008)

Kludge  said:
			
		

> I've been looking at what Convair did on the 204 where they routed the exhaust the length of the nacelles and out through some fairly healthy sized pipes. (Of course, it had R-2600s which are ever so slightly larger.) Aside from being slightly quieter, it also apparently acted like a tuned exhaust system since it boosted the available power as well.


 Oh yeah! The Convair 240/340/440 series is a favorite of mine. BTW, the engines were actually R-2800's

The trick "augmentor tube" exhaust you're referring to actually mixed the hot exhaust gases and outgoing engine cooling air with cold outside ram air in a large tuned exhaust pipe. The tremendous heat caused the cold air to expand and rush out the pipe and actually created a jet-like extra thrust effect, good for 8 - 10 knots if memory serves. It increased the cooling airflow as well.

As far as being quieter? It may have been my sensitive young ears but my memory of them was a bellowing racket like a herd of enraged bulls. My family lived near Memphis Int. Airport in the fifties and I still remember those glorious sounds. They would absolutely rattle the whole house, not just the windows! ;D

Connies? Now there was a smooth, relatively quiet machine. Those turbo-compound '3350's made a delightful sound. Only problem was that many times only 3 of them were running when landing after a long flight. Wise-guy pilots quipped "The Constellation is the world's best 3 engined airliner."

As far as your Ryobi mods, I remember an article in Model Aviation or RCM a while back where the author hopped up some weed-eater engines. If memory serves, the biggest power increases came from a properly tuned expansion chamber exhaust system, slightly larger carb., smoothing the inlet ports and altering the ign. timing. He may have replaced the reed valves as well. Raising the compression would definitely help but don't they have a one piece cyl. and head?


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## Kludge (Jul 29, 2008)

Bogstandard  said:
			
		

> The reason I asked about water, was that I envisioned one of those far eastern shallow water boats, engine at one end of a long shaft, prop at the other.



Hmmm ... we have a few very shallow waterways here that would work nicely on. With. Something. On the other hand, the local canoes are shallow draft too and I'm not much into rowing. 

BEst regards,

Kludge


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## Kludge (Jul 29, 2008)

DICKEYBIRD  said:
			
		

> Oh yeah! The Convair 240/340/440 series is a favorite of mine. BTW, the engines were actually R-2800's



Oops.



> The trick "augmentor tube" exhaust you're referring to actually mixed the hot exhaust gases and outgoing engine cooling air with cold outside ram air in a large tuned exhaust pipe.



Ah, okay. I couldn't remember all of why they did what they did. But then, I haven't touched an airplane for over 30 years and the mental facilities aren't quite what they used to be.



> As far as being quieter? It may have been my sensitive young ears but my memory of them was a bellowing racket like a herd of enraged bulls.



Outside, maybe. Supposedly the cabin was quieter - or at least the cockpit. On the other hand, a flight deck full of R-1820s (S2Fs) and R-3350s (AD-5s, and -6s) isn't exactly quiet. It is fun, though.



> Connies? Now there was a smooth, relatively quiet machine. Those turbo-compound '3350's made a delightful sound.



Sure did on the flight deck. I'd love to figure out how to create a blowdown turbine for the Jeep. Or one of the Ryobis. 



> Only problem was that many times only 3 of them were running when landing after a long flight. Wise-guy pilots quipped "The Constellation is the world's best 3 engined airliner."



Why not shut down one on the opposite wing to balance the thrust? ;D

Of course, a go around might be a bit hairy ... 



> As far as your Ryobi mods, I remember an article in Model Aviation or RCM a while back where the author hopped up some weed-eater engines.



Back issues. I need back issues. I guess I should subscribe too. Or renew.

The heads and cylinders are, in fact, one piece. But milling a bit ever so slightly can do fun things to compression ratios, and that milling can be done at the mating surfaces between the cylinder/head assembly and the block. Shaving a few thousandths can have interesting results. Worked on shaving heads, at least. But then, we had a bit more room to play with and ever so slightly stronger engines as well.

One other way I've been thinking about is ram air but Idon't know how well that would work, especially during climbout when I need the power the mostest.

Walbro (who makes a number of the original weed eater carbs) makes some larger almost bolt-on carbs which can boost performance rather neatly. A phenolic or something spacer between the block and the carb seems to be a consistant suggestion to isolate it from engine heat to some degree. Porting and polishing just takes me back to the good old days, just in a smaller scale. 

I shall endeavor to dig up that article and see what the author did.

Many thanks.

Best regards,

Kludge


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## Jadecy (Jul 29, 2008)

If you can use a 4 cycle you might be better off. The four cycle usually has better torque at the low end. The small 2 cycle engines usually don't produce power until you get in the upper rpm range and I guess one solution to that would be a gear reduction setup. I think you already know this but the exhaust pipe is critical to getting power out of a two stroke. I race karts and RLV makes all kinds of specially tuned pipes to produce peak power at a specific RPM. With the two cycle you get into things like squish band, port heights, reed type, pre mix ratios, tuned exhausts, intake length, ....

Looks like a fun project.


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## Kludge (Jul 31, 2008)

Jadecy  said:
			
		

> If you can use a 4 cycle you might be better off.



I tend to prefer 4-stroke but that may be a combination of experience with a number of them and bad times with the old Lawnboy mowers. Besides, a 4-stroke would let me play with an idea started on the Yahoo MICE group regarding fuel injection. A floatless carb as used on the lawn trimmer engines isn't that far removed from an aircraft pressure carb on one side (No altitude compensation as a major difference and a few additional minor differences) and the fuel control for a throttle body injection system on the other. Of course then we get into the whole idea of how complex an injection system can get vs how complex it doesn't really have to be on an engine that operates at a more constant speed/load situation. 



> The small 2 cycle engines usually don't produce power until you get in the upper rpm range and I guess one solution to that would be a gear reduction setup.



Or a different propellor pitch and/or diameter. There's somewhat a compromise situation where I can have lots of power with a low pitch prop (sweet with a 2-stroke engine) - nice for takeoff and climbout but lousy for cruise, or a higher pitch that stretches the takeoff and flattens the climbout but makes cruise nice and economical which fits a 4-stroke better. With at most a half gallon of fuel on board (internal and external stores), I have to decide how to burn it to achieve optimal performance. Or, stated another way, do I want something that will climb like a homesick angel but have to come back to roost fairly quickly or do I want something that climbs a bit more sedately but can stay in the air longer. 

A reduction setup may allow me to swing a larger prop but, in the end, I don't know that it's a solution. 

As a note, most airplane modellers - including giant scale - tend toward 2-stroke engines but I've long suspected that this is because the vast majority of model airplane engines are 2-stroke and that's what they know best. I'm more comfortable with 4-stroke ... but then I've never been known to be all that conventional either. 



> I think you already know this but the exhaust pipe is critical to getting power out of a two stroke.



And a 4-stroke hence the tuned headers on a number of racing machines. With a 26cc engine typically operating in the 6000-8000 RPM range, I suspect the thrust augmenter idea from the Convairs may be a righteous act to allow for tuning as well. (Now, if I could also get a more throaty roar instead of sounding like a runaway weedeater ... )



> Looks like a fun project.



Every aspect of it is - power plants, airframe, electronics, pneumatics (landing gear and brakes), and so on. I'm looking at using some "unconventional materials" in the airframe to lighten it as much as possible without messing with the structural integrity. It's design max gross weight is 35 pounds and I'd rather that be in on-board goodies than elsewhere. It's also capable of flying in the 100+ mph range so having the airframe such that it doesn't come apart in the air (or on the ground, come to think of it) would be a pleasant situation as well. Part of the electronics will be "watchdogs" monitoring things like cylinder head temp, ambient air temp, exhaust gas temp, rpm etc. I hope to add intake "manifold" pressure to that if I can.

Best regards,

Kludge


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