Texas Two Step Taco

John Murray

Been Around the Block
View attachment 230588
So Frits Overmars commented on this picture of an expandable two stroke chamber on Facebook but it and it needs to be saved and archived on a forum so I am adding it here. The following is his comment: I've posted the following text before, but judging by the above reactions not many have read it, so here it is once more:

There are several options in lengthening an exhaust pipe. You can move the end cone, or you can lengthen the header, like a trombone.
The gas pressure generates a force that is proportional to the cross section area of the moving part and proportional to the pressure difference at either side of that area. For a moving end cone this force can be up to 4 times larger than for a sliding header. That is one reason to go for the trombone system rather than the moving cone system.

The second reason: sealing. The circumferential gap that has to be sealed, is three times shorter for the trombone system. That means three times less leakage and three times less friction.

The third reason: say you wish to lengthen the total length of the pipe by 10 %. If you do it by moving the end cone, you will also enlarge the pipe volume by a little over 10 %.
But in a good pipe configuration the header length is about 1/3 of total pipe length, so in the trombone system, lengthening the pipe by 10 % will be done by lengthening the header by about 30 %. That gives a far greater variation in the pipe's Helmholtz frequency than a 10 % volume change.

It is true that the length percentages of all pipe components should be in a rather fixed relation to each other. Varying the lengths of all components by the same percentage would be the theoretical optimum, but that is not feasible.
Lengthening the belly will disturb the optimum relations, as will lengthening the header. So the pipe in its lengthened version will not be the optimum for the low resonance rpm dictated by the length. But it will be a hell of a lot better than using an exhaust power valve that spoils the 180° effective exhaust timing, necessary for true resonance.
And a pipe shortened beyond its optimum may not show the optimum length relations between its components either, but it will be a lot more effective in overrev than artificially raising the exhaust gas temperature by retarding the ignition, or by weakening the mixture strenght through closing a power jet, which has the disadvantage that not all inhaled air is used for combustion.

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I've seen some engines using variations of the variable length chamber - mainly karts and outboards. Which kinda makes sense with applications like these that lack a sufficient spread of gear ratios, or any at all. It's not something I'd want to do unless there were no better alternatives.

It reminds me of something I discovered by accident a few years ago. I was running the 370 Pursang engine in the Metralla on the street and was trying to quieten it down - it was obnoxiously loud with a garden variety packed silencer. I'd made quite a few different types of silencers and was testing them to see if I could find something better. Anyhow, one that I built - a baffled design - was a total failure at noise reduction, but what got my interest was the effect it had on the performance below the powerband. The engine was very noticeably stronger and smoother below the powerband, but a bit weaker when on the pipe. It turned out that the baffled muffler flowed better than the packed ones - I later discovered that the packed mufflers can flow very poorly with the pulsating flow, much worse than the straight-through design might suggest. The reduction in backpressure slightly reduced the top end but improved the bottom end by impeding the pipes ability to function out-of-phase when below the powerband. I was never able to make the baffled muffler both quiet and free flowing but it made me wonder whether a dual outlet, dual muffler pipe might work. At speeds below the powerband both would be open, and the lack of gas density in the pipe would make it relatively ineffective at reducing power, while once on the pipe the larger primary outlet would close off, allowing full power. I've haven't tried it though..

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