Muffler Modifications and backpressure

[don]

I tried to do this once before and ended up burning the valves from a muffler modification. I piped it like I see today. How did the people on this site who perfoem these muffler modifications to increase power know what is sufficient back pressure to allow proper valve seating? Is this process so sophisticated you have dyno equipment? Did you have to make your own test equipment? Do saws today need allot of backpressure for proper valve seating?

 

[davefr]

Exactly what 2 cycle valves are you referring to??

I think some older 2 cycle designs might have had some reed valve arrangement, but all I've seen is a hole in the side of the cylinder with the piston serving as the "valve".

I'd like to understand why any exhaust backpressure is needed in modern 2 cycle engines? Is backpressure needed to slow down the piston and prevent an over speed condition?

 

[John in MA]

The backpressure helps keep too much of the fuel/air mix from blowing out the exhaust. Note that some older chainsaws didn't have mufflers or anything--just a 2" pipe. I would assume it doesn't matter that much, at least with the slow saws.

The hostsaw tuned pipes are as good as you get. No real backpressure, but they form a shockwave that acheives the same thing.

Yeah, what valves did you burn?

 

[Dragoon]

Going on the principle that 2-strokes are 2-strokes, I messed around racing snowmobiles and Dirtbikes for a few years. More backpressure moved the power band lower in the RPM range, and less raised it. This worked great if all you needed was wide open throttle, but unless you let it rev there, and held it, the engine would bog. I've had reed valves fail, or float, but never burn, unless there was a SERIOUS problem with the timing ( CR-125 that ran backwards). I can't think of any small engines that are 2-stroke, and use valves as per a 4-thumper. There were some 2 stroke Diesels that had valves, but they are long gone, and MUCH bigger than even the V8 Predator saw of fame.

 

[don]

Back around 1978

Back on the farm we messed around with some Poulans and took off the mufflers to get them to run better( so we thought) (It was back in the times when you would flip the top cover on the air intake on a chevy 350 truck to hear the dull roar of the 4 barrell kicking in.) Anyway we would drill holes in the mufflers or take them off all together on the saws to get more power. When a couple of them quit on us we took em to the local shop and said we burnt the valve. Nothing close to the level of sophistication you guys do today with a saw.

 

[don]

So why the long exhaust pipes

I see some of the pics of these hot rod saws and it looks like an exhaust pipe from a motorcycle. These pipes are around 3 feet long. Do they do this in order to remove the exhaust from the area or are they looking for the shockwave effect?

 

[Dennis]

Don..those are expansion chambers...they are a tuned race pipe...taking the unburnt fuels from the exhaust and returning them to the combustion chamber at exactly the right time...there was a file posted on the site somewhere of the expansion chamber at work....
Some very serious gains can be made when you put a tuned pipe on the saw...

there are still some 2 stroke deisels around that use valves..like the Jimmy engine....

 

[Dragoon]

Very true Dennis, I stand corrected on the demise of the valved 2-Stroke diesel. I seem to remember reading in Popular Mechanics (?) a while ago that the auto guys were fooling around with 2 stroke "ultra high efficiency" engines. Some were traditional port type 2-strokes, and some were more like 4-strokes, with cams & valves. All used EFI, and some type of boost & scavenge systems. ASounded reasonable enough: Little engines, little cars, big power to weight. I'm not sure any would meet EPA emissions though.

 

[Toddppm]

http://www.maximumsled.com/mountain/garage/2strokeprinciples/two-stroke_animation.gif
Hhhmmmmmm, wonder if I can use this as my screensaver?

 

[Dragoon]

Very Cool Toddppm! That about says everything.

 

[Toddppm]

I think Ken? posted that a while ago, made alot more sense after seeing it in motion

 

[Dragoon]

No arguement from me. Worked the same way with my Bikes & Sleds, but I never really figured out what was going on in those fat cans. Careful, you may be taking away some of the Magic & Mystery from the Pipe building Wizzards.

 

[eyolf]

Hello folks...the last time the tuned pipe thing was floating around, I chimed in about a book by Gordon Jennings, which had a great deal of theory and results of experimentation with racing motorcycles. I dug up my old copy, and figured, roughly, the angles and length for a tuner for a 70 cc chain, with a desired power peak at about 10,500 rpm (pulling hard in the cut, rather than no-load). Turns out you want about 25 3/8" from the piston to the theoretical focus point in the convergence cone, but that the convergence cone wants to be a lot steeper than the ones I'm seeing on your pics.

Question 1) Is Jennings out of date, or is it just that you guys have been limited to using what you can get at reasonable price that dictates the gradual end cone?

Question 2. Jennings talks about inverting the stinger (sticking it back into the expansion chamber) in order to shorten the system's exterior dimesions (to get inside an AMA rule change) and that he found insignificant change to power characteristics, but significantly reduced noise in certain frequency ranges...seemed to partly dampen or cancel the expansion chamber ringing. You guys ever fool around with this?

 

[Dennis]

Todd...thank you ...exactly the animation I was referring to...

Drag...yeah that Jimmy engine is still very common up here...

Eyolf...I cant say that I personally have fooled around alot with the different lengths of diffuser cones vs baffle cones..I know Ken has and hopefully he will pick up on this...but generally we use a longer diffuser...with a steeper baffle...the steeper the baffle...the less rpms, technically a govenor....the length of the pipe is a formula..depending on several factors....

i wouldnt say Jennings is wrong...but engines have improved. and changed.....but the theoretics are still right

I have heard but never tried inverting the stinger...but have played with some "stinger" lengths...which is really is a "pressure bleed"

 

[kdhotsaw]

expansion chamber dynamics

Here is how the basic process of a expansion chamber works; as the piston decends in the cylinder after combustion,the exhaust port is uncovered. this allows exhaust gasses still under considerable pressure to burst into the exhaust tract, forming a wave front that moves away from the port at high speed seeking less confinement. after traveling down the header pipe it reaches the diffuser cone. the positive wave expands in the diffuser cone and inverts itself and sends a negative wave back up toward the exhaust port. the initial wave continues moving the diffuser still returning a negative pressure wave of substantial amplitude and duration. at the same time oblivious to pressure waves but adding to their effect, the exhaust gases quickly move down the header pipe and expand into the diffuser. They have a inertia which create a vacuum back to the exhaust port. this vacuum is quite strong, producing an effect of -7 psi at its peak. together with something like +7 psi pressure produced by the pumping action of the crankcase to send a fresh intake charge up the transfers into the cylinder. the combined effect is about 1 atmosphere, and very strong. it helps to sweep the cylinder of exhaust residue and pulls the new intake charge up the transfer ports (continued on next post)
.

 

[kdhotsaw]

Expansion chamber # 2

part of the wave energy has been inverted in the diffuser and sent back up the pipe as a negative wave, but enough of the original positive wave's strength is left to rebound off the baffle cone and send the still positive wave back up the expansion chamber, the header pipe, to the exhaust port where the wave blocks the loss of fresh charge out the port. the positive reflectived wave is even strong enough to reverse the flow of any mixture which may have made its way out of the exhaust port, stuffing the mixture back into the cylinder under pressure as the piston now moves up on its compression stroke to close off the exhaust port opening. the overall result of all this activity on the part of the expansion chamber, first pulling then pushing at the fresh mixture to hold it in the cylinder, is a huge boost in power. in fact, the expansion chamber is the most important power-producing system on a two- stroke engine. power increases of up to 50-60%are common.
The stinger is a pressure bleed, it can come out the rear as a stinger, or can just be a hole in the center or anywhere inbetween.
ken, kdhotsaw

 

[TheTreeSpyder]

Very nice picture and explanation showing how a tuned cone can create a sound impulse that reverberates back to compress further the chamber gasses into the cylinder!

i think also that the system has to be warmed up a few strokes, so as to be warm. Because it all works better in the thinner air, with less resistance for the movement of the sound waves doing all this work for you. All this works so well that the one of the limiting factors to it is the size of the pipe/flare that you can put on for the application i think.

i really wouldn't take a muffler off any engine without at least putting a straight pipe on it, as i would expect to burn up the exhaust side. A muffler doesn't just carry and baffle sound; it also is necessary as a heat sink to drain heat off from the explosions in the cylinder.

 

[bwalker]

i think also that the system has to be warmed up a few strokes,

Temp is very important in a two stroke expansion chamber. As temp changes so does the frequancy of the pulse that KD mentioned. The stingers outlet diamater is the way the internal temp is regulated. A pipe with to small a stinger will make excellant power in short burst, but will detonate the saw to death if operated at open throttle high load for any length of time.

 

[don]

Bearing load with much more power

If you use one of those tuned pipes wouldn't you stress out the bearing too much on the crank? Or is the bearing designed to handle much more load than normal operating parameters or do you modify the bearing?