G'day

How important is the outlet size on a muffler and how do you tell whether you need to open up a muffler?

I've measured those that I have and the differences are quite marked. Also, the factory cast mufflers can appear to have reasonable outlets but, in the OS E-3030, the diameter at the very end of the outlet is a little over 5/16" but a 1/4" drill will not go through the hole where the outlet meets the muffler body.

The outlet area controls the 'final' amount of back pressure exerted at the exhaust port of the engine.

If the outlet area is smaller than the engines exhaust port (in a non tuned system) it will act as a throttle and be part of the rev limiting system stunt fliers can use.

In a tuned system the outlet can be much smaller than the exhaust port area but obviously it will act as an extraction system in its own rite.

The area of the outlet is complimented by the internal volume and structure of the entire exhaust system, and indeed its possible to have a simple looong length of pipe with the outlet being the same size as the engines original exhaust port and still have good back pressure and silencing.

Conversely, it is possible to have a complex internal structure and the outlet area = engine exhaust port area and also have  good pressure with silencing.

I would think that you would open up the outlet on a standard model engines exhaust if you are dissatisfied with the power output, but you will suffer the obvious trade off from decreasing pressure with a decrease in silencing capacity. This would only really be effective if any of the internal passages are more free flowing than your outlet -there is no point having the most restrictive passage before the final outlet.