Megaphone Theory 101

[Ram4x4]

Old thread, but...

The way I've always understood the difference between a megaphone and a chamber is that the megaphone is basically 1/2 of the system represented by a chamber.

An expansion chamber has two effects: 1) the suction (opposite polarity wave) created as the pulse expands into the chamber followed by 2) the returning pressure wave (same polarity) created as the pulse encounters the convergent cones. The suction wave helps pull more air/fuel into the cylinder (and actually out into the header), then the pressure wave stuffs it all back in. The entire system is dependent on timing as the pulse travel speed is relatively fixed (it actually depends on the system temperature as the pulses and waves travel faster when it gets hot, so always measure at operating temp). You tune the chamber to provide the waves at the desired time for the RPM range you want it to work at.

A megaphone is the first half of an expansion chamber, it's missing the convergent cones to create the second pressure wave. You get the suction wave to help pull in more fuel/air, but there is no returning pressure wave.

[SeaBat]

This is in relation to the megaphone pipe on the modified OMC 15. Does changing the length move the effective RPM power gain and what RPM range are you shooting for in a Mod 200 OMC? I realize the OMC is not a high tech engine and attempts to improve it's performance are limited by design and the rules.

[Ram4x4]

Changing the length and angle of the cone will affect the RPM range. As long as the pulse has a space to expand it will create the back suction, but it's not quite that simple. The design of the engine itself, as mentioned in previous posts matters as well. Port placement, shape and size (cross flow vs looper) also helps determine which type of pipe works best. A weak pulse will create weak suction. A large angle will create a quicker climb in the back pressure pulse (the outgoing pulse expands quicker in less distance traveled), but it will be less in time duration.

[SeaBat]

If I am modifying the existing OMC megaphone and shorten it, will this move the effective RPM range up or down? Also, does cutting the outlet on an angle broaden the effective RPM range. As I understand, the stock OMC is most effective around the 7000 RPM range whereas the mod should run in the 7500+ range.

[Fast Freddie]

Shorten the pipe and the RPM range moves higher; but possibly narrower. Some of the best 200cc Mods are running the OMC pipe at stock shape and length. Keep the power band and you are more competitive.

[Ram4x4]

Yes, the shorter the cone the shorter the wave duration, but you must also consider any header pipe length as well. A short, straight section of pipe leading into the cone delays the wave. The longer the header, the longer the delay, generally speaking. The length of header and cone are adjusted for RPM range you want the magic to happen.

The greater the divergent angle of the cone, the stronger the suction wave, but keep in mind there is only X amount of energy in the exhaust pulse itself, so too wide of divergent angle will use it all up quickly which severely narrows the effective power band as well.

Think of this like sipping a drink from a straw. You can take a long, slow sip, or you can take a quick, hard sip, or somewhere in between. Which you choose completely depends on what you need the engine to do. Do you need something with a wider, more linear power band, or do you want something "peaky" that slams into power mode right at the end of it's RPM range? Even knowing that, it's impossible to really tell you to do this or that as each engine is different. Even engineers have a really hard time with exhaust systems because there are just too many factors and none of them remain constant.

The basic theory on exhaust is sound, the numbers work, the flow works, the results are real, it's just **** hard to get it right the first time with a calculator only. In most cases it takes experimentation. Basic theory can only get you near the ball park.

[SeaBat]

Thanks everybody I was worried that I was missing something I should be addressing in my mod. You've answered my questions very well and I am grateful for your input.
Bill