Source:
http://www.hpt-sport.com Tuned Pipes
Exhaust pipes are a major determining factor in an engine's performance. But a pipe is a pipe is a pipe, right? Wrong! Despite what you may think, exhaust pipes aren't installed merely to emit beautiful mechanical choruses. Why do you think companies like Kawasaki Honda Yamaha and Suzuki employ full-time staffs to research and develop precisely tuned exhaust systems for their racing teams? There's definitely something there, and they know it. So should we. It's not necessary to know calculus or be a mechanical genius to install or tune an exhaust pipe, but it is mandatory that we have at least a basic knowledge of the operating principles behind the tuned systems.
One of the most common questions asked about exhausts is this: "Do aftermarket pipes make an engine produce more power?" Yes and no. No, pipes will not extract horsepower from an engine that didn't have it in the first place, but they can reshape the engine's powerband to make more power at different points, as well as helping get every bit of available power out of the motor.
Different engines have different powerbands which are evident at different rpm. Personal watercraft tend to run best from 3000-6700 rpm in stock form, with race machines getting performance all the way up to 8000 rpm. Unlike other machines, watercraft have no transmissions to help them make use of a narrow powerband. This makes it one of the most challenging pieces of equipment to get improved performance from, especially for mechanics with limited experience. For the benefit of those just getting into watercraft, let's review some basic principles concerning where the power is coming from. We need to know what power is in order to make use of every ounce of it.
WHAT'S WHAT
Force (F): The pressure on top of the piston due to the combustion force can be measured in pounds. The up and down movement of a piston is called Linear Force.
Work: If force causes motion, then that motion is called Work. A common unit is foot-pounds; if a force of 50 lbs. is carried a distance of 10 feet, then 500 foot-pounds of work has taken place. (ft/lbs=force*distance)
Energy (E): Work = Energy. If you lift an object weighing 25 lbs. 24 inches, you have done work and expended energy doing it. 50 ft/lbs. of energy, to be exact. Let's say you put this object on a shelf. You used heat energy to lift the object, energy which transformed to static energy when you set the object down. Energy never disappears, it merely changes form.
Torque (T): The linear force on the piston causes the crankshaft to turn. The force depends on how far it is applied from the center of rotation. For example, if you use a short wrench to loosen a bolt, you need lots of force, but if you use a longer wrench, you need less force to do the same work. That's because the longer wrench provides more Torque. Torque is not work, and it is not power; it is simply rotational force. (Example: 22 ft/lbs applied 10 feet from rotation center = 44 lb/ft of torque) The formula is the same whether the path is curved or straight: Work Distance * Force.
Power (P): Power is the amount of work done over a specified time. Work is the force across a distance, regardless of the time it takes to travel that distance. If you pick up an object every 15 seconds, and your friend picks up one every 30 seconds, then you're working twice as hard. Using twice as much power. Power = Work / Time, or Power = ft/lbs / seconds. What we use, horsepower, is what it takes to move 33,000 pounds a distance of one foot in a period of one minute. It's easier to use this formula: One Horsepower 550 ft/lbs per second.
So try and follow me. An engine creates Torque, the torque performs Work, and the work requires Power. Okay?
Here are a couple of formulas that may help you out. A crankshaft turns in a circular motion, therefore the work done is proportional to the distance traveled in that circular path (the amount of rotation). So Work = Torque * Rotation (W=T*R), where rotation is measured in degrees.
Power = (T*R)/minutes. This can also be stated as P=T*RPM. Since T is not the same at all speeds, we're forced to develop a separate formula. Remember the hp formula. If you and your machine have a combined weight of 550 pounds, and the engine has 20 hp, it could lift the both of you straight up at a rate of about 20 feet per second, or about 15 mph.
GET UP ON THE DOWNSTROKE
The prerequisites for combustion are: Induction, Compression, Expansion, and Exhaust. A two- stroke motor performs double duty. On the upstroke, when the piston moves upward to compress the existing charge, the piston is also acting as a pump at the bottom, drawing a fresh mixture into the crankcase. On the downstroke, the piston both compresses the mixture in the crankcase and, after the exhaust port is partially open and the cylinder pressure has dropped, helps push a fresh charge up into the port to refill the cylinder. This happens on each and every revolution of the engine.
The port timing of a particular engine can be stated by its port dimensions, degrees, or port time area (TA). One of the hardest workers in the engine is the exhaust port, because it has to pull from the front and push from the rear. It gets help from a properly tuned pipe and the incoming fresh mixture which tends to push the exhaust out of the cylinder. This operation is called "scavenging," and is one of the factors that has greatly improved two-stroke engine performance.
At the end of a power stroke, there is still around 100 lbs. of pressure in the cylinder, and even 20-30 lbs. at the point where the bust port opens. This calls for a carefully selected blow-down interval in order to avoid the entry of exhaust pressure into the transfer port area. This interval will vary from engine to engine. A tuner's ability to match the cylinder port time area with an exhaust system at a specific rpm range is critical. And you can't always be sure that the tuned pipe specs from the manufacturer is right for your application. So it's very important that you find someone who knows what they're doing and stick with them.
Port time area is the most important factor in choosing the powerband in which we want our engine to operate. On highly modified machines, mistakes of 0.010" can completely fail to produce power at the desired rpm. This means working with a degree wheel is like measuring piston clearance with a yardstick - it just doesn't cut it. Using some one else's dimensions is okay, but can still be a disappointment unless you have a total map of the port layout including the height, width, radii, angles, in between port width, and direction of flow. It's just plain easier to do it right yourself.
If the tuner/porter doesn't know how to figure port area using formulas, he'll probably try to do one of those "secret" porting jobs. Then, if you wind up with less power than you had with a stock setup, get ready to hear him tell you there's something else wrong with your motor. If this happens, you should do these two things: buy a new cylinder (because you can't make a big hole smaller) and have it ported correctly; and have the first mechanic shipped off to pick bananas somewhere.
Scavenging is the most important process in a two-stroke motor, because if it doesn't happen, nothing else will. At 8000 rpm, we have roughly 0.0036 seconds for the exhaust port to open. In this short time we need to evacuate the residual gas of the previous charge from the cylinder, refill the cylinder with a fresh charge, and make sure that the returning pulse wave arrives at precisely the right time (if it arrives too late, we can lose up to 15% of the fresh charge; too early the piston may over- heat and cause real headaches).
A LITTLE CHAMBER MUSIC
The expansion chamber plays a very important part in producing usable power in any high performance two-stroke engine. (See diagram A).
The expansion chamber's purpose is to control pressure (sonic waves) and put it to work in helping to balance both mixture pressurization and exhaust scavenging in the combustion chamber. The design of a chamber control s peak power output, the torque curve, and even the rpm limit of an engine. When the piston begins its power stroke, it uncovers the exhaust port first. At this point, exhaust gasses retain high pressure and burst into the exhaust track, forming a wave front which moves away from the port at high speed.
After traveling a short distance through the head pipe, the wave reaches the diffuser, where it actually reverses itself, becoming a negative wave and beginning to travel back toward the exhaust port. As the initial wave continues moving down the system, it's followed by hot exhaust gasses quickly moving down the header pipe and expands into the diffuser. Both waves have an inertia which creates a vacuum back at the exhaust port. This vacuum is on the order of about -7 psi at its peak. Combined with the +7 psi pressure produced by the pumping action of the crankcase, this sends a fresh charge up the transfer ports. The combined effect is approximately one atmosphere, and very strong.