Do You Have a Problem?
We have discussed pipe tuning and how different expansion chamber designs can effect your watercraft's power. This time around, we'll look at how a well-informed mechanic can use some of this knowledge to correct problems with his or her powerband.
Many people proclaim to know the secrets of porting, and while some can admittedly get by, few actually have a thorough understanding of what's really going on. Fortunately, most personal watercraft engines do not require the precision of, say, a snowmobile or a roadracing motorcycle. That is why there are so many "expert tuners" in our wet world.
My modifications always begin by evaluating the existing powerplant. The first step taken is to measure the exhaust system; accurate knowledge of a tuned pipe's limits is mandatory for properly shaping the ports. Next, I measure the compression ratio, which tells me the full story regarding fuel octane requirements. Then the cylinder deck height is recorded, followed by the transfer port height. While inspecting the transfers, I note their width, shape, angles and corner radii.
Exhaust ports must be measured precisely, including height, width and top and bottom radii dimensions. Exhaust ports determine the powerband of the engine and are therefor the first thing that I want to get properly focused. The precise performance of an engine is measured in BMEP (Brake Mean Effective Pressure) inside the cylinder. This pressure ranges from 55 to 190 psi, regardless of engine size; whether it's 50cc or 500 per cylinder, all engines fall under this pressure range.
Horsepower output is related to the total area of an exhaust port - not just its height. For example, a Yamaha 701cc engine has a port height of 39.12mm, which is timed for a 90.6 degree opening. The port is 48mm wide and has 1lmm radii on all corners. The time area of this port is 11.4728 at 88 BMEP, or 2012mm. This engine produces 29.7 horsepower per cylinder at 6250 rpm, without the employment of a tuned pipe. Let's say that we wish to run this engine at 7200 rpm. This means we need to increase port area up to 12.2028, or 2417mm, which is good at 99 BMEP. This allows 39 horsepower per cylinder. With a quality tuned pipe, this engine could produce nearly 100 horsepower.
My work begins by widening the port to its limits, and then - only if necessary - raising it as well. Next, I time the transfer ports within the powerband limits of the pipe and/or the desired power range. This is an important consideration on most engines, though water-cooled pipes are less sensitive than air-cooled units. Cooled pipes permit a wider range of power selection, by simply changing water flow.
If your tuned pipe is functioning properly, you will be a happy person indeed. Unfortunately, not all pipes will produce such a positive result. There are many hidden problems which may be built into a tuned exhaust system by an aftermarket shop or an engine builder with limited expertise. Let's take a look at some possible hidden problems that may contribute to less than optimum engine performance.
Modern computer programs make life worth living. What was a dream just years ago, has become reality today. Nevertheless, let's not forget that these machines can only synthesize; it is still up to the human to analyze the information. It is possible to accurately predict performance characteristics of an engine using computer analysis. This makes for considerable savings in budget, and avoids unnecessary wear and tear of the engine and dyno.
Dyno testing is very expensive. Using my computer program, I can analyze each and every change made to the engine. If a result is satisfactory, I can back it up with the dyno and then go on to the next step. Have you ever tried to hand-calculate the data regarding exhaust systems? I can measure an existing tuned pipe, and predict its performance with great accuracy.
It is also possible to re-tune some of the parts of an existing system to your particular applications. If the pipe is made of several sections, you can alter it greatly, even a one-piece cast pipe could be changed at the header pipe and stinger area, while the water injection to the pipe could be altered to comply with the new powerband.
Performing such changes is work better left to an expert mechanic than a novice tuner, as making a mistake can leave you worse off than you were to begin with. Still, even if it's not you doing the work, it's helpful to know what's happening, and more importantly, what will happen.
George Grabowski HPT Sport USA