R/C Aircraft Proving Grounds - Canister Exhaust Setup

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Canister Exhaust Setup

I've been getting some private queries about header length, style of canister mufflers and other things to think about when setting up this style of exhaust Here are a few pointers (others please chime in).

First off.. EVERY "cc" rated canister is TOO SMALL. For instance, a "50cc" canister is too small for a 50cc engine or cylinder. Go 30-40% larger than the actual cylinder size, so for a 100cc engine go with at LEAST a 60cc canister per cylinder, and a 70-75 cc rated canister is even better. The guys running the 3W106 are being advised by the dealers to use the Pefa "70cc" canisters meant for 120's and 140's, and 140-150 engines should use 75-90cc canisters. The BME 102 will work well with the 70cc canister, and the 110 almost NEEDS the 75 cc canisters. Same goes for the DA 100, 70cc canisters will let it breath and make power, any smaller and you start restricting the exhaust too much.

Okay. . WHY the oversize of the canister? You have to remember that the "rated" size of the canister is for a EUROPEAN airplane, which is subject to a lot of noise restraints. The Europeans intentionally restrict rpm and power, but swing props with a lot more blades (3-4 blades is the norm, not the exception as it is over here) and really slow the motors rpm down in order to reduce the sound footprint. Systems "Engineered" for American engines, like the DA150, based on what the Europeans thought was necessary, almost always have proven to be restrictive and to not boost power. The first generation Pefa system for the DA150 did this to the one I owned. About the only exception to this general rule seems to be the 3W150 and 75cc canisters, which work very well together. This setup seems to have been designed in cooperation with 3w/Pefa, rather than Pefa calling the shots entirely, and it makes very good power for the 3W engines.

Header length. . .a lot of people ask about this. . First off, you are GOING to have to work to get your header length set up for YOUR engine/muffler combination. After picking out a canister for your engine, start with an overly long header, on a test bench, and take a base RPM reading after tweaking the needles for best performance. I recommend starting with a 9-10" header. Once you have an established "Baseline" rpm, compare the power to the "Norm" for that engine as seen by the cross section of modelers in the Web forums. For instance, the DA-100 makes about 6300-6500 rpm, normally, with a Mejzlik 28-10 and stock exhaust diverters. If you are using a 10" header, and 70cc canister on this engine, it may actually DROP RPM due to the excessive header length. You need to know the target rpm you are trying to achieve, then beat, with your combination.

You will probably always come out with a lower rpm than the normal "baseline" for that engine, so now it's time to start "tuning" the exhaust. The first thing to do, after establishing where your engines power is, is to open the high end needle about 1/8 turn. This will "crutch" the mixture a little when the power increases, more readily revealing where the power is going. As you shorten the headers, 1 inch at a time, be sure to compare the current rpm reading to all others, making a quasi-graph of the power curve. The power will start to increase. Make sure the engine is thoroughly warmed up, so that you get HOT readings each time. It will make everything more accurate. Pay close attention to the high-end needle too. As the power increases to the "Baseline" power figure, then goes past that, you will need more fuel. At a certain point the engine will noticeably "Wake Up" and make a serious jump in RPM. When that happens, stop and take a look at things. . How much rpm did you gain? How long are the headers? how long do you NEED them to be? Can they safely go shorter? Now is when you decide to start leaving the comfortable area of "normal" exhaust scavenging and wander into the realm of power increase. It pays to be conservative at this point. Gaining 200-300 rpm is normal. Much more than that requires a lot more work. If you slowly cut back header length, and play with the high end mixture till you get a steadily repeatable 200-300 rpm increase over "stock", you have just about achieved as much as you can. Pushing for more, at this point, will most likely start LOSING rpm, or start losing tractability in the mid-range, so it's best to cash in your rpm chips and be happy with what you have.

One other factor to consider. . if you are setting the exhaust up with a low drag prop, such as a Menz Ultra or thin bladed Mejzlik or AM propeller, when you install a draggy prop, such as a Menz-S, MSC or other wide blade, the tuning that gained you such a great RPM increase with the less draggy prop may not provide a substantial power increase with the draggier prop. This is why it pays to be a bit conservative on the "ultimate rpm" tuning for these setups. A drag induced drop of 500 rpm, simply because of a different prop, can bring the engines power curve down "off the pipe" a bit, and you will not see the actual 200-300 rpm gain you experienced with the thinnner prop, and may realize only a 100 rpm increase over former readings. It's best to double check the exhaust, using several props of different designs and sizes, to make sure that you are getting increases across the upper 1000 rpm of the engines power curve. On my BME110 I set up, I got a great jump for the Mejzlik "setup" prop, about the same for a draggier Menz prop (though the motor turned less rpm it still gained about 200 or so from baseline) but at the top end a very low drag prop (Menz 27-10) gained only about 150 rpm. Since I was wanting to turn bigger props in the mid-6400 rpm range, this setup worked as I wanted it to, but it's possible to over-tune the exhaust and wind up with a "lightswitch" for a power band, with a soggy mid-range and fantastic top-end power, with almost no transition. This is great for Pylon, but not for the everyday flyer or Aerobatics planes.

I did this process with my BME110, using 75cc Pefa canisters and starting with 10" headers. Power started at an anemic 6400 rpm with a Mejzlik 28-12. At 4.5" header length I had gained 400 rpm back to the "baseline" and by shortening the headers by a mere additional 1.5" I gained another 250 rpm. This gives you some idea of how quickly things change. I might have gained more power with an even shorter header, but it was impossible to shorten it anymore. As it is, though, I have a very smooth running engine, with GREAT power, smooth midrange, very good muffling of the exhaust, and a nice idle. Conversely, the 70cc canisters on this same engine, with a 4" heaer, run about at "baseline" rpm, and give almost no power boost, though the system is very quiet. A Menz-s 28-10 woke up to about 6450 rpm from a 6250 baseline, and a 27-10 Menz was spinning in excess of 7300 from a 7150 baseline, while the Mejzlik 28-12 went from 6800 to about 7050. I want to spin the prop, static, in the 6400-6800 range, so this tuning with the 75 cc Pefa's worked out perfectly in this case.

The off-the-shelf canister "systems" available work very well,and I recommend following the suppliers advice about them. But, if your engine does not have a dedicated system designed for it, or you want to play with a system or have to custom make a system to fit YOUR plane, then have at it.

Hope this helps all interested.
Kristen Welter