Well I finally dug it all out, looked it over again, changed a few things slightly, and wrote it all out. I know a picture would help greatly here, but this'll have to do for now. I've converted everything to inches since most of us will be dealing with that, especially when getting pipe.
The chamber I'm envisioning would be a "two-stage" expansion chamber, which would essentially have two cones at the engine end, instead of just one.
Here are the spec's I've come up with. These are based on an LJ20 engine, but should be the same for an LJ10 as well. There are a total of six sections, consisting of the header, 1st divergent cone, 2nd divergent cone, belly (the largest diameter part of the chamber), convergent cone, and stinger.
1. Header - this is the part that comes directly off the exhaust port. In most cases, it begins with the exhaust manifold. For a true two-stage expansion chamber, a custom header would need to be built, as this section should only be 2.5" long, and use 1 5/8" pipe. The pipes coming out of each of the two ports should be exactly the same length, so it'll need to be a true "Y", not angled backwards. (note: better check for clearance issues here!) If one is longer than the other, then one piston will have more power than the other, and will thus be forced to "pull" the less powerful one.
2. 1st Divergent Cone - most, if not all, of this would likely need to be incorporated into your header, as the 2.5" isn't even enough to converge the two pipes into one. The length of this section is 10", and it is a very slight angle: the pipe diameter would only go from 1 5/8" up to 2", a difference of only 3/8" over a 10" span. It may work to have the cone only 6" long, with a 4" long piece of 2" flexpipe at the other end (remember that flexpipe?), I'm not sure how much effect that would have on power output.
3. 2nd Divergent Cone - This cone would take the diameter from 2" to 3", and be 3.5" long (measured longitudinally, not by measuring a side of this cone!
4. Belly - the belly of this chamber is simply a 2.7" length of 3" pipe.
5. Convergent Cone - This cone brings the diameter from 3" down to 1 1/8" (1.125"), and the total length would be 5.9". Again, this length is measured not by a side of the cone, but the length from the center of the middle hole to the center of the output hole. In other words, the measurement would be taken with the large hole flat on the ground, and your measuring stick inside the cone measuring the distance to the smaller hole.
6. Stinger - This is a 5.9" length of 1 1/8" pipe on the end of the convergent cone. The small diameter of the stinger allows the backpressure necessary for the expansion chamber to work. This backpressure creates a denser environment for the sound waves in the chamber. Behind the stinger, you can run whatever exhaust you want without affecting the power, as long as it is not more restrictive than the stinger.
The overall length, measured from the center of the pipes/cones and including the bends for the exhaust manifold, would be about 24.5". Based on my calculations, this exhaust would allow an LJ10/20 engine to create 39hp (as opposed to 27-28 stock), with an average exhaust temp of about 700*F (380*C), assuming your LJ engine is perfectly tuned and ported and your carb is perfectly jetted for this chamber. That's doubtful, but I would expect at least a moderate gain by using a two-stage chamber, as well as a much broader power curve. (Remember, even an additional 3hp is a 10% improvement!). My thinking is to run 1.5" exhaust behind the chamber, which is about stock, and just run it all the way to the convergent cone, thus covering up the stinger completely.
I should point out that this is purely theoretical; I've never tried it, but the formulas are similar in the various books and websites I've read, so according to theory it should work. I'm hoping to put one of these together over the winter, but I can already see that time will be an issue getting the V ready for next year's get together in Colorado, so I'm posting it here in hopes that someone else has more time to try it.
Finally, in response to other questions, everything I've read indicates that it doesn't matter what you put behind the stinger, as long as it is less restrictive than your stinger. Thus, if you cut the rest of your exhaust completely, your LJ might be a little louder but the power output should be about the same.
I'll see if I can make up a diagram and scan it, as it would probably make this much clearer!
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