Will I Need to Rejet When I Change my Pipes?

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July 30, 2010 | By: Scott Holton

As moderator of J&P’s online forums as well as technical adviser here at the company, I get the opportunity to exchange posts about a variety of topics and interact with hundreds of people each week. And in both venues, I find one of the most asked questions is, “If I change my pipes, will I need to rejet?” The simple answer is “Yes, you do need to rejet.

And now I’ll explain why. But first, a little motorcycle 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 engine – space forces 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 thermal pulse reacted at the same speed, but they don’t. When tuning, you usually have to optimize  one or the other.

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 883s, 1200s and 1340s need a 1 3/4-inch 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. And 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 an A/R cone or Tork valve) 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 include 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 airflow 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 an rpm at which this happens, 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 isn’t a math course, I won’t bore you with the formulas, but if you’re interested, leave a question or comment below and I’ll provide additional information.

Now that we’ve pretty much outlined the basics, it’s time for some conclusions. If every pipe was made the same way, we wouldn’t 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 for which they are tuned, and they hurt performance everywhere else. For stock displacement engines, bigger diameters are not better. The shorter the pipe, the more effective in the higher rpm range. Reversion cones help. The longer pipes are better for low-end power. Baffles or mufflers help. Different shaped tips help.

Come to think of it, a chopper I once saw when I was growing up had long, up-swept fishtail pipes. Considering what we just discussed, whoever built that thing had a pretty good idea of what worked. So what I’m talking about isn’t new by any stretch of the imagination. These are proven, physical principles practiced at every drag strip and racetrack in the country.

Now that I’ve climbed off my little soapbox, 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 they figure that’s what you, the customer, wants. 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 decide between maximum performance and looking and sounding cool.

So that concludes today’s post. As always, I look forward to your questions and comments.

Comments: 9 Comments | Categorized Under: Tech Tips

Comments (9)

Hey,
I am riding a 1990 FLSTC with an aggressive cam, s&s Super E or G, Kuriyakn hyper charger, and Samson Extreme Reeper Slashers, and an ignition kit, and crane coil. I just rebuilt this engine putting .020 bored over cylinders with Wiseco competition pistons. the exhaust is new. The bike is having a rough ride when ever I get to temperetures and ride from 35 mph down. It idles fine, and has good response at highway speeds but fights the throttle when trying to cruise at low speeds. I also hear a tapping from the front head while driving at the lower speeds. Could you help me figure out what I need to do? I am assuming a jet kit is needed, but is the tapping a normal symptom as well or is that possibly something else. Thanks

would like more info on tuned lengths.

Scott,

When I lived in Montana in the late 70′s I ran a 45″ WL with drag pipes (Unbaffled). The pipes were 20″ long, and I ran a 34mm Mikuni. We rode most of the year at below freezing temperatures, and I do not know how many times I had to change the exhaust valves and guides to stop warping the exhaust valves, until I started plugging the pipes when I would turn the engine off. Is it true that the cold air rushing up the pipes when I would turn it off caused the warping of the valves? I eventually had a friend manufacture me a header that I liked the looks of and stopped the warping of the exhaulst valves. Thanks. I now live in MD and am building an 80″ Flathead and would like to know how long Drag Pipes should be without hurting the motor and give the best performance.

Big Don

Scott,

Thanks for your articles. You guys do a great service to the motorcycling community.

Pat

Marlon, send your e-mail to me at sholton [at] jpcycles [dot] com and I’ll get you some information.

i would like more information on calulating the tuned rpm and tuned length of pipes.

If jetted correcty, yes, it will improve at the RPM you ride most often.

Hi, I,m running a 883 sportster engine in my bobber with short 2inch baffled drag pipes and I,m thinking of changing to a pair of paugho upswept drag pipes with slash cut goose neck ends. (I might be able to manufacture short baffles for these if needed.) Will the longer 1 3/4 pipes improve the overall running of the bike.
Cheers Tam B.

Good explanation of how exhaust pipes work, but the title of the article is will need to rejet when I change my pipes. I must have missed where that was in the article.

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