Making Drag Pipes Work

If you insist on using drag pipes on your bike, there is something you can do to improve the low and mid range power produced by the engine. Even with the improvement listed here, the streetable engine power is not going to match power output of a good 2-1 or 2-2 exhaust system. Motorcycle Performance Guide does not recommend drag pipes or porker 2" pipes for serious street engines, but the performance fix listed here will improve the power of your drag pipes. Results have been confirmed by dyno results.

If it is loud, it must be fast?

The first item to get modified or changed on most new Harley-Davidson motorcycles is the exhaust system. Getting the proper Harley sound always seems to require increasing the decibel level out the exhaust, with many riders installing drag pipes as the exhaust system for the proper sound. The rider often believes that by reducing back pressure in the exhaust system the engine will also increased power. This is wrong. As a resulting of changing a stock exhaust system to Drag Pipes, most engines promptly lose 5-10% of the  power the engine produced.

By properly re-jetting the carburetor and adding a free flowing air cleaner to an engine with drag pipes, the maximum horsepower produced will improve over the stock engine. But there is a difference between usable power and maximum horsepower. The maximum horsepower of two engines may be similar, but the horsepower torque curves may be different. The area under the horsepower and torque curves defines the "power" the engine produces. The more area that is under the curve, the better the power.

A typical drag pipe produces a horsepower curve that initially rises very slow. As the RPMs start to rise above mid-range power, the curve begins to rise at increasing rate until maximum horsepower is achieved. Once RPMs have passed maximum horsepower, the curve drops of rapidly.

The horsepower curve of a typical 2-2 pipe like the Cycle Shack Slash Cuts produces a curve that may actually be closer to a straight line from low RPMs up through the rpm that maximum horsepower is produced. Once maximum horsepower is achieved, the curve drops at a relatively mild rate.

The horsepower curve of a typical 2-1 pipe like the SuperTrapp starts off slightly lower than the 2-2 pipe, but rises at a rapid rate in the mid rpm ranges. As the rpm range approaches maximum horsepower, the curve flattens out. Once maximum horsepower is achieved, the curve drops of rapidly.

Ok. It is loud and it isn't fast!

If your taste in bike styling requires that drag pipe must be used, there is some hope to getting back some of that lost low to mid rpm power. Here is a poor boys trick that will make your drag pipes work much more effectively. This setup can actually be tuned to meet the performance needs of the bike.

Make a tunable baffle by purchasing a 1/4"x1" thumb screw or taking a 3/4" outside diameter washer and weld it to the top of a 3/4" x 1/4" bolt. Now Drill a 1/4" hole about 1" from the end of the drag pipes. Take the tunable baffle and place a 1/4" nut and a lock washer (away from the large washer) the on it. Now insert your tunable baffle into the exhaust pipe like the example below.

Visitors to the Bike Tech WEB site constantly provide improvements to this idea. If you don't have access to welding equipment, try this handy little trick sent to me by Nick What I used was 1/4" x 1" thumb screws which can readily purchased at any local hardware store for about $ 0.50 a piece. They work great. The "football" shaped head on the screws measure approximately 3/4" x 1/2" so they have about the same surface area as the 3/4" o.d. washer because of the lack of the hole. And they do not require any fabrication. It is amazing what a little creative thinking can come up with.

You can tune the baffle by changing the angle of the thumbscrew or washer to the exhaust pipe. For maximum torque, the washer will be at 90 degree angle to the pipe. For maximum horsepower the washer will be parallel to the pipe.

How well does this work?

Bike Tech received a dyno run sheet from Gene P. that just showed what happened by changing the angle of the thumbscrew. Three runs were done. Thumbscrew parallel to the pipe (run 7), 45 degrees to pipe (run 6) and 90 degrees (run 5) to the pipe. Judge the results for yourself. This also shows you the benefits of dyno tuning something as simple as your exhaust system.

Run 7 (open or parallel to the pipe) Makes the most power, but has a huge hole in the power band just above 3000 RPMs

While run 5 makes the lowest horsepower (approximately 68), the 2500-4000 RPM power is the highest. This is the RPM band that most riders spend there time riding.

Run 6 improves the 3000-4000 RPM horsepower drop over run 7 without any significant drop in maximum power. 

Where did this come from?

This isn't some new technology that was just discovered. Any rider who remembers the Honda 250 and 305 Scramblers of the 70's should be quite familiar with the technology. In addition to tuning the power band, it also adjusted the amount of noise that came out the exhaust pipes. It was a well know trick when it was done by Honda.

For those of you who are willing experiment more, the size of the washer can be increased as can the size of the hole in the washer. A strong spring can be used to hold the washer in place in relationship to the pipe and a "wing nut" arrangement can be used to adjust the angle of the washer.