Go back and actually read what Dar said. Use all of the words.

Going back and actually reading ALL of his words, I found that this is what he said:


I may be wrong, as my knowledge of English is rather elementary, but he is saying that tuned exhausts give a bigger improvement over the regular silencer (25-30%) than with the open exhaust (15-20%). That is, he is saying that open exhaust has better VE and torque compared to a regular silencer.

Have I used all the words & do my posts make sense? Never mind if they dont. Cause this discussion is serving no purpose other than to marshall more and more opinion against me, the poor newbie who is supposed to ask questions like "D-Uh, I got an OS and I dont know the glow plug from the backplate", as opposed to talking about gas flow and Doppler anemometry.

Pipe, your English is far better than a lot of my fellow Americans and you use capitals and puncuation accurately.

The best way to hop up an engine is to add cubic dollars. Send it to someone who knows what they are doing and let them fix it up for you.

You are not wrong because of your English, but because you are partially technically incorrect.

An open exhaust can certainly have a higher exhaust gas velocity than an untuned muffler & often does. There may be fortuitous times in an engine's operating regime when an "untuned" muffler exhibits tuned behaviour, but on average this is not the case.

As you very well know, 2-stroke engines are extraordinarily sensitive to exhaust backpressure. A truly untuned muffler results in excessive backpressure, lowering exhaust header gas velocity (relative to an open header), thus reducing scavenging & VE. Both a resonant volume ("tuned" ) muffler, and a tuned length & volume pipe can rectify this problem in a specific RPM band, by either inducing low-pressure pulses at the exhaust port, resulting in higher gas velocity & improved scavenging, or by inducing carefully timed high-pressure pulses at the port, resulting in a pseudo-supercharging effect & hence, higher VE.

An untuned muffler can do neither. In an untuned case, it is more beneficial to have minimum backpressure, as would result from an open header. Dar is quite correct in what he said. He is also inadvertantly in error where fortutitous circumstances conspire to cause an untuned system to accidentally result in either a low, or high-pressure pulse, with the correct timing & magnitude. The chances of that happenning with any usefull regularity are quite slim.

Here's an article some of yaz might find interesting...

http://www.rccaraction.com/articles/htpporting_1.asp

I have done the outside of several sleeves, putting a radius on the lower lip of the ports, and adding guide channels...
It works...

all this tuning talk triggered my interest. I have done porting to many of my buggy engines, but never an aeroengine. well I just did the outside sleeve porting on my asp xls 52. I camfered all around the lower part of the sleeve and camfered and added guide channels to the intake ports on each side of the exhaustport. the intakeport right across the exhaustport was camfered from the factory, so I left that one alone. While I was at it I also removed one of the two headshims to raise compression because I will try to run some faI fuel.It all took just half an hour. I will run my engine by the weekend and compare tachreadings to my previous results. I`m very exited about this and will be very happy if I gain 2-400 rpm.If not successful I will put back in the headshim and run my regular 15/15 fuel and compare again.

I get it now. I said I might be wrong about glow engines and it turns out that I am.

What you said does make sense. That being said, I have never observed this effect with the engines that I have worked on. Maybe its because of the large volumes that even untuned mufflers have on full-size two-strokes.

Is the muffler volume on glow engines restricted to provide reasonable fuel tank pressure? And folks who remove the baffle realize an immediate gain in engine performance?

I have modified the sleeves of several engines as Asmund described...

I typically see about 4-500 r.p.m. increase with all else being the same as before the mod.

Example: O.S. .61FX was 11,100 before, and 11,500 after. 10% powermaster, APC 13X6 sport, slimline muffler and #8 plug.
(it was in a 9 lb. Corsair, hence the big prop )

Fuel tank pressure is not really a significant consideration, unless carb-throat sizes are excessive & the engines have poor fuel draw. There are a few examples of this situation, but not many. Muffler size is primarily restricted on model aircraft engines for weight & drag considerations, plus the sport-engine manufacturers seldom do any significant engineering on standard silencers for cost reasons. There are a couple of notable exceptions -- OS has actually spent time & money on their "box" muffler & it is the chief reason that the 46 AX engine modestly outpowers its predecessor. Another exception is GMS/Tower, who offer a resonant volume muffler on some of their engines, with good results in a particular RPM band.


In most cases there is an advantage to removing the internal baffle(s) & drilling out the outlet pipe.

we typically see 300-600 rpm`s gain just by removing the baffle and drilling the outlet. this will give less pressure in the muffler and tank. Maybe that`s why I dont experience any rich midrange problems on my asp xls 52 ???as numerous other reports. I love mine so far. Hope to squeese even more power out of it due to my grinding and polishing.I will report back on this during this weekend

If that is the case then I bet using a stub exhaust will definitely be an improvement over both open exhaust & silencer?

That s how we tested our engines. A stub of tubing the same dia as the original header and about 6 to 8" long (on a 125cc engine) would remove the undesired reversion of exhaust gas during the upstroke (& the resultant messing up of jetting/carburetion), and also improve exhaust velocity over open condition. This was the baseline condition for checking changes in intake duration, intake length and carb dimensions.

Anyone willing to try it out?

Gentlemen, thanks for all your replies! I've been away all week on business and this is the first time since last weekend I've had to view the thread

I'm going to have a go at porting an older Leo 25 engine but I've also taken on board the other stuff about bearings, carbs, fuel and exhausts etc!

To answer a few of the posts....

Ed Cregger you mentioned that I didn't state the use for the engine - I did in post 13! They are used in r/c spad combat where the rules allow stock motors of .30 or below with NO tuned pipes because of noise. If I was a little cryptic it's because I'd like to keep it quiet that I'm going to modify my engines! This isn't because it's against the rules, it's purely because I'd like an edge in combat. I've been using an MVVS 26 with a mini pipe and it's making about 17.5 rpm on an APC 9x4, however it runs on a pipe which isn't now allowed.

Kweasel cheating is out of the question, I want to do this legitimately or not at all.

Keep the discussion coming!! I'll post back when I've done some modifications and got some results.

You are correct -- I quite agree -- except that either an open exhaust, or a stub pipe are unacceptably loud in most jurisdictions. Hence, muffled tuned pipes, resonant mufflers & unbaffled stock mufflers.

I could quite easily do it with one of my rear exhaust engines by running with and without the pipe header which is 3.5" long. However, noise constraints won't really allow for that

Sorry about that. It is my bad memory at fault. <G>

One of the things that I haven't seen explored with 2-stroke model airplane engines is intake-tract tuning. Exhaust tuning is well understood, but intake tuning is also a potential source of additional power. I suppose that geometric constraints may be the reason that it it isn't prevalent.

Has anyone heard or any attempts at intake tuning?

I have been flying my asp 52 today and my grinding and raising of compression allowed me to run straight faI fuel with only a 200 rpm loss compared to stock engine on 15 % nitro. when I switched to 15% I gained an additional 100 rpm over the previous 15% readings.. So it works, just not to the degree I was hoping for. I think I will go back inside the engine and dremel some more since I left one port untouched. All in all 13100 rpm on asp xls 52 with 12.25-3.75 apc on o% nitro is quite good. fuel cost is under half of running 15%, so I`m happy so far. Next step is either to put on my rossi tuned pipe or ordering a tower muffler. Last but not the least, a buddy of mine ran his 61 fx today on the 12.25 apc and 15% fuel. This combo did 13100 bone stock, after removing the baffle and drilling the outlet to the maximum inner diameter rpm`s reached a very high 14000 peak. Alot of free power as you can imagine

I wonder if Kostas is reading this?

tomorrow I will fly my mvvs 49 on two different fuels(5% and 0%) and carefully note tach readings.When I come home I`ll pull out the sleeve and dremel on the 3 nice big ports and chamfer the lower edge around the sleeve. It worked on my asp so I think it will work on my mvvs also. Time will show.

Asmund,


If you do any of this, you would lose your engine's warranty.

So just make a note of that, if something fails to run as you expect it to.

I suggest that you wait with your engine, until it develops its maximum performance potential, on its own.

You guys all talk about hom much power you gain by hammering on your engines but no one ever mentions what it did to the idle or reliability. You don't get something for nothing.

I am of the, if you need more power, get a bigger engine, school and in the case of Saitos and MVVSs, on certain engines you can get a bigger one that will be lighter.

I am aware that any modding will bring my engine out of warranty. I`m just qurious and like modifying engines,that is the only reason I do this.I have never ruined an engine so far ,and this modifying has only made good engines better. My asp certainly liked the treatment and idles and trottles just as well as before. remember all grinding and polishing is done on the outside of the sleeve. No messing up on the chrome or ports itself, just chamfering the lower part of the ports and sleeve.Its no big deal really and the engines wake up a bit

asmund...if you have or can get one try an APC 12 X 4...it will put less load on the engine than the 12.25 X 3.75...although IMO 13,100 is a good R.P.M. for that prop on that engine.

The Tower muffler will also work better at a higher R.P.M. and my TT .46 Pro will spin a 12 X 4 @ 14,000+ on 10% and O.S. #8 plug.
The highest I ever saw was 12,800 w/ the 12.25 X 3.75 (stock )


loughbd...I haven't noticed and problems with idle or transition as of yet. I think mainly because the mods that I have tried aren't really that extensive.
The highest R.P.M. increase after the work on the outside of the sleeve is about 400 give or take a few. (that's an aircraft engine...however I have done a couple of car engines, and they sound like they are revving up more than that after the mod?)