Some manufacturers, notably Suzuki, continue to employ - and get good results from - a single pair of transfer ports. Nevertheless, the main thrust of development today is toward increasing use of multiple transfer ports. This trend began in the design of engines having other than piston-controlled intake ports, simply because such designs have an uncluttered rear cylinder wall and it was inevitable that someone would add a third transfer port in this otherwise unemployed space. MZ's Kaaden incorporated a third transfer port in his 1962 GP engine, fed from a window high on the piston skirt, just to cool the piston crown and carry oil up to the wristpin bearing. But while improved reliability may have been the motivation behind this change, it also resulted in a power increase. Others found much the same result, and by 1965 the triple-transfer layout was standard for engines having rotary-disc - intake valves.
Isolated examples of piston-port induction combined with multiple transfer ports are to be found throughout the two-stroke engine's history. Still, I am inclined to assign credit for stimulating real interest in the layout to Hermann Meier. Meier was hired by Royal Enfield to design and develop the engine for that firm's short-lived “production racer”, but was not allowed anything - approaching complete freedom in this endeavor. Management told him to work around the then-popular Alpha crank assembly (it was a more-reliable replacement for the Villiers crank, rod and bearings) and forbade anything as tricky (and expensive) as rotary-valve induction. In short, he was asked to design a cylinder that would magically produce all the power of a genuine GP engine without any of the latter's complication. Understandably, Meier's best efforts could not overcome the obstacles placed in his path by Royal Enfield’s management, but he did develop an engine that was a notch up, in terms of power, over the twin-cylinder Yamaha production racers of that time -which was quite a neat trick. And his Royal Enfield engine did have more than the usual pair of transfer ports. The presence of a large intake port on the rear cylinder wall prevented Meier from positioning a third transfer port there, so he simply crowded the primary transfer ports forward, closer to the exhaust port, and added a pair of smaller, auxiliary transfer ports between the main transfers and the intake port. The auxiliary ports were fed in part directly from the crankcase, but a pair of windows adjacent to and slightly higher than the wristpin, in the piston, also fed mixture through matching holes in the cylinder into the transfer passages.
The 250cc “Grand Prix” Royal Enfield, handicapped as it was by marginal reliability and wretched handling, soon passed into oblivion, but hard on its heels came the now-famous “five-port” Yamaha -with a virtually identical transfer port system. Initially, the Yamaha fed its auxiliary transfer ports entirely through windows in its pistons, and the transfers themselves were merely a pair of grooves flanking the intake ports. This arrangement soon, however, gave way to the now-established Yamaha practice of providing all of the flow for the auxiliary ports from the crankcase; it is only in the recent reed-valve engines that windows once again appear in Yamaha pistons. Without really knowing, I would suspect that Yamaha dropped the windowed piston concept because it adds a complication in manufacturing and weakens the piston - and because Yamaha's needle-roller wristpin bearings survive very nicely the conditions of scanty lubrication and high temperatures that caused problems for Meier and Kaaden.
In any case, multiple transfer ports are much in style, and do appear to give superior results if properly managed. But be advised that simply hacking an extra pair of holes up through the cylinder does not constitute proper management of the multiple transfer concept, for the auxiliary transfer ports must act in concert with the main transfers or they will only contribute to turbulence - which means an increase in charge dilution – and / or outright short-circuiting. Unless you have the equipment, and dexterity, to cut extra ports with great accuracy, you would be better off confining your efforts to improving your engine's single pair of transfer ports -if a single pair is what it has.
The best basic shape for a transfer port is a sweeping arc, but many are (for reasons of manufacturing cost or because of space considerations) substantially straight, with a sharp turn where they meet the port window. There is little difference in these two approaches in terms of bulk flow, but scavenging- stream control is superior with transfer passages that sweep wide to join with the port windows less abruptly. Consequently, the peak power obtained will be much the same with either transfer passage configuration, but the less abrupt shape gives better performance below the power peak. Obviously, it is much easier to tell about the difference than to make any real change in the ports' shapes - especially when the engine in question is a twin, with its cylinders crowded together - but it also obviously is good practice to do everything possible to make the straight port a little less straight. This can be done by carving away metal from the port's outer wall, and adding a similar thickness of plastic Devcon filler on the wall nearest the cylinder bore. Remember that the transfer passage should taper down from its entrance at the cylinder base to the port window itself, and that it is virtually impossible to make the entrance area too large. Enlarging the transfer passages reduces an engine's primary compression ratio, of course, but we have already established that crankcase compression is not terribly important - and mixture delivery into the cylinder most definitely is!
If, or when, you perform the modification just described, be careful not to alter the port window's direction, either upward or to the side. You might improve upon the manufacturer's handiwork in this regard; the chances are that any change will be for the worse. In practice, the effective area of the transfer ports becomes greater when they are angled more toward the middle of the cylinder and less toward the rear cylinder wall, but while a change in the order of 10-degrees will sometimes yield a boost in peak power, this increase will be purchased at some considerable cost in power range and I cannot recommend it to anyone who is unwilling to A) build a flow-pattern test bench and B) dynamometer-test a number of cylinders with incremental realignments of their transfer ports.
Traditionally, the basement-workshop experimenter's scavenging-flow test method has been to feed an engine plenty of oil, and inspect the pattern on the piston crown after aperiod of running. That is better than nothing, but it provides one with only a very crude picture of what is happening inside the cylinder while the engine is running. Better methods have been developed: One is the “water-analogy” test, in which a cylinder is fitted with a Lucite cylinder head and water forced up through its transfer ports and out the exhaust port. Air is injected into the water, in small bubbles, just below each transfer passage entrance, and these bubbles make the flow patterns inside the cylinder visible. Another, less messy method also entails making a clear Lucite cylinder head, but instead of water being used, air is aspirated from - the exhaust port (with a large vacuum cleaner, if you have one) and smoke or fine dye droplets are injected into the air stream as it enters each transfer port. Only by such means can you actually see asymmetrical scavenging streams, and short-circuiting, and such techniques are essential if you plan any major alteration in the porting your engine's maker has provided.
There are, as it happens, some broad rules concerning the size and direction of multiple transfer ports. One of these is that in a “five-port'” cylinder the auxiliary transfers should have an outlet area between 0.33 and 0.40 that of the main transfer ports. Unfortunately, nothing as clearly defined exists as regards the upward tilt of the transfer ports. In most engines having rotary intake valves and an auxiliary port or ports on the rear cylinder wall, the main transfers will discharge at angles ranging from straight across the cylinder, 90-degrees from the cylinder wall, to as much as 15-degrees of upward tilt. The third port (or ports) will always be tilted upward, but this can range from as little as 45-degrees to as much as 60-degrees. Maximum power is obtained, in most engines, with the least amount of upward tilting - as this makes the port windows effectively larger - but power range tends to suffer as the upward bias in the scavenging stream flow is reduced. Thus, in a motocross engine, one should start with a considerable amount of upward bias and reduce this until the engine's power range verges on becoming too narrow; the opposite approach, that of starting with “flat” ports and adding epoxy filler to the port roofs to provide upward bias, might more profitably be applied in road racing engines. In either instance, changes should be made in one-degree increments until the optimum is found.
There would seem to be no rule at all in determining the upward angles for the kind of paired transfers Yamaha currently employs. The TD2 road racing engine has its main transfer ports angled upward 15-degrees, while the smaller, auxiliary ports discharge straight across the piston crown. But the otherwise very similar, 350cc TR2 engine's main transfers have a zero upward angle, and the auxiliaries angled upward very sharply at 45-degrees. Yamaha's latest road racing engine, the 350cc TR3, combines these, with its main transfers directed upward about 15-degrees and the auxiliaries even steeper at about 45-degrees. In all cases, both main and auxiliary open at the same time in the Yamaha engines, although this is not universal by any means. Some designers have obtained best results by either advancing or delaying slightly the opening of the auxiliary transfers, with respect to the main transfer port timing, and the old GP Suzuki 50cc single had its third port open before its exhaust port - which caused crankcase pressure to rise very sharply just before the exhaust blow-down and subsequent transfer phase. Of course, this rather radical method of improving mixture delivery was effective only over an extremely narrow power range, and even then it must have had other serious shortcomings for neither Suzuki nor anyone else has employed it in recent years.
Now that reed intake valving has come into fashion, there will be a lot of “sixth and seventh” porting done to various engines. There are two different approaches to adding extra rear-cylinder ports in reed-valve engines: That chosen by Yamaha is simply to carve a slot leading up from the intake port to link the port cavity with the cylinder when the piston is down at the bottom of its stroke. This “sixth” port is not fed from the crankcase; mixture moves up through it and into the cylinder due to the ramming action of gas inertia in the intake manifold, and also because of the partial vacuum formed in the cylinder by exhaust system dynamics. Yamaha places the top of this extra transfer ports on a level with the four already found in their five-port engines, and the roof of the port is angled upward very sharply - about 70-degrees, in fact. Apparently, such “tuning” as is done with this kind of port is mostly a matter of varying its width, but not enough experimental work has been done with it to provide us with any guidelines. The other approach to this particular kind of transfer port is to cut windows in the rear of the piston to permit part of the charge compressed in the crankcase to blow up into the intake port, from where it is then able to escape through the boost port and into the cylinder. From early reports, the effect of this modification is to improve very substantially an engine's power range at some expense to peak power. This is, of course, precisely the effect to be desired in a large-displacement scrambler or motocross engine, but hardly what one would be looking for from a road racing engine. My own experience with reed valves is still too limited to permit me any firm conclusions, and the potential benefits of the porting they make possible, but I am inclined at present to believe that they fall something short of being the answer to the two-stroke tuner's prayers. Indeed, it may be that their principal contribution is to keep carburetion clean over a wider speed range than is possible with piston-controlled intake porting. I very much doubt that reed valving will ever equal the ordinary piston-controlled induction in terms of maximum horsepower - though I recognize that reeds have their uses in engines intended for general, all-purpose motorcycles.
Far more promising as a mean of completing the ring of ports around the base of the two-stroke's cylinder is a method being tried by Aermacchi, in Italy. Engineers these have been experimenting with a taller-than-usual piston, which provides them with enough space above the intake port for a trio of very short transfer passages fed from three holes drilled through the back of the piston, just below the ring. To get room for these holes, without pushing the piston crown up absurdly high above the wristpin, Aermacchi's engineers have gone to a flat-crown piston, with a single Dykes-pattern ring located at its upper edge. A similar modification might be possible in other piston-port engines, without getting into too much special machining or prohibitive costs.
No matter what kind of transfer system you ultimately employ, remember that while adding port area will improve bulk flow of the air / fuel charge up from the crankcase, it will all be for nothing unless the flow is controlled after it enters the cylinder. In a cylinder with main and auxiliary transfer ports, the upward pitch of the auxiliary ports is used to literally herdthe main scavenging streams together and direct them up into the cylinder. This can be accomplished by shooting the auxiliary streams under the main flow, and bending it upward - or by aiming the auxiliary streams upward and using them to drag the main flow along. The latter is to be favored when it is possible, simply because - as I have stated repeatedly - tilting a port upward reduces its effective area and reducing the area of the auxiliary ports has less effect on the total flow characteristics. On the other hand, if your engine has its existing transfer ports angled upward at, say, 15-degrees, and is constructed so that only very limited space is available for an extra pair of ports, then it might be better to aim the auxiliaries straight across the piston crown - as was done by Yamaha in its early experiments with the five-port concept.
No greater futility exists than to invest heavily of one's time and money in building an engine with mirror-smooth ports, the best available pistons, rings, etc., and then approach the problems of carburetion and ignition armed only with enthusiasm. It is an observable fact that some inherently mediocre engines have been made to prevail in racing simply because the men charged with selecting jetting and spark timing knew their business, but never in the history of the sport has there been an engine so splendid in its internal arrangement that a tuner's ineptitude could not render it entirely ineffectual. As a rule, two-stroke engines are both more difficult to tune and more sensitive to fine-tuning than their poppet-valve cousins - especially as regards spark timing, with some two-stroke engines being so extraordinarily sensitive to ignition advance that a shift of only a single degree either side of the optimum will result in a measurable loss of power. Yamaha's TD-series racing engines, for example, must be timed plus or minus only 0.0012-inch of piston travel from the specified distance of TDC. These engines are also very sensitive to spark intensity, and magneto maladjustments or other defects too slight to produce perceptible misfiring will cause a marked deterioration of performance. The two-stroke engine's air / fuel mixture is a somewhat less touchy matter, as over-rich mixtures tend to compensate for their reduced effectiveness in burning by improving internal cooling, but there is an optimum here too. Finding these optima for spark and mixture can be an exasperating, time-consuming chore, but there is no escaping the necessity for putting forth the effort.
While most tuners would agree that much depends on the basic tuners' arts, they are nonetheless inclined to seek magical solutions to any problems not instantly resolved by changing a main jet or replacing a set of points. Sadly, seldom does mechanical exotica - magic-box ignitions and the like - truly solve a problem. More often they merely replace the existing difficulty with another, or multiple, even more mysterious problems. For instance, most engines have ignition systems entirely capable of producing sparks at a rate adequate for the speed range anticipated by the engines' makers, but may collapse into a fit of misfiring at higher revs. Most tuners will assume that the root cause of this distress is that they have built themselves such an incredibly vigorous engine that nothing weaker than God's-Own-Lightning is enough to make it run properly, which sends them scurrying away in search of some transistorized, magnetically-triggered system with enough sheer zap to fracture atoms. Often as not, the system they get will clear up the misfire and convince them that they have done a Good Thing-even though the new system is impossible to time ,accurately and is prone to sudden, inexplicable failures. Actually, the fundamental reason for the misfiring probably was that at the higher speeds, the point cam profile was too “sudden” for the breaker-point springs, leading to point-float. The proper cure would have been to modify the cam profile to open and close the points more gently, and / or substitute the standard points-set with one having stronger springs, instead of abandoning the straightforward mechanical problem in favor of a black box of troubles best left to electronics engineers.
Details vary according to the fashion of the moment, but much the same series of events often occur subsequent to the discovery of a carburetion problem. A persistent misfire frequently leads people into the error of assuming that the carburetor at hand must be replaced with something more splendid. Today, the most frequently chosen replacement is a Mikuni carburetor, and for once the dictates of fashion are substantially correct, as the Mikuni is a very fine instrument and when used in the appropriate throat size it can be made to give excellent results. Unfortunately, it is distinctly possible to employ even the Mikuni carburetor badly, unless you know what you're doing and are prepared to work at it diligently.