BillFitzmaurice.info

Where to begin, well I have been thinking. I have a hobby interest in hotroding, always have. I have always wanted the most I could get out of my equipment whether sound system or performance enhancements for my vehicle. Atv, dirt bikes, motorcycles cars truck you name it I want to build it to its peak potential.

I suspect many of you here who carry the DIY spirit are in the same boat.

When it comes to exhaust and intake plumbing, size and length constitute flow efficiencies or restrictions. Larger dia plumbing does not always mean more efficiency. Shorter length or larger dia exhaust often hinders performance especially in the lower end of the rev range. Larger dia carb and intake function similarly. Something to do with flow efficiencies. We get to the limit of my claimed expertise quickly here. I see correlations between engine building and sound engineering concepts. Engines are just pumps of a sort and the game is about making them flow. Now I have yet to see talk of resonances exactly but I am pretty sure that the effort to increase flow ability of the inlet and exhaust of an engine starts to overlap a lot with a lot of sound sciences.

I wonder if I may start this thread and try to initiate some thought experiments among the group maybe we can come up with some new ideas. For example, on a typical Harley Davidson v-twin configuration the exhaust systems rarely establish equal length piping which is less than optimal. I wonder if this little issue could be easily resolved by simply using a slightly smaller dia pipe on the shorter runner. Should be able to generate the same port tune in a manner of speaking. My experience building bass reflex speaker enclosures has taught me that the smaller the port the higher the port velocity, there is a point where port noise occurs if the port is too small. There is a range of port velocity that is considered optimum and the dia and length interact with each other.

I have seen some pretty cool systems deployed by PORCHE, and other high end auto manufacturers as well as the sport bike industry with active intake tubes and exhaust valving but Harley in particular seems to neglect such technologies. I own a cheap busted old sportster just prime for some make shift mod-ing. I have been thinking about how to build an intake and exhaust system that is light weight unique and with some inspired ingenuity.

How do I maintain maximum flow but attenuate the spl to reasonable levels? What is the difference between gas flow and the flow of sound waves created through the combustion process of an engine more to follow as I try to develop my thoughts.

Hey Acua,
You posted twice

Air and sound isn't the same as air and engines.

Sound waves don't flow per se, they excite air molecules that pass the sound energy on by exciting the next, then the next molecule and so on.
Since it's not a perfect medium, air will lose sound energy proportional to distance, and we know that will occur at quantifiable rates.
You've already done it. The highest efficiency is from horn designs.
Speaker cabs can be capable of high SPL, but any attenuation stuffs that.
So if you want a lower SPL, build a cab that runs full tilt to produce that SPL, so you don't have to attenuate it. And lose efficiency.

Air flows in engines because it is forced to. Air intake occurs as the piston lowers in the cylinder with intake valves open, they close, compression occurs, add fuel and spark and just before top dead centre, ignition occurs, exhaust valves open, upstroke finishes and the cycle begins again.
Or, put another way; suck, squeeze, bang and blow.
There are inherent savings to be made by manufacturers to not put in the inlet manifolds and exhaust systems (and everything inbetween) to let the engine perform as it could. The compromise is it will perform well enough, yet save millions in production costs.
So most engines aren't as efficient as they could be.

When you talk to old timers about engines lots of things can be done that increase engine horsepower without major mods, such as aligning the spark plug tips in the same direction, electric fuel pump and thermal fan instead of mechanical pump and fan blade.
Changing exhaust is often the first thing many people do, because the engine "breathes" better. Throw in headers as well. Cold air intakes help. There's much more.

Unequal length exhaust tubing will broaden the torque curve, ie, less peak-ish.
Have you looked into bifurcated exhaust? It helps by broadening the torque curve by using longer tubes, and one pipe helps evacuate the other pipe to help increase power output. A resonator at the end of the single pipe could be used to quiet it down, possibly.

If it were me, I'd skip all the small gains to be had where you're looking and go straight to a rotary muffler

By rotary do you mean turbo charging, that does dampen the noise doesn’t it.

I am interested in a baffle design that can efficiently absorb/attenuate spl level without restricting the flow of the exhaust gases. See my attached, there is so much snake oil regarding this topic.

My bike is the rough unpainted job, the other picture is of an bike with an exhaust system that I really like, the issue is that those LAF pipes are just to impractical. How to builds pipes like that that cut 15db and still flow sufficiently!!!!

Like two subs that are out of phase with each other, could one slightly shorter exhaust pipe somehow be manipulated into being 180° out of phase, like noise cancelling headphones? Or more appropriate for the forum: a sub in one pannier could be fed the audio of the exhaust and reproduce it 180° off ala noise cancelling technology.

Now your thinking my language. This kind of thoughts keep me up at night.

ACUA wrote: ↑Mon Jun 06, 2022 5:20 pm By rotary do you mean turbo charging, that does dampen the noise doesn’t it.

Absolutely. They are very effective at muffling... and making horsepower.


^not mine

A lot of this stems from my tinkering with the oem exhaust system on my sportster. I started out with it as used and abused 883cc. I installed a basic 1200cc conversion kit with a capable programmer. I started with tinkering on the exhaust system by gutting it and then using the various parts I removed to establish some experimental baffling. I have attached a crude drawing depicting a variety of configurations that are easy-ish to achieve. It would be fun to be able to hear the sounds the engine makes while actively adjusting the dimensions of the different parts to achieve an optimal desired form factor or a least better understand the possibilities. For a stretch it would be fun to actively dyno the engine at each change just to see how the performance is affected. I realize that of all the ways to pinch extra power out of an engine that this is a low potential return on investment but still my curiosity is peaked.
The drawing I attached is a result of the possibilities I have from modifying or altering the oem parts but the amount of options are just vast enough to make me want to experiment.

I have thought about this as relating to gun silencers as well. Most of the product I have seen in this realm seems to be based off of trial and error until a viable product is identified and not based on a correctly constructed math equation and consistent variables. For this reason it interests me to bring such things up here. I think about the design of the exhaust pipes on 2cycle engines in that they are tuned to bounce a pressure wave back up the plumbing timed to hit the cylinder outlet functionally during the compression stroke to limit bypass for higher compression. Than I think about how two loud speakers 180degrees out of phase cancel each other out and I wonder if we could similarly design a plumbing circuit capable of the same thing in a silencer and an engine exhaust circuit? It would be cool to know what it would take ti do it even it the form factor was unreasonably large or otherwise expensive.

Rich4349 wrote: ↑Mon Jun 06, 2022 7:34 pm Like two subs that are out of phase with each other, could one slightly shorter exhaust pipe somehow be manipulated into being 180° out of phase, like noise cancelling headphones?

That's done internally in a resonator. The drawback is that it has a narrow attenuation bandwidth where it's effective, but that's what you want to reduce drone at the car's normal cruising RPM. Since it has a narrow bandwidth it's used in combination with a muffler, which doesn't reduce levels as much but operates over a wide bandwidth.

One thing I do know about sound engineering and combustion engines, is that it is important for engine performance on a naturally aspirated 4 stroke for purposes of scavenging during the intake and exhaust overlap.
When it comes to 2 strokes such as motocross bikes, specially the 125 cc 2 strokes.. you take that pipe off and the engine falls on its face in power output. Not only does the principle apply for scavenging like on 4 strokes, but it also pulls extra air and fuel into the chamber that goes into the expansion chamber for a split second before the sound wave bounces back and shoves the mixture back in after the intake port has been sealed by the piston just before the piston closes off the exhaust port. There is a reason the expansion chamber looks like a horn lol.

I added a picture of my Honda CR500 with a good view of the pipe for reference.

Subsonicspectre wrote: ↑Tue Dec 26, 2023 2:30 pm I added a picture of my Honda CR500...

That's a sound you can never grow tired of..

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That's a sound you can never grow tired of..
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And I never do this thing produces Music!