Wednesday, 5 April 2017

A simple way of Calculating a Tuned Pipe Length

I managed to acquire a brand new in box OS61VR.

There is very little information on this engine on the OS archive, in fact I cant find any reference to it. I have a choice of a number of tuned pipes I can use  - one came with it, but it looked rather small compared with the other 60 sized pipes I have so it may be the wrong pipe. The supplied header also appears to have been used as a hammer - it was very badly creased so I ordered a new one from Just Engines.

They also had no information on this engine.

It  has a large inlet and exhaust port. This suggests that the engine will be rather powerful and probably very thirsty as well.

Now, forgive me if I am teaching you to suck eggs here, but I need to set the pipe length and this can be calculated using the following formula.

Pipe length (in inches) = Exhaust Timing (in degrees) * 1650 / Desired RPM.

I am hoping to get 12000 rpm on a 12x8 prop (in the absence of any other information).

The exhaust timing is calculated by placing the engine at TDC, turn the crank (in the direction of rotation) and look for the piston crown just appearing through the exhaust port (at the top). Note the angle on the timing disc. Continue to rotate through BDC until the piston crown disappears again on the compression stroke - i.e. you have timed the exhaust open window.

I made up a simple timing disk and attached a pointer using a jubilee clip as shown.


Using this technique, I measured this engine to have an exhaust open timing of 145 degrees.

Substituting the numbers above,

Pipe length = 145 * 1650 /12000 = 19.9375". The pipe length is measured from the centre of the piston crown to the baffle in the pipe where the diverging and converging cones meet.

There becomes a point where the exhaust timing becomes too long for a tuned pipe to become effective. A longer exhaust open time will give the user an easier handling engine but the benefits of the tuned pipe diminish - reducing to zero when the timing increases towards about 175 degrees.