Will I Need To Rejet?
By Scott H. – J&P Cycles® Tech

 As moderator on the J&P Cycles® website, www.jpcycles.com, I get a chance to exchange posts about a variety of topics. As a technical advisor at J&P, I talk with hundreds of people each week. I am surprised how often I answer the same questions in both venues. The most asked question is "If I change my pipes, will I need to rejet?" In the last issue, I touched on some performance basics and I was going to expand on that, but I feel that due to the sheer volume of times the rejet question is asked, it should be answered. Yes, we do need to rejet, and I will try to explain why.

First, a little exhaust education. Exhaust technology is the reason two-stroke engines work. Volumes have been written about the principles governing expelling gases out of the beloved internal combustion engines – space will force me to be brief. There are three things we can tune for an exhaust: the sonic pulse; the thermal pulse; and gas velocity. The length of the pipe controls the first two, but gas velocity is controlled by the pipe diameter.

A SONIC PULSE is created when the exhaust valve slams shut and a strong wave is made that starts down the pipe. When it reaches the end, it turns around and heads back to the combustion chamber. The hot gas leaving the combustion chamber creates the THERMAL PULSE. It, too, travels to the end of the pipe and sends the wave back up to the combustion chamber. It would nice if the sonic pulse and the thermal pulse reacted at the same speed, but they don't. When tuning, you usually have to choose one or the other to optimize.

Ideal gas velocity is 300 feet per minute and a variety of factors influence this important goal. Remember, too fast is as bad as too slow. Without going into the math involved, the 883's, 1200's and 1340's need a 13/4" pipe to stay near this gas speed. A larger pipe on the same size engine actually slows gas velocity, and when it slows down, we are effectively putting a cork in the engine, and that hurts performance. Larger displacement engines require larger diameter pipes to maintain that 300 fpm.

The sonic pulse is similar to a piano. You strike a key which, in turn, strikes a string to create a sound, the note you want. The longer the string, the deeper the note. Pipes react the same way. The valve slams shut, sending the sonic pulse on its merry way. The shorter the pipe, the quicker the wave returns and the higher the sound. The longer the pipe, the slower the wave returns, and when it arrives, it is weaker.

This is important because if the wave gets back to the valve while it's still open, the wave will go back into the combustion chamber. Since the sonic pulse carries some residual exhaust gas back with it, a dilution of fresh air/fuel mix occurs, robbing you of maximum power.

Another thing that can happen during this valve overlap is that the wave will travel through the open intake valve and down the intake tract carrying some fuel out of the carb (creating a condition known as fuel standoff). This accounts for the poor low speed performance of short drag pipes. When we add a device known as a reversion cone (also called A/R cones or Tork valves) we are fooling the pipe into believing it is longer. This critter is a cone shaped doodad that is put near the head to catch the sonic wave and decay it. When the decayed wave returns to the valve, it will have less amplitude just as if the pipe were longer. Other ways to effect the sonic pulse includes the shape of the tip of the pipe, baffles, stepping the diameter of the pipe, expansion chambers and cyclonic tips.

This effects jetting because every pipe has different dynamic characteristics that change the air flow through the engine. In some cases, the waves blow the airflow down or dilute the mixture. In other cases, the wave arriving at the right time helps pull the spent gases from the combustion chamber. For every different length of pipe, there is a RPM that this happens at and that is known as the TUNED RPM. For a specific RPM, there is an effective length that is known as the TUNED LENGTH. Since this is not a math course, I will not bore you with the formulas, but if you are interested, call and I will let you know.

Now that we have laid out the basics, it's time for some conclusions. If every pipe was made the same way, we would not need to change jets. But each is different and the airflow needs are changed, depending on the tuning characteristics.

Drag pipes are effective at the RPM they are tuned for and hurt performance everywhere else. For stock displacement engines, bigger diameters are not better. The shorter the pipe, the higher in the RPM range it is effective. Reversion cones help. The longer pipes are better for low-end power. Baffles or mufflers help. Different shaped tips help.

Gee, when I was growing up, the chopper I saw had long, up-swept fishtail pipes. Going back through what we just discussed, those guys had a pretty good idea of what worked. What I'm explaining is not new by any stretch of the imagination. These are proven, physical principles practiced at every drag strip and racetrack around the country.

Now that I have gotten on my little soapbox for a bit, let's examine the state of the aftermarket exhaust industry. The folks making pipes today are smart. They know what I have outlined here. They make those big diameter drag pipes because that is what you, the customer, seems to want. And the Harley® riding public buys them. The bigger the diameter, the lower and louder the tone of the pipe. The longer the pipe, the deeper the tone without necessarily being loud. When you pick your pipe, you have to make the choice between max performance and looking and sounding cool.