Part 1.

Many have asked me in recent times about setting up an engine for a tuned pipe - perhaps its now time to share with you some of my knowledge. This works for me and will certainly do the same for you.

I include an approximate calculation of how long a pipe should be to develop maximum power for a particular exhaust timing at a particular propeller speed (RPM).   Alternatively this equation can be turned around to predict the optimum propeller speed for a fixed pipe length.

The formulas are:

pipe length in cm= 3750 exhaust timing in degrees/speed in rpm.


speed in rpm=3750 exhaust timing in degrees/pipe length in cm.

I must stress that the equations are a start point only and to properly set up a pipe you will need to get your hands dirty.

Selecting a  propeller pitch is a complex subject that would consume a whole column.  It is something that needs to be done for every model/engine combination to get maximum performance..... use either the engine manufacturers instructions or a propeller that you know will work properly.

An example of pitches is that a .46 engine will generally run very well on a 6-8 inch pitch whereas a large petrol engine could need a 20 inch pitch.  The cleaner the model is aerodynamically, the higher the top speed you can expect to get.  Alternatively, a low pitch will give you a lower speed but better acceleration.

A lower pitch is more appropriate for draggy biplanes and aerobatic models, giving better vertical performance and, for aerobatics, more chance of performing those all important constant speed manoeuvres.

When selecting a propeller type, different propeller shapes have different characteristics. Propellers of the same nominal size from different manufacturers can vary quite widely in performance. The performance pilot will probably choose an APC propeller- they are both efficient and quiet. With these propellers though please be aware that they are delicate and do not like too much rough and tumble when landing.  Little white stress marks do appear and that is the time to throw them out and get replacements.  Select a single pitch of a single type of propeller from one manufacturer and learn how to use it. If at the end of the setup you are not happy then start altering the pitch.  Playing around with different props and pitches does take a long time but if you record all your propeller experiments  you will very quickly build up a prop history chart and find that selecting your props  will become much easier to figure out.

With a fixed length pipe the only way to change the engine output is to change the propeller. Firstly, use the equation to calculate the speed you need to develop maximum power. Then try different propellers until you find one that turns the engine at about 85% of that speed. In a single session try the diameter either side. Fly all three  through the same manoeuvres and select the one thats suits best.

Engine and propeller performances change in different weather conditions. The optimum set-up on a hot, dry summers day is unlikely to be the best on a cold damp day in midwinter. Other factors can also be experimented with such as using different fuel. If your engine goes well on a particular plug  without the tuned pipe on, you may find that adding an extra head shim or switching to an extra head shim, or going to a cooler plug further improves performance.                                                                                                  

Big T.

Part 2.

Some have asked for more details relating to the use of tuned length pipes- specifically around setting up the pipe, and how to assess what to do next when things do not go quite right.

Overheating / and hot weather.

·         increase head clearance

·         decrease propeller load

·         improve cooling to both combustion and crankcase areas

 Cool running / and cold weather

 ·         increase compression

·         increase propeller load

·         partially block some of the cooling inlets

 High humidity

·         suggest you reduce the propeller load

·         increase stinger outlet size

 A guide to pipe length- ideally the shorter the pipe the better

 So a quick-guide to help your next move :

 Pipe too short

 ·         frequent burning out of glowplugs

·         sand blasted head- sign of pre-ignition

·         over heating engine

·         sounds harsh

·         excessive carbon build-up inside the head

 Pipe too long

 ·         very easy to needle

·         smooth and quiet running

·         could sound rich or be burbling

·         lacking in power

·         change your propeller- perhaps go bigger

·         generally a small prop requires a short pipe- so conversely a bigger prop will be suitable for low revs

 Most of your pipe problems will be able to be resolved by using the above as a guide, but never forget that the more you experiment then the more you will find the ultimate answer.

 Nothing is ever easy but if you take a logical approach to solving any problem then  solving pipe problems will become easier.

 Or if you are having problems then give me a call, regards Trevor Henderson.


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