Flow Like a River
Cylinder porting is and always has been one of the most delicate subjects on the planet, at least among engine builders. A two-stroke engine is deceptively simple; in reality, it's one of the most complex systems (along with exhausts) a tuner will deal with. Companies have invested millions of dollars and practically uncountable man-hours searching for the perfect port timing and flow area for their engines. The past 15 years have seen drastic changes in the materials and structural design of two-stroke cylinders, but the search for the holy grail of a perfectly timed and flowing cylinder goes on.
In case you're thinking that this search is something recent, take a quick look at some of the history of the two-stroke engine. The first patented two-stroke was a vertical twin patented by Alfred Scott in 1904. Scott also developed a rotary-valve engine in 1912. Schnuerle patented his ported engine in 1925; this engine had two bypass ports (the loop-scavenging method). In 1957, Walter Kaaden added a third transfer port opposite the exhaust port.
Erns Degner defected from East Germany to join Suzuki, and in the 1960s Suzuki won World Championships using his exotic porting. Bill Wisniewski applied the three-port, or Schnuerle porting, to a miniature engine, and in 1964 he won the FAI World Speed Championship. Later, Yamaha added a pair of auxiliary transfer ports alongside the main transfers. Dr. Hans Lipptsch added a pair of small auxiliary ports alongside the main port. In the 1980s, the Society of Automotive Engineers began to publish Dr. Gordon P. Blair's two-stroke engine technology, and in 1991 Tom Turner's (TSR) computer-aided designs were made available to the public. In 1992 George Grabowski contributing editor to PWCI began to publish tech articles thus ending the era of secrecy. So you can see that the process is an old, and an on-going one. One that we're trying to add a little bit to.
It should also be known that porting methods vary drastically from one tuner to another. The easiest method is to use the degree-wheel technique, where the timing of a port is performed by selecting a given angle-of-opening from top dead center (in exhaust ports this usually averages from 80-90 degrees; in transfers, usually 115-124 degrees). This isn't the most accurate method (as you can tell from the averages), and can involve a lot of hit-and-miss testing, especially if the person doing the porting is anything less than expert.
In my opinion, a tuner must have a thorough knowledge of port time area formulas, have the means to execute precise calculations, and have the ability to design the ports before he starts machining. A mistake of as little as 0.010" in the transfer port height can move the power band of a two stroke engine, resulting in a spongy power band.
The selection of a flow-through port is determined by the height, width, and vertical angle of the top of the port, as well as the horizontal angle of each side of the port. The power band's characteristics are set by the function of these vertical and horizontal angles, which can vary from 0-45 degrees vertically, and from 45-90 degrees horizontally. If you were to calculate all the possible combinations of all these ports and angles, you would begin to understand why we may never find the perfect settings in our lifetime.
A port's timing is actually the duration of the port's opening (which has little to do with the flow rate of a cylinder), sort of like the shutter speed on a camera. If your camera's shutter speed is set incorrectly, the pictures may turn out, but they won't be perfect. Likewise, if your port's timing is off, your engine won't work as well as it could.
The port's timing must complement the exhaust system. It's easier to adjust the timing to the exhaust than vice-versa, so it makes sense to select an exhaust system first, and then set the timing to match. Each exhaust is designed to operate at a specific power band width, which must be known if you're going to build a good engine. Instead of just hacking away at your engine, make intelligent choices based on the known limits of your engine's power band, and what you're hoping to get from that engine. For instance, don't install a pipe just because you like the ads, or there was a sale going on at the shop that day, or because of the salesman's claims regarding horsepower. Before you buy, get as much information as you can and then get what fits into your plans and desires. There are a lot of excellent products on the market today, but none of them are that great if you try to make them do things they weren't designed to do.
I As a rule, race pipes are built to operate at high revs, usually from 6800-8500 rpm. Limited systems do best in the stock range from around 6200-7000 rpm. And the sport recreational pipes fit some where in between, delivering maximum power in the 6500-7200 rpm range. It should be obvious that you need to match your pipe to your power band if you want the best results. The exhaust port controls the upper rpm range, and the transfer ports control the power band's width. On some units, this power band is very narrow (from 2500-4500 rpm), so if you install a pipe that doesn't work until you reach 7000 rpm, you'll have missed the boat completely.
The flow rate through the engine determines the power output of a unit. The intake area of the carburetors, the flow rate through the transfer ports, and the size of the exhaust port must all work together for optimum results. Installing big carbs or carving out a huge exhaust port won't do much good if the transfers restrict the flow rate of a mixture, and vice-versa. The flow through the port begins at the entry, down in the crankcase. Any restriction through velocity and power, and the biggest problem areas are at the port's roof and the neck size just below the port window. If the charge is misdirected or restricted at the window. where the incoming charge's velocity is high, many problems may occur (like fuel separation). Also at this point, the direction of a charge is determined by the deflection in the horizontal and vertical angles (the power band is controlled by this function of these angles).
To illustrate what has been discussed so far, let's look at the Yamaha WaveBlaster, which I consider a superbly designed craft. The Blaster's handling is revolutionary, however, by using the cylinder housing from a 650cc engine, I think they wound up creating more work for the owners of porting tools than anything else. The larger sleeves and higher port windows on the 650cc engine restrict the flow at the roof of the transfer ports, which is a big handicap for somebody trying to build a limited boat that still has some muscle. Matching the ports to the sleeves yields major improvement in the flow rate of this engine. Please note, however, that the power gain is not accomplished by simply modifying a single part, but is the result of properly matching all the engine's components.
Now, for further illustration, let's compare the port layout of a Yamaha 701cc engine with that of a Kawasaki 750cc engine, and we can begin to understand why the Kawasaki can deliver more horsepower. Pay special attention to the layout of the angles, as this gives a good example of the difference between the engines' designs.
George Grabowski HPT Sport USA