A major problem with cylinder heads on high-output engines that began life as low output engines is persistent leakage around the head/cylinder joint. The combined increases in temperature and pressure seem always to be too much for the joint, and you will find evidence of fire puffing past on the surfaces after disassembly even though you may not have observed anything out of the ordinary when the engine was running. This leaking will occur even if you have retained the engine's stock compression ratio, and it may become very serious if the head has been thinned to get a compression ratio increase. Many manufacturers, perhaps most, feel some awful compulsion to skimp on section thicknesses when they make a cylinder head, a habit that often stands revealed as a questionable economy when you test their handiwork on a dynamometer: first, the thin sections often do not have the cross-sectional area required to transfer heat away from the head's lower surface quickly enough to keep the spark plug temperatures stabilized; second, most of these cylinder heads are secured to their cylinders by only four widely-spaced bolts, which presumes heavily on their beam-strength to maintain a tight seal at the joint.
This last situation becomes especially marginal when metal has been machined away to raise the engine's compression ratio, and the stock head gasket (usually cut from light-gauge, soft aluminum) will in many cases not be strong enough to hold even the pressure increases involved in a simple switching of exhaust systems. Shave the head (which both weakens the head's beam strength and increases the forces acting upon it) and you'll very likely find that it becomes impossible to hold the head/cylinder seal- the gasket will fail after only minutes of running. Also, attempting to use the stock cylinder head, in either standard or modified form, often will increase the heat input around the spark plug to such extent that the engine becomes impossibly fussy about plug heat range. Use a plug cold enough to avoid trouble at maximum output, and it will foul at anything less than full-throttle operation. There is nothing like masses of metal to equalize the temperature gradients through the cylinder head, and - sad to say - those masses are not provided in many stock cylinder heads.
Cylinder head design also can strongly affect overall cylinder cooling. When the cylinder head's lower surface is cooler than the cylinder itself, heat will be drawn away from the latter; conversely, a cylinder head can also put heat into the cylinder if the situation is reversed. All things considered, the engine's best interests probably are served by isolating, to such extent as is possible, the cylinder and head- which means restricting the contact area at the cylinder/head joint to a narrow sealing band which bulges to encompass the hold-down bolts, or studs. In that way any cooling problems will be isolated, and can be dealt with separately. That, of course, assumes that it will be possible to improve cylinder cooling should such improvement become necessary. Actually, making a new cylinder head is fairly easy (it can be either cast or simply machined from a block of aluminum) while the cylinder itself presents a far more difficult problem in fabrication. So you may very well want to use an oversized, deeply-finned cylinder head to help cool a particular engine's stock, cast-iron cylinder. And if that should be the case, remember that you'll need a maximum contact area between head and barrel, and surfaces that will seal without any kind of gasket. There is a very sharp temperature gradient across any joint, and even a solid copper gasket presents one more pair of surfaces across which heat must flow.
You may find that providing a seal between the head and barrel is one of the more difficult facets of the overall job. As I have said, stock aluminum gaskets are almost certain to fail, being a bit weak at ambient temperatures anyway -and impossibly frail at the temperatures to which they will be subjected. Copper is a better material, for while it is nearly as soft as aluminum at ambient, its hot-strength properties are better. Copper is soft enough to make a good gasket in the annealed state, but hardens in use, and must be re-annealed frequently to keep it soft and thus retain its properties as a gasket. Brass should never be used as a gasket material, but steel may be used if it is very thin and has one or more corrugations rolled, in rings, around the bore - in the manner of the head gaskets used in some automobile engines. You can also get a good seal by machining a narrow groove in the cylinder's upper face and inserting in it a soft copper ring (made from wire) to bear against the head's lower surface. Other, even better seals may be had with gas-filled metal O-rings, piston rings (they'll work here, too) and one of the best sealing rings I've seen has a V-shaped section, laid on its side, with the V's point aimed away from the bore. Gas pressure tries to force the V open, bringing one arm to seal down against the cylinder while the other is pressed against the cylinder head. Another sealing ring that works in roughly the same fashion is a hollow metal O-ring with vent- holes drilled through from its inner diameter to admit gas pressure from the cylinder - which expands it outward and thus creates a seal even between somewhat uneven surfaces.
Nominal compression ratios, as I have said before, have little meaning in high-output two-stroke engines. However, you can work with trapped compression ratios almost as effectively as by measuring cranking pressures. An engine's trapped compression ratio is the ratio between the cylinder volume at the moment of the exhaust port's closing and the volume with the piston at the top of its stroke. To find this, you must first measure the combustion chamber volume, with the piston in position at top center. The job can be done with the engine assembled, using n graduated cylinder and pouring in oil until the level comes up to the spark plug hole. Or you can calculate the volume. When the combustion chamber has a simple shape (part-spherical, conical or cylindrical) I prefer to do the job by calculation, but more complex shapes send me scurrying for a can of oil and a graduated cylinder. In fact, the process of actual measurement may appeal to you as a regular thing, because you will need a graduated cylinder for more than this single task, and a slide-rule may not be a part of your basic equipment. In any case, remember when figuring the compression ratio, that it is not the ratio between piston displacement and combustion chamber volume, but between cylinder volumes from the point of exhaust port closing to top center, as in the following formula :
Where CR is compression ratio
V1 is cylinder volume at exhaust closing
V2 is combustion chamber volume
Traditionally, compression ratios have been measured "full stroke". That is to say, V, would represent the combustion chamber volume plus piston displacement from bottom center to tog center. Thus, a combustion chamber volume of 28cc and a piston displacement of 250cc, calculated full-stroke, would be
But a far more realistic figure is obtained when V1 represents the cylinder volume above the upper edge of the exhaust port, and if we assume that our hypothetical engine has an exhaust port height equal to 45-percent of stroke, then V1 becomes 55-percent of piston displacement plus V2, and calculation goes like this:
Coincidentally, that compression ratio (5.91:l) is very nearly all a non-squish combustion chamber will permit in anotherwise fully-developed two-stroke engine. With small-bore engines you may push the compression ratio up to perhaps 6.5:l without serious consequences, using a non-squish cylinder head, but that is very near the limit. Good squish-band cylinder heads, on the other hand, permit compression ratios up to as much ns 9.5:l in motocross engines with exhaust systems that provide a wide boost without any substantial peaks, but for road racing engines I cannot recommend anything above 8.5:l even when unit cylinder size is only 125cc. You will find that higher compression ratios than those suggested can produce marvelously impressive flash readings on a dynamometer; as soon as the engine has a chance to get up to full temperature, the output will drop well below that sustained by an otherwise identical engine with a lower compression ratio. Sustained, and not flash horsepower, is what wins races.
Walter Kaaden was chief engineer of MZ's racing department through that firm's glory years on the Grand Prix circuit, and in that capacity Kaaden advanced the state of the art with regard to expansion chamber design very considerably. And one day while discussing the subject he remarked, only in jest, “You'll know when you have the design right, because the chamber will then be impossible to fit on the motorcycle without having it drag the ground, burn the rider's leg, or force the relocation of one or more major components”. Of course, all present had a fine laugh, but the joke contained a large and bitter kernel of truth. In point of fact, that odd, bulky bit of exhaust plumbing we call an “expansion chamber” (a poor term for the device, but widely used) is exceedingly difficult to accommodate neatly on a motorcycle. Routed underneath, it is an acute embarrassment in terms of ground clearance even on a road racing machine and fights a losing battle with rocks on an off-road bike. Curled back along the motorcycle's side, it can force changes in the position of fuel tanks and frame tubes - and always roasts the rider's leg and/or forces him to ride bow-legged. Just as bad, it fiendishly assaults the ears of everyone for several hundred yards in every direction. and has done more to make the motorcycle - and the man astride one – unpopular than all the Wild Ones movies, and tabloid headlines of One-Percenter's misdeeds, put together.
Attended as it is by these manifold inconveniences, one almost (but not quite) wonders why we bother with the expansion chamber. Unfortunately, damnable nuisance that it unquestionably is, there is nothing else in the engineer's bag of tricks that comes anywhere close to matching the boost a two-stroke engine gets from a properly designed expansion chamber exhaust system. For that reason, it has become the ubiquitous helpmate of the high output two-stroke engine, and for that reason it will be with us until we all change over to electric motors or gas turbines. And until that time, experimenters will be tossing away stock mufflers and trying different expansion chambers as a major part of their endless quest for ever-higher performance.
Actual percentage improvements between engines fitted with their standard mufflers and the same engines with expansion chambers will vary greatly. A lot depends on how good (or bad) their muffler happened to be, and on carburetor size, porting, etc. -any of which can impose limits that cannot be totally compensated by even the best of expansion chambers. In most cases, however, the improvement will be in the order of 10- to 25-percent, and when the expansion chamber is given a bit of help from changes in timing, and the rest, it then becomes possible to get improvements ranging from 50- to (in some instances) over 100-percent. This difference is widely appreciated, even by those who know absolutely nothing about the expansion chamber itself and have no direct experience with the device, and that accounts for the brisk sale of accessory chambers as replacements for stock exhaust systems. It also has led many an enthusiast to construct an expansion chamber of his own design.
Sadly, the real result of most people's shade-tree experimental work is simply to discover that it remains possible to bring down on one's head all of the expansion chamber's considerable disadvantages without being compensated by an increase in performance. Or, as I heard one experimenter comment, looking bemused at the chamber he had cobbled together for his motorcycle, “It doesn't make much power… but it sure is noisy.” He was being funny, but I didn't laugh, because the only thing that distinguished him from his fellows was that he was honest about the results; most of the others do no better - but aren't willing to admit that they have made a big mistake.
Where does everyone go wrong? Usually, it is the result of simple, uncomplicated ignorance regarding the inner workings of the expansion chamber, which-all the folklore surrounding the device notwithstanding-are absurdly uncomplicated. Using a mixture of sonic wave behavior and controlled backpressure, the expansion chamber helps pull exhaust gases out of the cylinder during the initial parts of the exhaust/transfer process and hauls the fresh charge into place- and then reverses itself to prevent the charge from escaping out the exhaust port. To illustrate the point, let's watch (in slow motion) the activity through a single operating cycle, from the time the exhaust port opens and through the transfer phase until the exhaust is once again closed. From beginning to end, the process takes only about 3-to 4-thousandths of a second.
THE BASIC PROCESS
When the exhaust port cracks open, gases still under a considerable pressure burst out into the exhaust tract, forming a wave front that moves away at high speed down the port and headed for less confined quarters. After traveling a comparatively short distance, this wave reaches the first part of the expansion chamber proper-which is a diffuser (commonly called a megaphone). The diffuser's walls diverge outward, and the wave reacts almost as though it had reached the end of the system and is, in the manner of waves explained in the first chapter of this text, reflected back up the pipe toward the cylinder with its sign inverted. In other words, what had been a positive pressure wave inverts, to become a negative pressure wave. The big difference between the action of the diffuser and the open end of a tube is that the former returns a much stronger and more prolonged wave; it is a much more efficient converter (or inverter) of wave energy.
As the initial wave moved down the diffuser, the process of inversion continues apace, and a negative pressure wave of substantial amplitude and duration is returned. Also, overlaid on this is the effect of inertia on the fast-moving exhaust gases, and the total effect is to create a vacuum back at the exhaust port. This vacuum is very much stronger than one might suppose, reaching a value of something like minus-7 psi at its peak. Add that to the plus-7 psi (approximately) pressure in the crankcase working to force the fresh charge up through the transfer ports and you will better understand how the transfer operation is accomplished in such a very short time. Obviously, too, this combined pressure differential of almost one atmosphere is very helpful in sweeping from the cylinder the exhaust residue from the previous power stroke. It's all a lot like having a supercharger bolted on over at the engine's intake side - but without the mechanical complication.
Years ago, the exhaust system ended right behind the diffuser. That was the arrangement on the old supercharged DKWs, and we saw stub megaphones used on the Greeves scramblers of the fairly recent past. Those devices did a job in clearing exhaust gases from the cylinder, and helped the fresh charge up from their crankcase, but their vacuuming effect was very much a mixed blessing: their problem was that they didn't know when to stop vacuuming, and would pull a sizable portion of the fresh charge right out of the cylinder. Horsepower being more or less a direct function of the air/fuel mass trapped in the cylinder at the onset of the compression stroke, this aspect of the pure megaphone's behavior was highly undesirable, and the two-stroke engine was not to come into its own in racing (where power is vitally important) until after a cure was found for the problem.
Here, our original wave reaches that “cure”. Following the diffuser, and after perhaps a couple of inches of straight-walled chamber, the wave encounters a converging cone that effectively constitutes a closed end to the expansion chamber. A part of the wave energy will already have been inverted by the diffuser and sent back to the cylinder, but there is enough of its original strength left to rebound quite strongly from that closed end, and it reflects with its original, positive, sign. In due course of time, this wave arrives back at the exhaust port itself, stalling the outflow of the fresh charge. Indeed, it will momentarily reverse the flow there, stuffing what might otherwise have been lost back into the cylinder. The net result of all this activity on the part of the expansion chamber - first pulling and then pushing at the fresh charge to hold it in the cylinder - is a big boost in power. In fact, it is the only thing you can do to a two-stroke engine that will clearly be felt in the seat of your pants; you don't need a dynamometer to find the difference.
As was mentioned before, the expansion chamber is not purely a sonic wave device: Back at the closed end of the chamber there is an outlet pipe, and it is too small to keep the pressures inside the chamber equalized with atmospheric pressure. Consequently, there is an abrupt pressure rise inside the chamber, toward the end of its operating cycle, which is felt at the engine's exhaust port and plays a very large part in preventing charge loss.
This entire process can work wonderfully well - and it also can fail miserably if the various elements of the expansion chamber are not properly dimensioned. All of the various waves and pressure sucking and surging about the exhaust port must operate in agreement with the engine's requirements. When they disagree, the result is worse than can be obtained at a much lower price paid in time and money with the stock muffler. As it happens, the motions of those waves are stubbornly tied to exhaust gas temperature, and supremely indifferent to what the engine would prefer in terms of their arrivals. The time intervals between the initial wave departure, and the return of its reflected components is a function of wave speed, and the system's lengths. Thus, as wave speed is subject only to the laws of physics and exists as something one must simply use without altering, the task of designing an expansion chamber for some particular application is to establish lengths, diameters and tapers that will use the pulsations within the exhaust system to the engine's benefit.
We may start by determining the proper length through the entire system back to the expansion chamber's closed end. That task requires that we know the speed at which sonic waves travel within the chamber, and therein lies a great difficulty. As noted previously, these waves' velocities are determined largely by the temperature of the gases through which they are propagated - and that factor, temperature, various continuously in the course of a single operating cycle. Exhaust gases emerge from the cylinder at about 1200° F and have very nearly (about 800o F) the same temperature back in the outlet pipe. But expansion within the chamber itself cools them (prior to recompression and reheating back in the baffle cone) to perhaps 500° F., or less, in the midsection, and a wave docs not move as rapidly through those cooler gases. It is possible to calculate fairly exactly the temperatures at all points throughout the system, but that is a very complex thermodynamic problem and certainly beyond the capabilities of the layman. Indeed, honesty compels me to admit that it is not a problem I would like to face without a computer and the assistance of someone experienced in that kind of work.
Happily, in this instance it is possible to arrive at a satisfactory solution to the problem by determining wave speed -by starting with the answer and working back. In short, you can measure a lot of existing expansion chambers known to be effective, and by comparing their lengths, exhaust port timings and the speeds at which the engines develop their power, eventually come up with a figure for wave speed representing a workable average for a whole range of high-output engines. My own research, conducted along the pragmatic lines just described, was begun in about 1960 and I arrived at a conclusion in 1964 that has required only slight modification over the succeeding eight years. That conclusion was, and is, that one may use a wave speed figure of 1700 ft/sec in combination with the anticipated engine speed at maximum power to arrive at a system length (measured between the exhaust port window and the point of mean reflection in the cone that constitutes the closed end of virtually all expansion chambers). That figure provides an excellent starting point for the system, as it represents a high average and any error will merely result in a lower-than-projected power peak. Actually, the addition of more examples to my charts in recent years make me inclined to think that something like 1670 ft,/sec is more accurate, but I still use the 1700 ft/sec figure as a starting point, and subsequently shorten the system slightly, perhaps an inch, if tests indicate that the power peak obtained with the chamber is too low.
Using that high-average figure for wave speed (or indeed any figure your fancy dictates, if your findings contradict my own) you may establish the exhaust system's tuned length by means of the following formula:
Where Lt is the tuned length, in inches
Eo is the exhaust-open period, in degrees
Vs is wave speed, in feet per second
N is crankshaft speed, in revolutions per minute
For example, in an engine with an exhaust-open period of 180-degrees, and a power peak at 7000 rpm, and using the 1700 ft/sec figure for wave speed, then,
That length is, I must again stress, measured from the exhaust port window back to a point slightly more than halfway down the baffle cone at the end of the system. The exact point, and how to find it, will be dealt with shortly, along with an explanation of why we use a cone to close the system instead of a flat plate - and how the taper of that cone influences an engine's power curve. First, we'll consider the size and taper of diffusers.