| Difficulty: Easy to Moderate (some welding) | Time Required: .5 hour | Cost: $0 - $20 | |
One of the first modifications most riders make to their motorcycles is a set of aftermarket exhaust pipes. Many opt for drag pipes, which are basically just a length of chrome pipe (by comparison, the stock mufflers on my 96 Intruder had three baffle plates and a core of fiberglass packing). These pipes make a LOT of noise, so people often assume that they make power also. But a completely open exhaust attached to a stock motor will actually reduce horsepower by 5%-10%. Usually a significant throttle lag develops also, making the bike hesitate when the bike is accelerated from highway speed, such as when you pull out to pass someone. Adding a free-flowing air intake system and re-jetting the carbs will make the motor work well with the pipes, and correct some of the throttle lag, assuming that you have: 1) the money, 2) the desire to further modify the bike and perhaps void the warranty, and 3) a bike with an external intake system or the desire to add one to your ride. If you do these other modification, and keep the open pipes, overall horsepower will be better than stock.
On paper.
The maximum horsepower on a motor with free-flowing intake and exhaust is impressive on paper, but maximum power is achieved at very high RPM. Power in the RPM range that street motorcycles live in is actually unimpressive, as the open system makes very little torque in the low and mid ranges. Open exhausts (and the matching open air intake system) are designed to work at high RPM and with a wide-open throttle, like at a drag strip. After all, that's why they are called "Drag Pipes!"
But you like the noise ( insert argument here: "Loud pipes save lives!", "I hate my neighbors", or "But chicks dig it') so the drag pipes are going to stay. So here is how you can make them work on the street.
2 * 3/4-inch washer
2 * 1 1/4-inch length of 1/4-inch threaded rod OR a 1 * 1/4-inch long 14-20 bolt
4 * lock nuts to fit the threaded rod (or regular nuts with lock washers)
1 * 1/4-inch drill bit and an electric drill (not a cordless one)
A friend or co-worker with a welder, or a bottle of beer for a guy at the local muffler shop
Some high-temp flat black paint (optional)
Step 1: Weld the washers onto the heads of the bolts, or to one end of the threaded rods. Then thread one of the nuts onto each from the other end, stopping about 2/3s of an inch from the bottom of the bolt head/ washer. Paint them flat black if you want it to be less visible once its installed. After a whopping two minutes of work, you should have two assemblies that look like the drawing below.
Step 2: Drill a 1/4-inch hole in each drag pipe, 1-inch from the end. Drilling up from the bottom is easiest, but if the pipes are off the bike you could drill them so the hole is towards the bike so no one can see it. Use a center punch or nail, and add a drop of oil or some WD-40 to the drill bit to keep it cool while it cuts.
Step 3: From the inside of the pipe, stick the bolts/ threaded rods through the holes in the pipes, and put a nut on each from the outside. Play with it a little until the washer is in the center of the pipe, and then tighten it finger tight. Looking down the pipe, it should look like the drawing below. DO NOT USE THREADLOCKER! You'll know why in a minute...
Step 4: Congratulations! You have just installed a set of baffles in your drag pipes that can be tuned for the type of riding you are doing. For maximum mid-range torque, put the washer directly across the pipe at a 90-degree angle to the exhaust flow and tighten the nuts. Fire the bike up: do you hear any difference in the noise level? Probably not! But take the bike for a spin- you can feel the mid-range torque! Doesn't the bike pull better, and more smoothly, when you roll on the throttle? Dyno tests have proven that this little device boosts low and mid-range torque by creating just enough turbulence and back pressure to keep the airflow continuous, which lets the exhaust system suck the gasses out of the combustion chamber more efficiently, improving airflow over a completely open system.
If you are planning on doing some racing on the dragstrip, loosen the nuts and turn the baffles so they are parallel to the pipes. This will reduce the back pressure and free up top-end horsepower at very high RPM, like drag pipes are supposed to do, but the turbulence helps the pipes work better than if you removed them completely.
If you have changes the final drive ratio or put on a very small diameter back tire, start with the washer at about a 45-degree angle to the exhaust flow, and a little trial and error will help you find the ideal angle to get the most out of your ride.
Well, there you have it: with ten minutes of work and about $2 worth of material, you have modified your drag pipes to work on the street while keeping the noise. The result is a quicker, more streetable bike that will still piss off your neighbors.
