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I asked a related question about PowerFlow Exhaust systems, which produced another question for me: How do they go about finding the resonance frequency so they know how long to make the exhaust pipe?

I guess I'm interested in the technical aspects of it. What formula do they use? Do they have special equipment?

I think it's interesting that they now offer a "short stack" for some mods:

“Classic” or Short Stack? The Classic is our original design from 1999. It is the “ideal” tuned length for this engine. Everybody loves the performance, only a mother (or an engineer) could love the looks. So, we came up with a compromise: The Short Stack is virtually indistinguishable from the stock Cessna exhaust and will give you about 85% - 90% of the performance gain of the Classic in a much more attractive installation.

Is this one of those instances of the 80-20 rule (approximately)? It seems interesting that a similar length gives you 85-90% of the gains. I'm guess then, that it's not just about the length of the new exhaust, but also the width?

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  • $\begingroup$ I started to formulate a concise answer but it started to get out of proportions. Basically its a combination of primary (header) tube diameter and length, how you bring the primaries together (4 cyl: 4 to 1 or 4 to 2 to 1, 6 cyl: 6 to 3 to 1 or 6 to 2 to 1) and the diameter and the length of the secondary tube. Thats about it for older type aeroplane engines, as there rarely are mufflers. The math is not very complicated, bu there is just too much to write down right now. Optimal flow is difficult, the 85% from optimal is not that hard to accomplish. Stock exhausts are traditionally crap. $\endgroup$
    – Jpe61
    Feb 11 '20 at 19:12
  • $\begingroup$ I went and checked out the answers to your previous powerflow question, and they kinda got it right, yeah, but the principle is way more complicated than just finding a resonance, since each cylinder does not have an individual exhaust pipe. I guess they were close enough to give the big idea. I'll have to get back to this later. $\endgroup$
    – Jpe61
    Feb 11 '20 at 20:05
  • $\begingroup$ Why wouldn't engine manufacturers just automatically put a tuned exhaust on? The prices don't seem like they add too much to the cost. $\endgroup$
    – Canuk
    Feb 13 '20 at 22:16
  • $\begingroup$ That is weird, but that's how it is. Tuned exhaust often means more expensive materials or/and tooling (mandrel vs. crush bend) and since you get satisfactory performance (or got, as these 0-360's are archaic), why go the extra mile... small costs add up in big series. $\endgroup$
    – Jpe61
    Feb 13 '20 at 22:30
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I think there is a misconception with how these systems work. These are 4-to-1 or 6-to-1 header systems, as used on cars, and the primary feature of a header system is that the pipes between the exhaust ports and collector are exactly the same length, which is why, on cars, they snake this way and that, because the port closest to the collector has to have a pipe that is just as long as the port farthest away.

The objective is the make sure the exhaust pulse from each cylinder reaches the collector at an evenly spaced interval to minimize interference between the pulses, and also to have the low pressure behind each pulse help pull the next pressure pulse into the collector (they are sometimes called "extractor" exhaust systems because the pulse timing creates a kind of suction effect).

The timing of each pressure pulse's arrival at the collector is the key factor. To the extent that tuning goes on, it's to make tiny adjustments in pipe length to account for small variations in pulse timing due to different pipe contours - a pipe with more bends may need to be shortened a tiny amount to because its pulse arrives a bit late because of its more convoluted path. In other words the tuning is the ensure the pulses are arriving at the collector evenly spaced.

From a performance standpoint, longer equal-length pipes are better than shorter equal-length pipes because the inertia of the flow stream is higher and this helps evacuate the cylinders better, enhancing the whole suction effect, kind of the inverse of the slight supercharging effect of long intake runners on the intake side.

The short stack Powerflow system gets 85-90% of the gain because it's still a 4-1 header, which is the primary feature; it's just that with shorter pipes there is a bit of efficiency loss from the shorter flow run compared to long pipes. You also save a couple pounds with less piping.

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  • $\begingroup$ Are the primaries the same in classic and short stack? Every discontinuation in the exhaust system sends a pulse of it's own along the pipes: collector, possible mufflers and x-pipes, and the end of the whole pipe. The short stacks secondary pipe's length is such that the pulse timing is not as good as with the longer one. $\endgroup$
    – Jpe61
    Feb 11 '20 at 21:07
  • $\begingroup$ Oh and this is a waaaay better answer than the ones in the previous question 👍🏻 $\endgroup$
    – Jpe61
    Feb 11 '20 at 21:09
  • $\begingroup$ Thks. I think people confuse 4 stroke headers with 2 stroke expansion chambers, where the resonance of pressure wave reflections in the chamber are what's going on. I'm not sure it's correct to say discontinuities send their own pulses, as much as the discontinuities alter the timing of the pulse. In any case, the difference I can see between short stack and long stack is the muffler doohickey is left off but I can't tell if the primary pipes are shorter as well.If you just download the IPC for each version and look at the individual pipe PNs perhaps it'll be obvious what the differences are. $\endgroup$
    – John K
    Feb 11 '20 at 21:16
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You set up a electromagnetic force driver (usually the diaphragm driver from a horn loudspeaker) in the engine end of the muffler and drive it with sine waves of varying frequency. You then put a microphone on the exhaust end and measure the loudness of the sounds exiting it.

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  • $\begingroup$ With this you get the resonance frequency of the muffler, but what does one do with that? $\endgroup$
    – Jpe61
    Feb 11 '20 at 19:58

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