Pipe design is complicated by the many variables. The art of pipe design is to combine the lengths, angles and diameters of the pipe to suit the characteristics of the engine, especially the exhaust timing and end use.

Some example are;

Small volume - high heat - narrow operating range (peaky)

High volume - lower power (if too big) - broad range

Steep angles - 'peaky' - high power

Shallow angles - broad range - moderate power

Shorter or larger diameter stinger - lower or higher pressures and heat.

There are a multitude of ways of quoting pipe length. The most accurate method (and most rarely used) is to quote the distance from the piston face to the Mean Reflection Point (MRP). The MRP is the average of the reflected wave forces. (see diagram). Many people quote a length of plug to the high point, which is easy when pipes have a clearly defined high point ... most modern pipes have a flat or mildly curved centre section which makes for a smoother, broader power band..

The 'pipe length' is only a guide to optimum performance. Only in pure speed events, ie, FAI and C/L Speed are pipes and engines used near optimum. In the case of R/C Pylon the engines must be 'on pipe' on the ground and in the air, ie.. 25,000 ground rpm, 29,000 air rpm on a pipe designed for 27,500 rpm.

The basic equation for pipe length is:- L = E x V rpm

where L = Length of pipe (to MRP), E = Exhaust timing and V = Exhaust wavespeed = 1675 feet / sec

In this equation the exhaust wave velocity can be regarded as a constant. From the equation we can tell that as engine speed increases the pipe length will decrease. High RPM, high exhaust timing and high power often go hand in hand as increasing the exhaust timing will enable the engine to rev faster, producing more power.

However, as can be seen from our formula, if we increase the exhaust timing, we need to lengthen our pipe, yet the usual result of increasing the exhaust timing is higher RPM, and the formula states that higher RPM requires a shorter pipe. If we consider the size of the numbers involved, it usual that a shorter pipe (rather than a longer pipe) will be required. Also see the pipe length chart.

Several other handy pipe design criteria, (all the equations on this page are in inches).

Max pipe dia = 2.6 to 2.8 x header diameter (ID)

Mini Pipe length = (755 x exhaust timing in degrees) divided by rpm.

Mini Pipe diameter = the square root of ((4.52 x capacity in cu. in. x 4), divided by (pi x length)).

The above are just a few of the basic equations, there are many rules of thumb for all sorts of diameters, lengths and angles, which are far to involved for a book of this type. For modellers wanting to learn more about pipes, pipe design and general 2 stroke principles, there are several very good books available, written primarily for 2 stroke motor bikes.

Shown below is the Bolly P42 pipe, designed for FAI pylon racing. An excellent performer.