A question:

With all this talk about exhaust (congrats Jeff & Kirby on the cheap fixes) got me to wondering why a three into one produces more power than a three into two.......
Why do the performance pipes for bikes go into one? It seems to me that would create more restriction. Any one have any clues?

Not many clues, but I have a few text books.

The reason the 3 - 1 produces more power than the standard system is to do with the way gas pressure waves behave.

In short, the length of the pipes before the bit where they come together, the diameter of the pipes and the angle of the collector will determine the power peak, the torque peak, and how much mid-range the motor will make.

Valve sizes and exhaust cam timing come into this, but these two things will complicate the explanation a lot, so I will leave them out.

Anyway, if the primary pipe (the bits before the collector where they come together) length is correct, it means the exhaust gas pulse from an open(ing) exhaust valve will arrive at the correct time to either assist the gas leaving the cylinder next to it, or to prevent more gas flowing out of the other cylinder.

This "extracting" effect is what helps the motor make more power. Some systems are called "headers" or "extractors".

Most 3 - 1 systems are made of single wall tubing, and most stock systems are double walled to prevent blueing of the chrome, and to cut down on noise. By using single wall tubing, the primary pipe is bigger in diameter, and this is where the less restrictive idea comes from. 3 - 1's are usually less restrictive for this reason.

The mufflers fitted to standard systems are usually reverse flow mufflers. That is, the exhaust gas has to pass into a chamber and then turn around 180 degress to get to the outlet from the chamber. Often there are two or more in a muffler. With the best design in the world, these are never going to be as free flowing as, say, a Kerker KR series pipe. They are straight through, and have very good flow characteristics.

So a 3 - 1 pipe, or a 4 - 1 pipe has less restriction in the header pipes and muffler, and if they are designed right, the primary pipe length is set so the exhaust pulses assist cylinder filling and exhaust gas removal.

As an aside, the current fashion is to use a 4 - 1 for maximum power, and a 4 - 2 -1 for more midrange while retaining good top end power.

If in doubt, look underneath any Honda CBR600 (Hurricane for the North Americans). They have a 4 - 2 - 1 system.

I currently use a textbook called "Performance Tuning in Theory and Practice: Four Strokes" by A. Graham Bell. It has a very good chapter on exhausts and how they work, and how to make them.

For more explanations, just ask.

Bernie.