Increasing 2 stroke performance - how?
 

I've got a question for all you 2 stroke engine gurus :D

If I wanted to take a brand new ABC .28 2 stroke and get the maximum performance out of it ie as high revs as possible at full throttle on an APC 9x4 prop what do I need to do to it?

I plan to run it in on an 8x4 on the same fuel I intend to run the engine (ProSynth 10%) on but should I be messing about with timing on the liner and/or fiddling with the exhaust?

TIA, Andy

RE: Increasing 2 stroke performance - how?
 

fidling with the exhaust is the most obvious to do first, as the exhaust on two strokes is almost as important as the engine itself in creating power. If you know your business you can port the sleeve and camfer the lower part of the sleeve to aid in breathing(I do this on my buggy engines to gain top rpm) about porting do some searching in car forums and you`ll find info and pics on the subject. good luck. Remember,the one who sits on the biggest pile of broken parts/engines will be the greatest tuner in the end:D

RE: Increasing 2 stroke performance - how?
 

Like most other fellas, I would love to find a way to tweak more power out my engines. So I think it begs the question: If an engine can be manipulated to produce more power, why doesn't the engine companies themselves do so, as alot of us would purchase these hoped up jobs, even if it ment shorter life-span of engines? Just my thought.

RE: Increasing 2 stroke performance - how?
 

asmund, thanks for that I'll have a look at some car forums.

RevGQ, I believe its because most manufacturers prefer to see longevity and reliability out of their engines rather than outright power, at least so it see's out the warratny period!

What i'm looking to do is find a way to get extra power out of a stock abc .28 without using a tuned pipe!

RE: Increasing 2 stroke performance - how?
 

If the manufacturers was doing the filing and grinding for you, the engines would be substantially more expensive and not a lot of people would buy them.(look at the top tuned buggy engines, they are very expensive even though they are only 21 in size) Manufacturers make a compromise between power and longevity and of course cost.

RE: Increasing 2 stroke performance - how?
 

Why dont you want to use a tuned pipe?

I am sure you can get MOST of the gains you get from porting by using only a tuned pipe on your stock setup. But, if you do both, then you could easily get a big gain in power, I would say 5000-10000 rpm above stock using same prop is not out of the question. Watch the tip speed of the prop your using though, it could disintegrate with the higher speeds your engine is now capable of.

If you do want to go the porting route, look up www.macdizzy.com, they have the basic info you need to get started.

You will need to set a baseline first with the intended prop, then set a goal in terms of RPM. Dreaming does not help, set a conservative realistic goal like say a gain of 2000 rpm to begin with. After that you determine the port time-area for that RPM, and start cutting! Building & matching a tuned pipe to the new porting will be a b*tch, let me tell you that.

RE: Increasing 2 stroke performance - how?
 

I'd like to see some VERIFIED RPM figures. I don't believe simply adding a tuned pipe to an otherwise STOCK engine will result in a 5000-10000 RPM increase. 500-1000 or 2000, maybe. I've run plenty of tuned pipes on stock engines.

Why don't you want to run a tuned pipe? It's a quick power boost. Ditch that 10% fuel, too. We used to run up to 50% in our Webra Speed .28s. They threw the occasional con rod, but screamed like banshees.

I wouldn't mess with timing unless you REALLY know what you're doing, in which case, you wouldn't be asking this question. When you change one thing, you usually have to change several others to make it work right.

Dr.1

RE: Increasing 2 stroke performance - how?
 

What have you got against just buying a bigger engine? Are you racing in an event that limits engine displacement to .28 cubic inches?

I was hanging out at our local flying field once and a friend was flying a new plane with an OS 1.08 in it. I said "well, that ought to put you in the gallon a day club". He replied, "actually, I'm burning less fuel in this plane than I did with my old tuned pipe .60".
It also wouldn't surprise me if the 1.08 was also lighter than a .60 plus a tuned pipe.

RE: Increasing 2 stroke performance - how?
 

Thanks for the suggestions guys. Engine is used in a competition that limits size to .30 with NO tuned pipes!!!

Otherwise of course I'd have gone and got a bigger engine and shoved a pipe on it. What I need is a .28 sized 'sleeper' :D

RE: Increasing 2 stroke performance - how?
 

Just get Webra 25gt and be happy. An 8/6 prop will work just fine.

RE: Increasing 2 stroke performance - how?
 

I did say "do both" (meaning porting AND tuned pipe) before giving the 5000-10000 figure, which was just a generic figure that I think is possible from my experience with full-size 2 strokes. I could be wrong as far as glow engines are concerned though.

What are the rules in your racing class Hercmate? Any curbs on the type of fuel; any restriction on the prop?

If not you should be using the highest nitro that your engine allows, with a prop that has the highest pitch and smallest diameter. I dont know what s recommended for a .28, but just as an example you could substitute that 9 x 4 with say a 8 x 5 or 7 x 6. Once your prop is selected, I think some experimentation with a tach and some head shims is in order when you are trying more and more nitro in your mix. Add head shims as you increase nitro.

Beyond that you should think about porting.

RE: Increasing 2 stroke performance - how?
 

Re-porting a two stroke shifts the rpm where the maximum torque occurs, it does not increase the maximum torque very much, it may even reduce it.

Don't overlook the fact that it is the "drawbar" horsepower that counts. (Drawbar horsepower is a tractor and locomotive rating that measures the power actually delivered to the drawbar.) Drawbar horsepower is shaft horsepower times propulsion efficiency. If your engine and propellor combination is delivering 5 pounds of thrust to the airframe while going 110 feet per second, it is delivering 550 ft-lb of energy per second to the airframe which is one horsepower. If it takes two horsepower to turn that prop, then the propulsion efficiency is only 50%.

Retuning an engine to turn super high rpm means you will have to use a smaller diameter prop which reduces the propulsion efficiency, perhaps canceling any benefits of the increased shaft horsepower. Increased propulsion efficiency is the reason electrics and a lot of full scale aircraft, especially the experimental planes that use auto engine conversions as well as the WWII propellor planes, used gear reduction drives to the prop. It is also the reason that I and a lot of others don't prop our engines for "maximum scream".

RE: Increasing 2 stroke performance - how?
 

It's not a racing class, it's a combat class. The reason I said a 9x4 is that outright speed is not the key but good acceleration and most importantly having the motor 'on song' in the turns.

I've had a long look at the car forums and it would seem that porting is the way forward for more rpm. However I have an old, but good .25 that I'll try it out on first :D

RE: Increasing 2 stroke performance - how?
 

Hercmate

You've gotten some really good advice so far and I'll throw my own two cents worth in which is based on racing motorcycles and karts back in the day.

Altering port timing can raise (or lower if not done right!) the maximum operating range and thereby increase horsepower. The downside to this is that usually the power band is narrowed. So, naturally if you are running at a higher rpm you have to run props with less load to enable you to actually get into your desired rpm range. Smaller diameter greater pitch props will produce a faster moving cylinder of air but less actual thrust. This works on airframes with low drag like pylon racers but works against you on high drag airframes like a fun fly. As has already been pointed out, the faster a prop turns the less efficient it gets so there is a definite point of diminishing returns here.

At any rate as you could probably guess it all comes down to experimentation with both port timing and prop selection. If you have a lot of free time and money you could probably come up with a real sleeper. Personally, I'd start with the prop selection first, trying different brands and sizes and maybe even playing with the configurations of the props. I'm betting that among the stock props APC's will perform best and MA's will make the engine scream (because they flex and flatten out at high rpm) but not pull as well. Wood props might have the most room for performance gains since they are so light. Carbon fiber props (if you can find one in your size range) will retain their shape at high rpm and be light as well. When you test props it's best to test them against a bench mark like a friend's similar plane. You can be easily fooled by a screaming enigne into believing the plane is actually performing better.

Finally, in any limited modification rule racing having a really well broken in engine is paramount. Less internal drag means higher rpm. Break it in at the mixture the engine will finally run at (or, at the most just slightly richer but not slobering rich by any means!). Also break it in with a low load prop to avoid blow by and the galling that come with excessive blow by. In the end you want the lowest internal drag and the highest compression possible. You might even try milling the head a little which will also require you to raise your nitro.


Good Luck,

Big Mike

RE: Increasing 2 stroke performance - how?
 

If you are going to run an 8x4 on a 28, the first thing you better do is find an RPM connecting rod. Next is to get ready to watch it fly to pieces after you lean it out. An 8x4 is a .10 to .15 size prop. I ran a 25 to destruction once to prove a point and I uesd an 8x4. Rod busted after about 10 minutes and the neighbors screamed about the noise. Engine modifying was what kept RPM in business.

Actually the manufacturers do just about all they can to make their products as powerful as possible. It is a COMPETITIVE business. The big problem is by trying to modify an R/C to produce more power something has to suffer. That something is the idle and dependability. When we modified two cycle engines back when I was racing C/L and running rat race we didn't care about idle. Didn't do it. Just ran flat out. Hi Johnson even had a set of tune up and modifying tricks he would send out in his newsletters. Of course the first thing he stated was that any warranty was instantly voided by modifying an engine. Half the time the mods didn't produce any chnage. I have a Johnson 36 Ball Bearing that will turn a 9x7 faster than on OS 40FSR. Of course fuel economy went out the window. 4 ounces in less than three minutes but boy does it scream.

RE: Increasing 2 stroke performance - how?
 

If your local rules allow a "mousse" can muffler then that is what I would start with. Set it up for peak rpm on the ground using an 8x4 and fly on a 9x4. If it does not come on in the air then lenghten the header by 5mm and try again.

Get that right using a low nitro fuel and then start adding nitro [ this might mean shimming the head to reduce compression].

Fast is good but in combat faster is always better.

MVVS 20 + mini pipe + 25% nitro = fastest kid on the block!

RE: Increasing 2 stroke performance - how?
 

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I find it curious that he hasn't stated the intended use of the engine. Is it strictly a speed event in a straight course or a circular course? Makes a difference.

OR, is the engine going to be used in R/C Combat? This makes an even bigger difference in prop selection and rpm.

As you have pointed out, maximum scream isn't always the answer.

What about throttle tractability? Is the engine going to remain at high rpm with the throttle only being used to shut off the engine? Will there even be a throttle? Too many variables.

Is there a fuel restriction? Do you have to use just one provided fuel? Or can you make your own witches brew?

How long does your engine have to last?

I like events where you have to finish with the engine you started with. That keeps some folks from getting too wacky.

RE: Increasing 2 stroke performance - how?
 

Hercmate Your looking for that magic wand to wave over your engine to make it faster and more powerfull than your fellow competitors. With everything in life it's a trade off. I had a drag car. Great for the strip but it sucked to go get groceries . Everyday drivabilty was no fun. Your club has rules so you can't do what you want to do. Your best bet is to sand your props really smooth (cut the air) and perfect balance along with prop spinner. And put in the best ,low drag, high speed bearings you can find. Most engines have cheap ruffer bearings in them. And take the baffle out of the muffler and RTV muffler joint so you don't lose pressure. Put a super tune and use the least (safest) oil content like cool power 17% and resonable nitro content.

RE: Increasing 2 stroke performance - how?
 

If you can run it without a carb then use a large venturi and crankcase pressure. Since it will not need to idle you can also cut rotor to close later, this will maximize the induction charge. You are not getting far out of the engines intended rpm range so any timing changes should be slight. Try a .010" shim under the liner and a little more nitro. It would probably be easier to disguise a 32 and cheat like the rest of the winners.

RE: Increasing 2 stroke performance - how?
 

Pipefather,


A tune pipe would typically supercharge an engine by 20%....
So getting 5,000-10,000 RPM, as you were already told by others, is very much out of the question!

Getting 1,000 RPM more, as in, say, 14,000 instead of 13,000 RPM, would require nearly 25% more HP and 16% more torque, at the higher RPM.

Spinning the same propeller at twice the RPM, requires eight (8) times the HP. And even this is correct only at tip-speeds of non-compressibility (up to roughly Mach 0.7).
At higher Mach numbers it becomes very much higher.

...You obviously never tested, or observed, back-to-back testing of normal and tuned exhaust systems on an engine, so please allow others that are experienced with this to express themselves and bring yourself to listen to them.


A good tuned exhaust will typically increase VE and torque by 15-20%, over the open-exhaust numbers and by about 25-30% over a regular silencer (RPM by 10-15% higher).
An outstandingly great tuned exhaust (like those of F2B racers and nitro boats, spinning 30-40,000 RPM), can achieve up to 27-30% over the open-exhaust VE numbers.

Believe this, or you are bound to be extremely disappointed...

RE: Increasing 2 stroke performance - how?
 

Darzeelon,

If you go back and actually read my post #11, you will find I said the following:

Meaning, in effect, that a well ported and piped engine WILL produce a 5000-10000 gain over a STOCK (meaning factory porting and silencer) engine.


How can you be so sure? I will quit being modest for the moment & tell you that I worked for the Engineering Department of a very prominent 2-wheeler manufacturer here in India (world's 5th largest in fact). During that time I have observed, AND tested dozens of combinations of tuned pipes & porting . This is not including the perhaps thousands of simulations we have run on software like MOTA, BiMOTION (generic 2-stroke simulation software) & AVL's Boost (which is not even available to people like you and me). Do you know why I got this job? Because I independently built my own tuned 2-stroke exhaust systems before that & my paper on the same won first place at undergraduate state-level, thats why. Therefore, while my experience with glow engines in particular is less than poor (as I have admitted myself), my experience with 2-strokes in general, IS NOT.

Please refrain from making arbitrary statements about people who you know only as someone who wrote a few out-of-the-ordinary comments on an internet forum. The purpose of my post on this topic was to help the person asking the questions. OTOH, the sole purpose of yours was to impose your apparent "know-all" attitude on me, or to start a pissing contest. If this helps the guy asking the questions, SO BE IT. I'm all game!

Coming to the point, while I do not know much about propeller dynamics, I beg to differ as far as your baseline of 'open exhaust' is concerned. That is not the correct baseline from which to measure. The correct way is to have a stub exhaust long enough to prevent undesirable reversion and the resulting impact on intake system dynamics. In case someone takes issue with this, I will repeat, this is how we do it on full-size racing 2-stroke engines. Which have piston speeds, piston accelerations and gas velocities VASTLY exceeding that of glow engines (aircraft only, not the marine or car engines).

Your comments regarding the comparision between open exhaust and factory silencer are blatant untruths.

You are implying that VE (I assume this is exhaust mean velocity?) is HIGHER with an open exhaust than a normal silencer. That shows you have only a surface understanding of gas flow in engines. The very fact that you have an open exhaust means you are LOWERING your exhaust velocity over the stock header. Show me temperature gradient data or Doppler anemometry data to the contrary, & I will gladly accept that I am wrong!

Open exhausts do NOTHING for efficient performance. Notice I said both "efficient" and "performance". You may get a few hundred rpm more with an open exhaust. But at what cost? Vastly decreased volumetric efficiency and cylinder filling. Meaning that half your fuel charge is being thrown out that open exhaust!

believe this.....or NOT, your choice.

RE: Increasing 2 stroke performance - how?
 

Mufflers do give a good increase in fuel economy over an open port in all sizes of cylinders that I have worked with. So did a smallish carburetor. The timing specifications for small glow engines are not even close to ones published for larger cylinders. And the numbers that come from tuning programs never match what I know ran best. Any tuning experience a person may have with motorcycles does not apply very well to model engines.

RE: Increasing 2 stroke performance - how?
 

Uh --- Pipefather -- VE means Volumetric Efficiency. Don't you just hate when that happens -- you know -- suddenly the other guy's argument actually means something? [:o]

RE: Increasing 2 stroke performance - how?
 

Are we talking about a 10,000 RPM increase on a .28 2 stroke engine?

RE: Increasing 2 stroke performance - how?
 

No it still doesnt mean anything. If VE is indeed volumetric efficiency, you dont get volumetric efficiency increases with an open exhaust as opposed to an untuned silencer, which is exactly what I said at the end of my previous post.

Kweasel, I have noticed that stock aircraft glow engines (atleast the couple I have had the chance to measure) have somewhat mild timing, on the exhaust side, compared with bigger engines. This may have something to do with the fact that actual port timing is less important than port time-area. Do notice that most glow engines have bridged exhaust ports which can be made very wide relative to the bore. So a milder timing can be used to make the engine more docile as far as idle and part-throttle efficiency is concerned, while still enabling a high gas throughput at higher rpms.