It's Kadenacy's fault.

Megaphone, Reverse cone, Siamese, Balance pipe, Expansion bulge and even the length of the inlet track are the results of design attempts to use the âKadenacy Effect.â in exhaust systems.

Designers try to improve engine efficiency by utilising the effects of âpressure waves in gasesâ by means of a TUNED exhaust system.

The exhaust systems and inlet tracks are of fixed length, in our types of engine anyway, but the frequency of the pressure wave varies with engine speed, therefore any increase in performance is usually to mid-range torque as this effect only works well over a limited rev range.

As the complete exhaust, thatâs pipe size & silencer type, are critical in a tuned system, replacements must be to the same specification as original if the improvement is to be maintained.

How does it work, well, there is a lot more to it than this but one effect is in simple terms.

When the travelling positive pressure pulse of gas created by the opening of the exhaust valve suddenly expands, usually when reaching the silencer, it creates a negative pressure pulse (vacuum) that travelâs back up the exhaust pipe.

The designers trick is to get this negative pulse to arrive at an exhaust valve, which on the induction stroke is open due to valve overlap, (both inlet & exhaust vales open at the same time) at the right time. This will have the effect of drawing in a greater than otherwise charge of petrol/air mix into the cylinder, more charge more power. Itâs the timing of the pulse that limits the rev range over which this idea works.

Ron C.

Better/fuller explanation welcome.