Differences in bend and header length and muffler diameter size of pipes should make it a little clearer to understand cold blooded is from a exhaust pipe issue or Pilot
jet issue. When the exhaust port first opens, the sudden release of pressure sends the exhaust gasses out through the pipe. At the same time the exhaust pressure is released a sound wave also travels down through the pipe. When the expanding exhaust gasses pass through the tapered cone (B1
) the expanding diameter cone creates a vacuum which helps draw more gasses out of the cylinder and also helps to pull more of the air/fuel mixture into the cylinder. This process is so efficient that it can draw some of the air/fuel mixture out into the exhaust pipe. The sound wave traveling down the pipe bounces off cone (B2)
and returns to the exhaust port where it creates a "Wall" which holds the incoming air/fuel mixture in the cylinder until the piston closes off the exhaust port. The returning sonic wave only reaches the port at the correct time when the engine is operating at a certain RPM
. The RPM that the wave will return at the correct time is determined by the length of the pipe. Sound always travels at the same speed (about 1700 ft. per second) so the length of the pipe must correspond to the RPM of the engine, that's why the pipe only helps over a narrow range of RPM's.
Shorter pipes - moves the power band to a higher RPM. Longer - moves the power band to a lower RPM.
Short pipes and steep angle causes a narrow but hard hitting power band. Long pipes with gradual angle causes a wider but softer hitting power band. This section determines the RPM where the pipe will help pull the exhaust gasses out and draw the air/fuel mixture into the cylinder.
Short, steep angle causes a narrow but hard hitting power band. Long, gradual angle causes a wider but softer hitting power band. This section determines the RPM where the sonic wave will return to the exhaust port. The lengths and angles of B1
and B2 will usually be very similar since they must work together to cause the pipe to work at the same RPM.
This length will be longer if B1
are long and short if B1
are short. The total length of B1, B2
equal length F.
Long dimension F
will give a wide soft power band and short will give a hard hitting narrow power band.
determines the RPM where the pipe will be in the power band. Shorter will put the power band at a higher RPM and longer will put the power band at a lower RPM. At high RPM's the exhaust port is open a shorter period of time therefore the distance the sonic wave travels must be shorter in order to return at the correct time.
The diameter of sections A & D
(the silencer and short section of pipe in front of the silencer
) are proportional to the size of the motor. If the diameter of section D
is reduced or the length increased (more back pressure) top end power can increase, if the diameter is increased or the length reduced (less back pressure) low end power will increase. Increasing back pressure also increases cylinder temperature and can cause overheating. Just remember that rule if a little is good a lot is better, that will get you in trouble every time !!!!
Example: compare an 80cc and 125 cc
the pipes being shorter is not due to the size of the engine, it is due to the RPM where the engine is designed to operate. The smaller motors need to make maximum power so they sacrifice low end power in trade for high RPM power. 500 cc Moto crosser’s are tuned milder so they make a lot more low-end and are more controllable on top end. A 500cc tuned like a 125cc would make about twice as much power as a stock 500cc and be almost impossible to control unless you were road racing. A 125cc tuned like a 500cc would have a power band suitable for a trail or street bike.