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. 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. 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. 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. It also has led many an enthusiast to construct an expansion chamber of his own design. 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. 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. any of which can impose limits that cannot be totally compensated by even the best of expansion chambers. A lot depends on how good (or bad) their muffler happened to be, and on carburetor size, porting, etc. 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.Īctual percentage improvements between engines fitted with their standard mufflers and the same engines with expansion chambers will vary greatly. 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. 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. 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. All of the various waves and pressure sucking and surging about the exhaust port must operate in agreement with the engine's requirements. 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. 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. Clearly be felt in the seat of your pants you don't need a dynamometer to find the difference.Īs 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.