HIGH PERFORMANCE ENGINES AND TUNED PIPES- AS KNOWN BY BIG T.


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 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

OR

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 setup a pipe you will need to get your hands dirty.

Selecting a  propeller pitch is a complex subject that would consume a whole column. its 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 ruin very well on a 6-8 inch pitch whereas a large petrol engine could need a 20 inch pitch. he cleaner the model is aerodynamically, the higher the top speed you can 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 the time to throw them 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 does take time but if you record all your propeller experiments then you will get very quickly a 'prop by history' and you'll find then everything should work well.

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.

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