Philip,

I got this from the Mac's website:

Tuning the exhaust system is actually quite simple. Tuning is determining the length of the system at which the engine achieves maximum RPM. In case you have an engine for which there is no tuning suggestions available, we suggest you first break the engine in properly, trying to have at least 30 minutes of running time. Some engines need to have special treatment at the beginning, in which case you need to consult the manufacturer's suggested break-in instructions. Next, run the engine at full speed, open-face (with nothing attached to the exhaust, although this may not be possible with some engines whose carburators require muffler pressure, in which case we have actually hand-squeezed a sealed fuel tank). Next, put the full-length header and tuned pipe on the engine and run it. If the RPMs are less than the open-face reading, cut 1" off the header (1.5" for engines over 1 cubic inch) until the RPMs are above the open-face reading. Then cut the header .25" until maximum RPMs. Or, when the next cut causes a decrease in RPMs, this is the best tuning length for static and vertical performance (fun flying, aerobatics and hanging it on the prop).


Example of theoretical engine XYZ .61

Open-face 14,000
Header full length
at 6.25" 13,800
5.25" 14,300
5" 14,500
4.75" 14,700
4.5" 14,900-
4.25" 14,900
4" 14,800

Unless you were really hanging the airplane on the prop, the 4" length would be close to be giving you the maximum vertical performance, and would therefore be an excellent tuning length for your airplane.

The exhaust system consists of the header (which attaches to the engine exhaust), the silicone tubing (connector), and the pipe. The exhaust system not only reduces sound, but, when tuned to the correct length, will increase the engine's RPM.

Hope this helps explain it a little better.

Jon