Pipe Dreams
Ask anyone what they plan to do to increase the performance of their brand-new personal watercraft, and more often than not, they'll answer, "Install a new pipe." Intake grates and impellers are also popular choices, but a high- performance exhaust system can make more of a difference in the real world. Of course it naturally follows that pipes are the item of choice among the many aftermarket companies that cater to the personal watercraft market, with dozens of companies touting their wares as being the best.
Properly designed and matched to cylinder port time area, a tuned pipe can up engine power by 20 to 50 percent over a powerplant with a mismatched or poorly designed exhaust system. A properly tuned pipe is supposed to operate with a two-stroke engine in the same manner that a supercharger works with a four-stroke motor. If utilized correctly it can reward you with truly awesome performance. However there is also the other side of this very complex equation, a side that can reward you with only an empty bank account and possibly, a one-way trip to the funny farm.
I met a very open-minded person at the Cincinnati trade show, and he tested 75 different pipe combinations on a dyno. When all the smoke had cleared away, he was flat broke and totally disappointed, not having gained one more horsepower than was available from aftermarket systems which were already available. What does this mean to you? Only that as a customer, your best course of action is to seek a proven product and to make sure your engine builder knows his stuff. If you yourself are an aspiring engine builder, your greatest chance of survival in the cutthroat financial real world ties with investing in a quality education. Most importantly, any experimental work you do must be performed on your own personal equipment not your customers. This advice may sound obvious, but not heeding it can quickly ruin a reputation.
I do not claim to possess knowledge of everything there is to know relating to tuned exhaust systems, but you can't help but learn something in 35 years of building engines. If nothing else my experience may assist you in avoiding some common mistakes. Technical information is difficult to comprehend without trying it yourself, but pay attention and you may save yourself some hassles in the future.
When a piston reaches top dead center (TDC) and begins its journey down to bottom dead center (BDC), it first uncovers the exhaust port. This point is at 90degrees of crank rotation on most stock engines. From that point on the power stroke ends and the blow-down phase begins, continuing through approximately 30+ degrees of crank rotation on the majority of standard motors. The transfer ports kick open at 122 degrees of crank rotation, signaling the end of the blow-down I period. This means that the hot exhaust gases have had very little time to escape from the cylinder. The transfer ports remain open from 122 degrees to 238 degrees, when a fresh charge of fuel mixed with oxygen in our case rushes into the cylinder. This process takes 116degrees of crank rotation to complete.
At this point, intake ceases and partial compression begins, but part of the exhaust gases still remain in the cylinder, mixed in with the fresh charge. This unfortunate phenomenon contributes to power losses. When the piston reaches the top of the exhaust port at 270 degrees of crank rotation, the compression cycle begins and continues until TDC.
During the down stroke, when the piston opens the exhaust port, hot exhaust gases rush into the pipe, creating sonic velocity and high pressure. This pressure greatly helps in drawing out the remaining exhaust, and at the same time assists in pulling in a fresh charge from the transfer case. At this time several very important functions transpire: As the sonic wave travels down the exhaust system, part of the fresh charge is pulled down through the pipe's first section called the "header." This charge could be lost forever, contributing to a lean condition and power loss within the engine. When the sonic wave reaches the end of a diffuser, part of the wave splits and begins to travel back towards the exhaust port, while the rest continues out towards the end of the pipe. The deflected part of the wave, which has changed its sine, is responsible for sucking the old exhaust and the fresh charge out of the cylinder.
The undisturbed part of the wave, upon reaching the end of the pipe (called the "convergent cone"), also reverses its direction and begins to travel back towards the piston. And this is where "super-charging" begins. If the backward traveling wave (called the plug pulse) arrives in the header section at the proper moment, it will push the fresh charge which is located in the header back into the cylinder at the optimum time of crank rotation (before the rising piston closes the exhaust port). This is where you either make the best possible power or suffer a serious power loss, depending on your tuning.
This action is limited to a relatively narrow band of rpm usually around 2500, and much less on high-rpm, high-Break Mean Effective Pressure (BMEP) engines. For example, GP road race machines may only undergo the process for 500 rpm sometimes even less!
The pipe's design is related to many things, including the preferred range of applicable rpm, the spread of torque, overrev rpm limits, exhaust temperature, port shape, port size, top and bottom port radii, port timing and the chamfer angle of the ports. The blow-down angle and the duration of the transfer port opening are two other very important factors.
I used to say that! knowing the operating temperature of the pipe and the proper position to locate a temperature probe could be worth a million dollars; that was with an air-cooled system. Now I say that knowing where to measure temperature in a wet pipe is worth more. And I suspect that no one has yet found that perfect spot even the factory boys with buckets of money and high-tech computers. Arriving at the proper operating temperature of a tuned exhaust system can be the key to success or a wide open gate to failure. Why? Because there are hundreds of possible combinations affecting each and every setup.
For instance, horizontal angles of transfer ports, which may be directed over a large area across the face of the piston, can determine the type of power you are going to get. These angles have a range of approximately 40 degrees. And the vertical angle of the transfers, which varies from 0 to 45 degrees, also participates in affecting your power. Exhaust port duration ranges from 175 to 208 degrees, transfer port duration from 116 to 142 degrees. That is a lot of combinations, my friend; now just try and pull off a porting design that is accurate to five thousandths of an inch (0.005") for an all-out racing engine! It's no easy feat, and there aren't many engine builders out there that really know what to do when it comes to precision pipe and port design.
George Grabowski HPT Sport USA