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Two Stroke Tuned Exhaust Systems

Article by Noel Davern 

We all know that attaching a properly ‘tuned pipe’ to our 2 stroke motor will give it an enormous power increase. But how does it do it? Read on. 

The tuned exhaust system, although it has no moving parts, assists in the scavenging (removing exhaust gases from the combustion chamber) and boosting or ‘supercharging’ (forcing extra fuel/air mixture into the combustion chamber) of the motor to produce this remarkable increase in power. It does this by utilising the properties of sonic waves. To understand how it works requires us to get into a little bit of theory. Follow me! 

First off, ‘sound’ is a series of pressure waves or pulses that spread out from a source (such as a vibrating guitar string or the opening exhaust port of a 2 stroke racing motor). The speed with which they travel is determined by various properties such as the density, temperature and pressure of the medium they are travelling through. We call this ‘the speed of sound’. In the case of hot exhaust gases it is about 1675 ft./sec (or whatever that is in metres/sec). 

Now, a sonic wave travelling through a medium enclosed in a pipe exhibits the strange property that part of it is reflected back along the pipe when it reaches its end. This happens regardless of wether this end is open or closed (the rest of the waves energy is lost to the surrounding atmosphere). Not only that, the ‘sign’ of the reflected wave is inverted when the pipe end is open (a positive wave is reflected as a negative one) but remains the same when the pipe end is closed (a positive wave is reflected as a positive one). Funny hey? (funny strange that is, not funny ha ha). This property of acoustics is very important to the function of the tuned exhaust pipe so if you are still a little confused, re-read this paragraph! 

Still with me? Good, lets keep going. 

Let’s look at the old ‘straight pipe’ exhaust system armed with our new knowledge. The moment that the decending piston opens the exhaust port, a positive pressure wave zaps down the pipe at the ‘speed of sound’ (don’t confuse this pressure wave with the flow of the actual exhaust gases, they’re not going quite so fast). When it reaches the end of the pipe, which is open, a negative pressure wave is reflected back along the pipe towards the exhaust port, again travelling at the speed of sound. If the exhaust port is still open when it arrives (it will be if we’re lucky, we’ll come to this later - for now let’s assume that it is), this negative pressure wave will assist in drawing exhaust gases out of the combustion chamber and in pulling the fresh charge from the crankcase up through the transfer ports. It may (if we’re really lucky) actually draw some of this new charge right through into the exhaust pipe. While all this is going on, the wave has again been reflected, however this time it encounters the combustion chamber as a ‘closed end’ and keeps the same sign (negative). It scoots back out the pipe and when it reaches its open end is reflected once more with opposite sign (positive). When this wave reaches the exhaust port, if it is still open, it crams the part of the fresh charge sucked into the exhaust pipe (we were really lucky) back into the combustion chamber just before the now rising piston closes the exhaust port. Hey presto! With a little help from acoustics and a lot of luck we have an efficiently scavenged and ‘super charged’ motor.

 OK, now let’s look at the ‘luck’ part. Like most things in life, it’s not luck, it’s good management that’s needed. All we have to do is set the length of the pipe so that these reflected waves arrive at the correct time. Easy. We decide what rpm we want our motor to run at and we know the speed of these waves (more or less). We adjust the exhaust port timing and the length of the pipe so that all the scavenging and boosting is occurring at the correct time and we’re there. Alternatively we can start with a long length of pipe and keep lopping bits off the end until she’s humming like a bee - or screaming like a banshee!

 Had enough of the theory already? Does it actually work? Yea, up to a point. The problem is that each time the sonic wave reaches the open end of the pipe, a hell of a lot of its energy is lost. The reflected wave is a lot weaker. By the time the poor old boost wave reaches the motor, it can’t blow out a candle.

 How do we get around this? Well, why not add a ‘megaphone’ or diverging cone to the end of the pipe. While we’re at it, lets call the straight bit of pipe a ‘header’ (and I’m not being rude here). The sonic wave on reaching the end of the header (and the start of the diverging cone) still encounters it as an open ended pipe, however the reflected wave is a lot stronger. Neat hey! I’m not sure why this happens, I think it’s done by magic or mirrors or something. Anyway, we now have a lot more efficient scavenging taking place. Another problem is that we are stuck with the same length of pipe to time the scavenging and the boost pulses. If the length is set so that optimal scavenging is taking place, the boost pulse may not necessarily also arrive at the optimal time. Well, now that we have started with cones and headers (but please, let’s steer clear of cone-heads), why not add another one? If we add a convergent cone to the end of the diverging one (to let the exhaust gases out we will need to add a bleed hole, called a stinger), the sonic wave will see it as a closed end of the pipe! Being a closed end the wave will be reflected with the same (positive) sign as the original wave. Note that even though some of the wave is reflected at the end of the header and does the scavenging, the remainder just keeps right on belting along the pipe to be reflected and do the boosting. If we join the two cones with a straight length of pipe, we can time the arrival of the scavenging (negative) and boost (positive) pulses independently of each other. We call this system an expansion chamber or full tuned pipe, as used on our beloved F3Ds.

 Now remember, these waves always travel at the speed of sound. For a given pipe set-up, there is a relatively narrow rpm range (time when the exhaust port is open or closed) when the reflected waves are arriving at the ‘right time’. This is called the ‘Power Band’ of the motor-pipe set-up. When the motor is operating within this rpm range we say it is ‘on the pipe’ (and we are usually smiling widely). Outside this rpm range the pipe is doing little for the motors performance and may in fact be hindering it.

 This is a much simplified explaination, to design a pipe set-up to deliver maximum power/useable power band requires a fair bit more delving into theory, calculation, and experimentation. I think we’ve all had enough of that for now. Hopefully, you have found this discussion useful in making the principles a little clearer. Remember: there is power in that there sound!

 By the way, most of the pioneering work in 2 stroke tuned exhaust systems was carried out in the 1950s by one Walter Kaaden, of the then East German motor cycle company MZ. Thanks mate!