Whats the difference between a tuned pipe and a muffler?

A tuned pipe is generally longer than a conventional muffler and is designed to resonate at a specific frequency (a specific RPM).

The result of this resonance is a suction at the exhaust outlet of the engine which helps to rapidly scavenge the exhaust from the engine and haul in a new fuel/air mixture. This allows more power to be generated.

The downside is that maximum power at only one RPM, a lowering of fuel economy, an increase in "fussiness", and an increase in engine stress.

The upside is a really cool sound, a really fast plane, and the admiration of all who watch you fly!

Doesn't the tuned pipe reflect the exhaust shock waves, pushing the unburned fuel/air left over from the last combustion cycle back into the engine as well?

http://www.howstuffworks.com/question636.htm

There can be a downside to using a tuned pipe - shorter glow plug life and often a much higher db reading.

I have a ST .61 ringed RE with a long tuned pipe 14-16"
with a header. I have never learned how to tune it other
than slide the pipe into the silicone sleve until it "hops
on the pipe". Any better way? From what I've heard you
can't tune them on the ground since the in flight leening
out is what will do it. Any help would be appreciated.

Frank Ward
Atlanta, GA

A full tuned pipe will have a low pressure wave reach the exhaust just as the piston uncovers it. This helps extract all the exhaust plus it draws some of the fresh charge through as well. But before the exhaust port gets closed another reflected high pressure wave reverses the flow of fresh mixture and pushes it back into the cylinder. This is where the "supercharging" effect comes from. Because the waves inside the pipe travel at a constant speed the engine must be running within a certain rev range for the timing of these waves to be right. By lengthening the pipe the interval between the high and low pressure pulses is extended and so the engine needs to run at lower revs to match this extended interval. OTOH by shortening the pipe then the engine must be run faster to be "on the pipe".

A muffler works mainly by allowing time for the pressure changes to get all mixed up and dampen out before going out the end. The bigger the muffler (more volume) the more time for the pressure to even out and the quieter it'll be.

I know the physics, I was asking for the methods of
tuning it.

Frank Ward

As far as I know Frank, that is how you tune them, the way you do it.

Sliding the pipe into the coupler til it comes on the pipe isn't the best way.. this is called tromboning and it is hard to set the needle doing this in my experience.


The best way is to run it without the pipe and check the RPM. Then connect the pipe, run the engine, set the needle and check the RPM. Then cut 1/4" off the header. Hook the pipe back up, run the engine, set the needle, check the RPM.

Repeat as many times as needed until the RPM drop. You can run it there, or back it back out 1/4".

A properly set tuned pipe won't "go lean in the air" under normal use...if you are running some very high RPM it might need to be set a little richer. Also, after the engine is "on the pipe" the needle reacts differently. Pinching the fuel line and letting the engine hit the pipe works good. Make sure the engine is rich enough, pinch the fuel line and see how long it takes the engine to jump in RPM. Usually about a second is about right. If you just screw the needle in til you get an increase you will be too lean. On the other hand, if you just back the needle out, the engine will just keep running until you throttle back....then it is way to rich to get back on the pipe. The pipe will take over the engine's needle. It takes some time to figure all this out.

I have pipes on a lot of engines, and I've set them all on the ground. Setting the pipe isn't the tricky part..setting the needle is.

Then cut 1/4" off the header .

This instruction sounds like I am to hacksaw off
1/4" of the header. If I have a silicone connector
connecting the header and the pipe wouldn't I get
the same results by moving the header end of the
silicone tube connector 1/4" closer to the exhaust
port?

Frank Ward
Atlanta, GA

The silicone connector is just to keep gasses from leaking. You can slide it maybe 1/4", but it's not for tuning. For one thing it will not last very long if too much is exposed to the exhaust, and it will sag and pulse if there is too much exposed in between the pipe and header.

Use tubing cutters

If you just buy a pipe and header, and put a 3" piece of silicone between them, you are gonna be way long..unless you might be running the absolute largest prop your engine can turn.

So knowing this..the header folks put extra tubing on the header..so you can cut it off until you get it set for any prop smaller than the absolute largest prop your engine can turn.

Don't sweat cutting the header..it's therapeutic to make a cut and see the engine perform better. It's the way it's done...

I have used a MAC'S Muffeled(sp?) Pipe several times in the past. I called them, told them what I was running, and they had a "Pre-Tuned" header and pipe already made for my application. I bolted it up and flew it!!! No fuss. They have worked great for me.

If you get a pipe and header, then you do need to "Tune" it. As stated before, set it, check it, shorten it, set it, check it again. It's the only way to get it right. As you do this the RPM will rise each time you shorten the pipe. At some point the RPM will drop off. That is the point where it is too short. Go back to the last length and secure it.

A pipe is best for a given prop/fuel/rpm combo. Once it's set up, it's done. If you then want to change props or fuel, you have to re-tune the pipe.

I have run a bunch of OS 46 engines on a Mac's pre-tuned set up, with a 10x6 prop. They are right on every time. Quick responce on throttle and good top end. If your wanting a more rounded set up, the "Sport-Jett" Muffler from Jett engineering may be better for you. It gives about the same top end, a better mid range, and you bolt it on like any other muffler. There is nothing to adjust. It's a crap shoot, so good luck.

As the plane gains speed the engine will unload and turn a higher RPM. I think Chuck is right, you need to shorten the pipe from the peak on the ground or the engine will be past the ideal RPM that the pipe is tuned to. This works great for straight and level speed. But then again if you want more power on uplines the maximum on the ground setting just might work. It certainly would be the best setting for hovering a plane.