Bisi - "sound wave enhancer chamber" - D-series.org

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Old 09-08-2003, 08:18 PM   #1 (permalink)
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Quote:
Mista Bone
(8/24/02)
sounds like a fancy name for a expansion chamber, similar to what is used on dirt bikes.
Bisimoto - "Actually, Sir Bone is correct. This is a type of expansion chamber. My hypothetical concept is as follows:
As a sound wave travels down the "enhancer chamber", it travels at the local speed of sound. It continues to travel until it encounters a cone at the other end of this chamber. This causes a weaker wave to be reflected up the chamber. Dependent on the length of the chamber, this weaker wave could be timed to reflect once again and "merge" with another strong incoming wave...thus amplifying it and assisting in the scavenging action of my header.
I am in the process of developing a similar apparatus for the individual cylinders. I will keep you posted in regard to my success, or lack thereof.
I couldn't weld if my life depended on it. This system is a collaboration between myself and Elite Motorsports in Montclair, Ca. I hope that helped. Long live the d series!
'Bisimoto D15 "
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Old 09-08-2003, 08:19 PM   #2 (permalink)
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The expansion chamber works for 2 stroke engines but the theory is for the most part inapplicable to 4 strokes since with a 2 stroke you actually want a positive pressure wave timed to push any A/F mixture in the exhaust back into the combustion chamber under pressure prior to the port closing.

What Bisimoto is doing is somewhat different I'm still trying to understand it. I don't think from his description that he intends to have positive pressure reflections return all the way to the exhaust port. Maybe I'm wrong.

Chas


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Old 09-08-2003, 08:20 PM   #3 (permalink)
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Chas, your assumption is correct. I desire to take advantage of the sound wave after my curved megaphone.
I continuously experiment on my powerplant to allow me to remain competitively in the pro stock/all motor racing series series, against those high dollar sponsored DOHCs. This leads me to unorthodox methods of extracting power. Even as an engineer, I refuse to limit myself to the confines of my academia: therefore I resort to different, and sometimes simplistic, methods of increasing engine efficiency.
The chamber was another one of these experiments that I tried of late. Fortunately, I gained a significant amount of power from it. As I mentioned in my earlier post, this is my hypothesis, a stab at how this assisted me so well...and also an explanation to the thought process that went into it.
As for my British Isle friend, I need more time to develop algorithm based designs...as Chas and Bone may agree with me, there is no "one size fit all" for any engine application. What works for my powerplant, will definitely require design modifications to work in yours.


'Bisimoto D15
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Old 09-08-2003, 08:21 PM   #4 (permalink)
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high pressure and velocity pulse from exhaust valve opening event reaches chamber (a). Due to impedence mismatch, part of pulse is inverted (low pressure) and reflected back as in an open header design (b). Remaining pulse is radiated outward into chamber. Part of that pulse (c) is reflected by inverted cone and reaches inside opposite corner to be reflected back outward again (d,e,f). If timed correctly, reflected pulse reinforces the subsequent pulse that enters chamber.


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Old 09-08-2003, 08:21 PM   #5 (permalink)
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Chas...I fear you!
You are absolutely on point. It is a pity you do not reside near me. A sound wave is what I try to utilize in my current design. An exhaust port opening pressure pulse is what I will try to take advantage of in my subsequent experiments...thus forming chambers on individual header pipes.

'Bisimoto D15
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Old 09-08-2003, 08:22 PM   #6 (permalink)
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Thanks Bisimoto. Although I am in So. Oregon now, I grew up in LA and my father is still in Lompoc so the place is not strange to me.

After looking at my crude diagram I can see that the inverted cone could work better if curved in an elliptical fashion such that when looking at it in a two-dimensional plane (like the above picture), one focus is at the exit of the header (entrance of the chamber) and the other at (or slightly in front of) the corner opposite of the reflecting surface. This will take the acoustic pulse radiating outward (in a somewhat hemispheric pattern) and refocus it where you want it. I can see some other tweeks that might also be helpful. This is very interesting theory, and I'm surprised that it actually works.

I wish you the best of success for your new improved tuning system that you are working on.

Best regards,

Chas
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Old 09-08-2003, 08:26 PM   #7 (permalink)
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posted by: D14a1
8/26/02
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Old 09-08-2003, 08:26 PM   #8 (permalink)
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The key to understanding what Bisimoto is doing is to understand the difference between pressure waves (or in this case acoustic or sound waves in free space) and actual exhaust flow. Also understand that the two like to travel differently. My lousy drawing makes no attempt to illustrate that.

Exhaust flow has momentum and doesn't take too kindly to sharp detours. Sharp detours causes turbulence and resistive losses, something you don't want. Exhaust flow if not molested prefers to travel in a straight line since it has momentum propelling it. Sound however will travel in all directions from the its source as long as the source is not significant dimensionally to the wavelengths of sound that eminate from it. I learned this a long time ago when designing loudspeakers. In this case, the sound coming out of the header and into the chamber is in the form of very powerful pulses that will radiate outward in all directions into the chamber. Normally in an open header design this energy is lost to the environment. What Bisimoto is attempting to do is to reflect that power back in such a way that it can be recycled to create more vacuum in the header and therefore more power from the engine. That's the hard part. There is an interaction between the pressure and sound waves and actual exhaust flow. I'll let you contemplate that. Please note that Bisimoto has an engineering degree so if you really want to be conversant in this kind of thing you need to do a lot of research. Even though I have an EE degree, the stuff that Bisimoto is doing is for the most part beyond me.


Notice that there is no restriction to actual exhaust flow, since it's allowed to travel in a straight line out of the chamber.

Chas
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Old 09-08-2003, 08:27 PM   #9 (permalink)
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Bisimoto: Like 2 stroke engines, do your chamber will work in a narrow rpm-band? I mean that at a certain rpm, the sound wave will create a vacuum that will help the flow, but if you are off that rpm-band, the sound wave will create a back pressure doing the reverse effect.

This is the first time I see that applied on a 4 stroke engine. It is really interesting.

I may be difficult for you to understand me, my english is reallly bad... Sorry. If there's something that's not clear, just tell me; I will try to clarify.

Fred Cyr
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Old 09-08-2003, 08:27 PM   #10 (permalink)
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Sorry to overwhelm you D14. I can get obfuscative (confuse the hell out of people). I don't really intend to. It has nothing to do with being smart or stupid.

Bisimoto can beat the pants off many a DOHC VTEC because he can go well beyond simply using bolt-ons in his car. His car is *engineered* to be fast.

Do you need to have a degree to design amazing things? Surprise! No you don't! My brother (RIP) was a phenomenon who designed switching power supplies in his sleep and saved the company he worked for from bankruptcy. He never got a degree.

Chas

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Old 09-08-2003, 08:28 PM   #11 (permalink)
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Heheheh...degrees are for terminology. As long as you understand the concept, you can talk about it, you'll probably just use the wrong language.

I was having trouble understanding how this wouldn't impede flow until the post about how it's just the sound that's getting reflected, not the actual exhaust.

It's an amazingly interesting concept. Just trying to wrap my head around the consequences of the RPM, the length of the headers and the chamber and stuff is hurting my brain without even getting into the shape of the damn thing.

This isn't really the kind of thing you could do as a bolt on, huh?

It'd be so dependent on every little thing about your engine, that it doesn't seem that there could possibly be a way to make it universal, and any approximation would probably render it relatively useless.

Hmmm...thinking about it more...wouldn't the sound pulses sometimes create destructive interference instead of constructive, and cause back pressure instead of suction? Well...I guess it'd actually cause less suction instead of actual back pressure...but still...

Now...if back pressure lowers the torque peak down through the RPM band, and less back pressure raises the peak upwards in the RPM band, it'd be a good thing if you could tune the destructive interference into the low end RPM, and the suction effect into the high end RPM right?

Or am I way off base?

Mojo Jojo Si
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Old 09-08-2003, 08:28 PM   #12 (permalink)
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There is a myth that back pressure is good for low RPM torque. I'll just say that there are many sites on the internet that try to dispell that myth. Do a search if you are curious.

On the other hand, increasing and maintaining exhaust gas momentum does help low-end torque by increasing the scavenging effect. To do that you need to run small diameter but uniform piping. Resistive restrictions that cause back pressure without increasing exhaust gas momonetum actually reduce low-end torque. In short, tuning a chamber to increase back pressure at low RPM's would be counterproductive.

Chas

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Old 09-08-2003, 08:29 PM   #13 (permalink)
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Quote:
Okay...so it's not that the "back pressure" of a smaller exhaust helps low end torque, it's that an excessively large exhaust has too much room for the combustion gases to "fill" leaving exhaust gas velocity slower than it could be with a smaller pipe? - Mojo Jojo Si
There's more to it, but in a nutshell yes.

Chas
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Old 09-08-2003, 08:30 PM   #14 (permalink)
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A new design will be tested in December. I am also almost finished with a sohc "strip" bolt on header for those weekend/street race drag battles. This will be a header with a merge collector that is similar to the one I ran all season, but exits out of the side of the motor...instead of over the tranny. I plan for it to be an easy bolt on for you guys.


'Bisimoto D15
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Old 09-08-2003, 08:30 PM   #15 (permalink)
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Very interesting - very interesting indeed. This seems like a very original idea for the 4 stroke engine. You guys have really been using your noodles

Now, would this system be tuned for a narrow rpm band as mentioned in an earlier post (reference to 2 stroke)? If so, if you are drag racing only (high rpm) you should lose low end power that wouldn't neccesarily be need in this case by having your chamber tuned for the high rpm range. But, there should be a way to make it work a varying rpms either by physically altering the shape per rpm (very difficult) or by the shape of the reflective portion so that it reflects different accoustic pressure levels (dBs) by varying the slope of the curve of exit. At some point though, there would be a shape that would have dimeishing returns for a very broad rpm range. Right?

And dual chambers, each tuned for a different rpm range would probably end up cancelling each other out

Thanks to Bisi for sharing this information - you could have been selfish and kept this to yourself as a "secret" for your own use. We really appreciate it.

Definately food for thought. Not that I will ever completely understand it - but it's yummy anyway.

civicminded92
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