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Wrench Tips and Tricks


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Bike Maintenance - tips and tricks from the experts


Benjie and John Joe with knees in the breeze

So...are you a garage mechanic? Can you tear down an old shovel or a pan and put it back together with your eyes blindfolded? Or, did you not even know that your Harley requires oil in the crank case? Whether beginner or advanced, this section is for you. We'll talk big bike maintenance here -- from simple stuff like oil changes and lightbulb repair, changing tires and wheel bearings, changing the oil, tightening the belt and primary chain, air filter cleaning, to more complex stuff like bleeding the brakes and changing the brake pads, re-jetting a carbeurator, changing a cam and ignition and more.

This Month:

SUBJECT: Drag to make 'em work

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.

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.

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!!"

Note from Papadad: This cheap trick really works!!! Try it. Works the same with the long "shot-gun" pipes like the Sampsons and the home-made pipes that are common over here too. Ever wonder why Screamin Eagle, Kirker, Bub, Cycle Shack and other "slip-on" pipes sound really good (just like pipes with the baffles drilled out) and seem to have WAY more torque than straight-though pipes? Take a look inside. See the bolt/washer/plate combo? Yup...sometimes welded in place. The MOFOMOCO knows something !!!! It's easy to drill an inconspicuous hole in the bottom of your straight-through pipes to insert a bolt and some washers for some added torque. Some companies are charging as much as US $40 for a set of "power cones" ... sheesh. Another yuppie-scum rip-off. Just buy a couple bolts for 10 pesos each at your local hardware store and you're all set.


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