Tuned / Megaphone pipe Tech Question

[Seagull 170]

Does anyone out there remember far enough back, to recall if anyone ever cut the ends of their open ended Megaphones at an angle as opposed to square, & would it extend the powerband or just weaken the pulse?

[SS 201]

To tune pipes I remember putting a fitting on each pipe taking the highest vacumn reading .Then cutting the pipe from the center of the exhaust valve to the end of the pipe.
Thats about 50 plus years ago.

[Seagull 170]

It seems to appear that most of the rules about megaphone design has long been forgotten, perhaps understandable as there is so much confusion about how they actually work.

So I'm going to have to reverse engineer the principals.

Is there anyone who's restoring a good performing megaphone motor who can supply me the following information.

Fuel type
Looper or cross scavanged
Racing use rev range
Best performance revs
Induction type, piston ported, rotary or reeds
Transfer port duration in degrees
Exhaust port duration in degrees
Center line length of pipe from piston to end
Diameter of pipe from start to finish of cone & length between both measurments
Any water injection

Any info would be real handy as it's crazey reinventing something thats been done before.

[Mark75H]

Harry Brinkman made pretty good diagrams in his How to Hot Rod C Mercs booklet and Gordon Jennings did likewise in the Two Stroke Tuners Handbook.

You did not ask the basic megaphone physics question before so I did not answer.

When a wave reaches the end of a pipe and exits, an echo of the reverse sine reflects back into the pipe. If the pipe has the correct length the echo will reach the motor end when the piston is near BDC, echo again and cause a suction across the piston via another new wave going out again ... echoing back when it reaches the outlet of the pipe again and traveling back in. This time we hope to have the wave hit just as the piston is closing the exhaust port and pushing a little of the stuff it pulled across the piston (perhaps a little too much stuff) back into the cylinder.

I'll see if I can come up with the diagrams and length calculation stuff. Its based on the same sound wave in gas at temperature stuff as expansion chambers.

The angle of the pipe affects how strongly the signal returns to the cylinder.

As a guess I would think that an angled outlet is not useful; probably more related to the pipe maker's flawed calculation of the pattern before rolling.