Numerous times I have seen people ask for the magic length for a particular tuned pipe system. The truth is that the "ideal" length is dependant of many factors. Things such as engine exhaust timing, type of boat, intended application for the said boat, propeller used, type of drive system, weather conditions, etc. So many things can affect something else that tuning must be done for the combination of hull, hardware, engine, pipe and prop as a whole. There is no way around this if you are looking for optimum performance from your boat. You can tune the pipe to the engine for a specific RPM range, but the ideal RPM range can change depending on the factors mentioned above.

In this article we will assume that everything other than the pipe's tuned length itself has been decided and can not / will not be changed, just to explain the basics of tuning the pipe. We will also assume that the tuned pipe system being used has been well designed for the engine. In real life, you must remember that you will have to experiment with different propellers, drive settings, etc to get the best possible performance from your boat...and every time you do so, you might have to re-set your pipe to get optimum performance for the combination you have chosen. Tuning a pipe for best performance at a said RPM range on a dyno and setting a pipe for best on-water performance are two different things. We want to set it for best on-water performance.

So let's start with what the tuned pipe does and why we need to "tune" it.

For more detailed theory on how a tuned pipe works scroll down in this article and read "How Two-Stroke Expansion Chambers Work".

Here is the simplified explanation, the way I see it.

Some Adjustable Tuned Pipes

Here are my recommendations for setting a tuned pipe system for the commonly used Zenoah G260PUM engines:
* please note these are only recommendations. They are for DRY TUNED PIPE SYSTEMS ONLY. You can start with longer or shorter header settings if you wish.

If the manufacturer of the pipe can give you a recommended starting length follow his recommendations.

Measuring Length

After you have measured and set the pipe for the starting adjustment of 13", mark the header and cut it so no more than about 1/2" extends inside the pipe past the second Oring. If you are using a steel pipe then cut it so you have the least amount possible extending into the chamber. See picture further in this article.

With the pipe setup at starting length, run the boat and note performance. Ideally you need a Tachometer installed in your boat for testing and a GPS isn't a bad thing to have also if you want to know actual speed. With experience you can do most of the initial testing by ear and sight but in the end the Tach and GPS are indispensable in my opinion.

When using aluminum pipes with water-cooled couplers, you should find a means of preventing the pipe from moving on the header. Pictured below is a pipe clamp that does just that.

After a good run, bring the boat in and note the speed and RPM readings. Now push the pipe in on the header, making it about 1/8" shorter. Run the boat again. Again, note speed and RPM readings. Also note how the boat performs on water. In the end it's not necessarily just about top RPM and top speed...it's also about the kind of throttle response and acceleration you want.

Keep shortening the pipe combo 1/8" at a time. In my opinion to do this correctly you should try and keep the part of the header extending into the pipe as short as possible. So in reality you should be cutting off the header every time you shorten the pipe. With water-cooled couplers such as found on aluminum pipes you may want to have it extend a little more than normal just for testing, but still keep it as short as you can. Keep it about 1/2" - 5/8" extending past the second Oring during testing.

Continue shortening the pipe and running the boat, every time taking notes on speed, RPM and performance. Do this until performance starts degrading. At this point you will back the pipe out to the previous length, to where it performed max before starting to go downhill. You can now trim the excess header inside the pipe if necessary. I usually keep it to 1/2" maximum past the second Oring. Headers are relatively inexpensive, so you can keep spares in case you have to run it longer on a different setup. If you have too much extending inside the pipe it will affect performance.

In the picture below you can see approximately where to trim the header. On a steel pipe you should try to trim it so none of the header is actually extending inside the diverging cone. On an aluminum pipe with water-cooled coupler, you must note where the Orings are and make sure the header sufficiently clears the second Oring for proper sealing.

Needs Trimming

You have now set your tuned pipe system for optimum performance with your boat setup! Easy isn't it?

In theory this should never change after you have set it, but in real life conditions you might find the need to lengthen it for a bit more torque or shorten it for more RPM. One is at the cost of the other unfortunately!

For example if you change to a propeller that puts more load on your engine, you might have to lengthen the pipe a little bit to get the required added torque. The pipe in this case will tune to a lower RPM range. If you change to a propeller that puts less load on your engine, you might want to shorten the pipe a little to gain more RPM since it is now requiring less torque. The pipe in this case will tune to a higher RPM range. This is what I was talking about earlier when I said many factor affect the "optimal" tuned pipe length for your setup.


Happy Tuning!

Read on for more technical information on tuned pipes!

How Two-Stroke Expansion Chambers Work

*This information is from Dave Marles of Prestwich Model Boats .

You know that changing the exhaust pipe and pipe length on your boat can have a marked effect on the engine's power characteristics, but do you by how much and why ?

How Much.  A two stroke 125cc engine with standard exhaust system can combust no more than 125cc of fuel air mix. A two stroke 125cc engine with good tuned exhaust system can combust approx 180cc of fuel air mix. 

Why.  Simply put, it's because the two-stroke exhaust system, commonly referred to as an 'expansion chamber' uses pressure waves emanating from the combustion chamber to effectively supercharge your engine.
In reality, expansion chambers are built to harness sound waves (created in the combustion process) to first suck the cylinder clean of spent gases--and in the process, drawing fresh air/gas mixture (known as 'charge') into the chamber itself--and then stuff all the charge back into the cylinder, filling it to greater pressures than could be achieved by simply venting the exhaust port into the open atmosphere. This phenomenon was first discovered in the 1950s by Walter Kaaden, who was working at the East German company MZ. Kaaden understood that there was power in the sound waves coming from the exhaust system, and opened up a whole new field in two-stroke theory and tuning.
An engine's exhaust port can be thought of as a sound generator. Each time the piston uncovers the exhaust port, the pulse of exhaust gases rushing out the port creates a positive pressure wave which radiates from the exhaust port. The sound will be the same frequency as the engine is turning, that is, an engine turning at 24,000 rpms generates an exhaust sound at 24,000 rpms or 399 cycles a second--hence, an expansion chamber's total length is decided by the rpm the engine will reach, not displacement.
Of course those waves don't radiate in all directions since there's a pipe attached to the port. Early two strokes had straight pipes, a simple length of tube attached to the exhaust port. This created a single "negative" wave that helped suck spent exhaust gases out of the cylinder. And since sound waves that start at the end of the pipe travel to the other end at the speed of sound, there was only a small rpm range where the negative wave's return would reach the exhaust port at a useful time: At too low of an rpm, the wave would return too soon, bouncing back out the port. And at too high of an rpm, the piston would have traveled up the cylinder far enough to close the exhaust port, again doing no good.
Indeed, the only advantage to this crude pipe system was that it was easy to tune: You simply started with a long pipe and started cutting it off until the motor ran best at the engine speed you wanted.
So after analyzing this cut-off straight-pipe exhaust system, tuners realized that pressure waves could be created to help pull spent gases out of the cylinder. Following this, The tuners realised that  these pressure waves could be utilised still further by using a divergent cone to increase the strength of the negative wave and then that a convergent cone added to this would increase power still further as explained next....

The exhaust opens on the down stroke and a pressure wave emanates from the exhaust port into the header pipe. This pressure wave travels through the exhaust gases that are in the pipe at the speed of sound... It’s the pressure wave that travels at this speed, not the exhaust gases themselves. (Imagine a stream and you throw in a rock. The waves from that rock will travel down the stream faster than the speed of the water.) Anyway, the wave reaches the front divergent cone and a weak negative wave (negative pressure or ‘suck‘) (laws of physics) is sent back to the exhaust port which reaches the exhaust port while the transfers are open helping to remove exhaust gases from the cylinder which in turn helps fresh mixture from the crankcase up through the transfers into the cylinder. (Some of which will enter the front part of the header)
   The length of the front cone and its distance from the cylinder (header length) determines the amount of time that the pressure reducing wave from the exhaust does it work in emptying the cylinder of exhaust gas and then assisting the fresh mixture up from the crankcase into the cylinder. If header is too short then the wave energy from the front cone is wasted because the negative wave (the ‘suck’) arrives at the exhaust port while the cylinder pressure is still high after combustion. It should arrive there when the pressure in the cylinder is low but there are still exhaust gases that need to be extracted. If the header length is too long then the wave is arriving later than optimum and the exhaust gases are not fully removed from the cylinder. The front cone needs to be long enough to generate a wave to help the fresh mixture into the cylinder but it also needs to continue working long enough to allow some fresh mixture into the first part of the header. This is the mixture which will be forced back into the cylinder. If it is too short, then it does not allow mixture into the header. If it’s too long, then it reduces the length of the rear cone and that needs to be long enough to force all of the un-burnt mixture in the header to be forced back into the cylinder. The pressure wave continues into the rear cone and immediately sends a positive pressure wave (laws of physics!) back down the tuned pipe towards the exhaust port forcing the un-burnt fresh mixture back into the cylinder. The strength of the wave increases as the rear cone gets smaller and the length is made so that the returning pressure wave from its very end at the junction with the stinger coincides with the point of exhaust port closure. When this most critical length (start of stinger to exhaust port) is correct, then maximum power is achieved. If this critical length is too short then the returning wave forces hot gases back into the cylinder, dramatically increasing cylinder combustion temperatures. If this length is too long the maximum power will not be achieved because maximum supercharging or cylinder filling will not occur, although power in the corners will be better because the tuned length will coincide more with the reduced rpm in the corners.