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Pro Tuning on a Shade Tree Budget


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?

If you are serious about making horsepower on the street with your Harley-Davidson, drag pipes will not fit into your engine program plans. Serious street power requires a serious exhaust system. Exhaust systems like the SuperTrapp 2-1, Hooker, White Bros. E-series 2-1, Vance&Hines, Khrome Werks and Cycle Shack 2-2 Slash Cut exhaust have a long track record of "making power". The familiar sound of a drag pipe may be music to a "bikers" ears, but the performance rider hears the labored acceleration as the motorcycle moves by.

Drag pipes do have their place. It is on the drag strip where the engine runs in a very narrow RPM band. On the street, stick with proven winners.

Dyno testing of exhaust systems for EVO and Twin Cam engines is now available.

EVO Exhaust
Twin Cam Exhaust

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.

wpeA1.jpg (1629 bytes) align= 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.

wpeA0.jpg (6708 bytes)

wpeA2.jpg (6368 bytes)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.

I want to thank Terry for remembering this trick and bringing it to my attention. He was looking for some additional power from this bike. The cost of a new set of pipes was not in his budget at the time so the following suggestion was made by the Motorcycle Performance Guide staff

"If you want to use the 'poor boy' trick to make your drag pipes work better, just drill a 1/4" hole about 1" from the end of your drag pipes then put a 1 1/4" by 1/4" bolt through it with the shaft inside the drag pipe. Place a about 1/2" of washers inside the exhaust on the bolt then add a lock washer and nut. Tighten it down and take if for a ride. This should improve the mid range power, and it might have cost you $2.00 . You can actually tune the power range a little by adding or removing washers, or altering the length of the bold. Cheap Trick!!"

Terry responded back with the following message:

"I tried that and it made some difference but not tremendous. I then took a 1/4" x1" bolt and welded a 3/4" diameter washer to the head of the bolt so it looked like a candy sucker. I used a nut on inside of pipe and a nut on outside of pipe with "sucker " inside the pipe. By turning the "sucker", could vary amount of resistance to exhaust flow. I ended up with washer flat across pipe for maximum resistance. That made a TREMENDOUS difference in mid range power and quieted it very slightly. Unbelievable difference! Don't have a dyno to provide numbers, but I KNOW it's much quicker."

We appreciates Terry's efforts. It's the little tricks like this that help all riders. If you have a tip that solves a performance or maintenance situation, E-mail us with your suggestion.


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Modification, Installation, Maintenance and Tuning Index  will help you find most of the information you want on one page.

How to get Professional Tuning Results at home
Testing the Innovate Motorsport LM-1 portable air fuel meter

Profession Tuning on a Shade Tree Budget

Veypor VR2 Data Logger and Instrument Panel
Video Installation and Demo
Purchase VR2

Engine Performance
How to Build a
TC96 2007 Engines
TC88 70HP Stage1  
TC95 128HP Stage 3
TC95 100HP Street
TC96 2007 Stage 1/2
EVO 64 HP Stage 1
EVO 74 HP Stage 2
EVO 82 HP Stage 3
EVO 95 HP Stage 3
883 to 1200 Upgrade
Shovelhead Modifications

New EFI for EVO and TC

Performance Gallery
Horsepower Gallery
Evolution 80
Twin Cam 88/95
Evolution Unlimited
Sportster Unlimited
Drag Strip Gallery
Land Speed Racing Gallery
CV Carburetor
Modifying the CV carb
Tuning a CV carb
Selecting a cam
Install a TC 88/95 cam
Install a Big Twin cam
Install Sportster cams

Camshaft Specifications
Twin Cam

Exhaust Systems
EVO Exhaust Testing
TC Exhaust Testing
Khrome Werks AR100 test
Making Drag Pipes Work

Shop Manual
Carburetor Troubleshooting
Finding Manifold Leaks
Cylinder Heads
Pistons and Cylinders
Belt Drive
Shop Manual Appendix
$20 Bike Lift
Plug Wires
Spark Plugs
Engine Tuning
Nitrous Oxide
Motor Oil
Stutter Box
General Information
WEB Links
Buy Books and Manuals
Performance Calculations
Estimate Horsepower
Estimate 1/4 Mile Time
Estimate Top Speed

Engine Displacement
Exhaust Length
Gear Ratios
Air Density

The Nightrider Diaries
The ramblings of a genius a, a madman and something in between.

Where is Sifton Cams?

Autocom Active-7 tested

Harley-Davidson EFI
-EFI basics explained
-EFI modifications explained

183 HP, 2 carbs, 2680cc

Copyright 1997-2006  Stephen Mullen, Oldsmar, FL -+-