Here’s the Lightest, Freest Flowing, Quietest Exhaust for the NSX

[Mac Attack]

Well, a combination of those and cost too :wink: ABSOLUTELY NO DRONING AT ANY RPM's (read post below).

It has a muscle-car low-end rumble, a slight hint of mid-range rasp, and flat-out screams at top end:

<object width="400" height="300"><param name="allowfullscreen" value="true" /><param name="allowscriptaccess" value="always" /><param name="movie" value="http://vimeo.com/moogaloop.swf?clip_id=10608517&server=vimeo.com&show_title=1&show_byline=1&show_portrait=0&color=&fullscreen=1" /><embed src="http://vimeo.com/moogaloop.swf?clip_id=10608517&server=vimeo.com&show_title=1&show_byline=1&show_portrait=0&color=&fullscreen=1" type="application/x-shockwave-flash" allowfullscreen="true" allowscriptaccess="always" width="400" height="300"></embed></object><p><a href="http://vimeo.com/10608517">New NSX Exhaust</a> from <a href="http://vimeo.com/user1744194">Dave</a> on <a href="http://vimeo.com">Vimeo</a>.</p>

Forgive the clamps. I'm currently learning how to weld so a lot of this will be redone :redface:

I could have gotten a higher pitch by removing the cats. But I care about the environment. Also, I wanted a center exit for my future custom diffusor instead of it dumping on the passenger side (which it did for awhile when I was tuning the sound).

Dave

 

[Mac Attack]

I recently saved 59lbs by swapping my OEM exhaust with the following:

Headers
(2) Magnaflow 2.25” metallic cats
~1.5ft of 2.25” piping
~4ft of 3” piping
(1) Flowmaster dual 2.25” to a single 3” Y-collector
(1) 6” long, Dynatech split-flow 3” straight-through muffler

As expected, it was loud, and had a very annoying interior resonance between 1900 RPM through 2400RPM.

It was a good-sounding loud, and in-cabin measurements with my Radio Shack dB meter confirmed it was OK for it’s DD duties (outside of the resonance region). As a DD, I do a lot of cruising 40-45MPH in 5th gear, or right in that resonance region.

First, I had to determine what frequency was causing the resonance….
Using my laptop, a good microphone with a frequency range at least 50Hz to 10kHz, and a sound analyzing program (capable of doing fast fourier transforms), this is what a steady 2100RPM sound was inside my cabin:

Specifically:

OK, so it was ~105Hz at 2100RPM. What was the cause of that? It could be the engine firing rate, the cylinder firing rate, or just my new exhaust tailpipe natural frequency due to it’s length.

Tailpipe frequency is the easiest to calculate, mine was either 82Hz just accounting for the 3” pipe, or 145Hz accounting for the total 2.25” and 3” pipe length. It basically doesn’t depend on engine speed – as long as it gets an 82Hz or 145Hz signal, it will resonate and increase it’s amplitude.

Next was engine firing rate: Basically RPM divided by 60 (convert minutes to seconds), and multiply by 3 (for a 6cylinder engine, there are three firing pulses per crankshaft rotation). At 2100RPM average, that turns out to be 105Hz! Cool.

Now that I know what’s causing it (engine firing rate in the exhaust), I can try to attenuate it by putting in a filter.

Unfortunately, 105Hz is a pretty low frequency with a long wavelength. To effectively attenuate it, you need sufficient length (using a quarter-wave pipe or a Herschel-Quincke pipe), or sufficient volume (with a Helmholtz resonator).

Here's a 350Z thread on a similar topic :wink:
http://www.my350z.com/forum/intake-exhaust/474684-advanced-resonance-tuning-art-pipes.html
What they are doing on test pipes:

The easiest way for me to cancel out the 105Hz was to use a straight tube capped on one end. I could have saved some weight with a metal Helmholtz resonator, but I would have had to fabricate it.

The math is pretty easy, I can describe it later. But, my pipe ended up being 36" long. It worked - no more resonance!

Dave

 

[Vic Cardenas]

Very interesting concept. I'll have to look more into this for my particular application.

 

[Tai]

This looks great, glad to see someone actually doing something about what I have always wondered possible.

What could be done to get it to be a little louder/higher pitch at the top end? Could a valve and short pipe be a possible solution?

- T

 

[tbromley]

This looks great, glad to see someone actually doing something about what I have always wondered possible.

What could be done to get it to be a little louder/higher pitch at the top end? Could a valve and short pipe be a possible solution?

- T

^+1, I'm with Tai- especially on the higher pitch side.

 

[Mac Attack]

In short, this could have a higher frequency sound to it by doing three things:
-Remove the cats
-Shorten the exhaust piping
-Remove my little 6" muffler

Other than that, since you really can't increase a particular frequencies volume except by resonance, it would have to be done instead by identifying the frequencies and tones that "sound good" and then reducing the volume of all the rest.

Sound is generated at the exhaust valves. Timing, lift, duration, air/fuel ratio, etc all play a significant part in this initial noise source. Then, you have header/collectors/scavengers, etc that drastically change the sound. Then, the exhaust piping. Material properties (pipe material natural frequency), exhaust length (due to standing waves formed in a particular pipes' length), and exhaust dump location all determine the tones we hear in the vehicle.

After all that, then you can play with the silencers and "tweaking" the sound For example, the 6" long muffler I put on should theoretically silence frequencies 600Hz and higher because of it's small size. It won't do anything for frequencies lower than that. Also, because of it's small volume, that little muffler will only reduce those frequencies around 6dB - not really enough for your ears to distinguish a difference in overall volume when the 600Hz and lower frequencies aren't being attenuated. I could remove it, but it was $90 and it only adds a pound, so I'll just leave it on....

Sure, I could change the sound - but that would involve time, fabrication skills (a lot is trial and error too), and an increase in muffler size/weight.

My goal was to have a light, free-flowing system that gave me a lot of space for my diffusor, was cheap, and had no droning. I was worried about it sounding too much like other Honda wet farts, but was pleasantly suprised for what it was....

Dave

 

[ChopsJazz]

Dave, that's a lot of hard work and good theoretical analysis you've done. Bravo.

I may have simply overlooked it, but is there a significant power gain over other set-ups?

Personally though, I'm not a fan of the sound. Especially the low end sound from a dead stop.

Keep up the good work.

 

[Vic Cardenas]

Could explain the formula you used to get from 105Hz to a 36" tube? You left this out. Is it all based on volume?

 

[Mac Attack]

Could explain the formula you used to get from 105Hz to a 36" tube? You left this out. Is it all based on volume?

Sorry - it's based on what frequency to reduce and what kind of resonator you're going to use to reduce that particular frequency.

There's three ways:
-Trombone style
-Helmholtz resonator (think intake resonator, but metallic to withstand exhaust temps and corrosiveness)
-Quarter wave tube (simplest and what I did)

All three have unique pros/cons in their complexivity, manufacturability, and effectiveness (what range of frequencies they'll attenuate and by how much).

I did the quarter tube. Basically, the exhaust pulse goes down your tailpipe. When it gets to this side branch, the pulse splits - a sound wave goes down the side branch, and another sound wave continues out the tailpipe. There's no exhaust "flow" through the side branch since it's capped on the other end. It therefore presents really no restriction to the exhaust flow - it just cancels out the sound. This is what Corsa uses in their mufflers and auto manufacturers use in resonators.

If the side branch tube is sized correctly, the reflected wave leaving the capped side branch will be 180degrees out of phase by the time it goes back to the branch - therefore canceling that frequency out.

The formula to calculate the length is:

L = v / 4f, where

L = Length of capped side branch in feet
v = speed of sound in exhaust gas (feet/second)
f = your desired frequency to reduce

So, you need to know your problem frequency first.

A quick online search said speed of sound in exhaust gas was ~1150 ft/sec. I back-calculated 1250 ft/sec after everything was done. This is a function of exhaust temperature. As there is no exhaust flowing through this bypass, it stays pretty cool (I can touch the capped end after hard driving with no burning). So, go with 1250 ft/sec starting out.

The factor of 4 is due to it being a quarter-wave resonator :wink:....

So, let's say you have a problem frequncy of 105Hz at ~2k RPM.

Your capped side branch length should be:

L = 1250 / (4*105) = 3 feet.

There's a little more to it.... Your side branch should be the same diameter as the exhaust pipe for maximum sound reduction. Mine's not since I didn't want to totally elliminate the 100Hz frequency. Engine firing rate is what gives an exhaust a powerful sound. My 2.25" side branch got rid of my resonance problem, so I didn't feel the need to go any larger. I could go smaller to save weight, but that's for the future....

Also, to attenuate a wider frequency range (but not as effectively cancel those frequencies), you can stuff steel wool in the end of the side branch. Kinda like stuffing filler in speaker boxes to make them "appear" larger.

I would like to trim another inch off my side tube, and then I may stuff a little steel wool in there too.

The nice thing about this is it's cheap and effective. Anyone can do this.

Dave

 

[Mac Attack]

Dave, that's a lot of hard work and good theoretical analysis you've done. Bravo.

I may have simply overlooked it, but is there a significant power gain over other set-ups?

Personally though, I'm not a fan of the sound. Especially the low end sound from a dead stop.

Keep up the good work.

Thank you. I had to do it because I don't have $2500 to spend on a muffler, especially when this will have custom twin-turbos on it soon.

I'm personally not a big fan of the sound either, but it's a compromise between sound, weight, and space. The nice thing about this setup is that I can change the sound pretty easily by putting in a conventional (but short) muffler like Borla, Flowmaster, Spintech, Magnaflow, etc. That will add weight though, and it's pretty much a $150 for every muffler you try out. Sound is pretty subjective so you could go through a lot of mufflers before settling on the one you like.

I will say that I was relieved after first fireup. I was worried it was going to sound like other Honda wet fart setups.

There should be a significant gain over other setups since it is basically a straight-through 3" exhaust. A little low-end torque is lost compared to more restrictive setups, but it should provide more than the others at higher RPM's (like 5k and above).

The best of both worlds would be an active system with a vacuum-controlled valve to open a free-flowing path.

Dave

 

[RYU]

This is a good technique to control the tone of the exhaust. Thanks for proving something i've wanted to experiment on for a while. You did a lot of hw there! HKS uses this method on their 350Z exhaust also. The 'Bosozoku' movement in Japan takes this to an extreme by adding multiple bypass cannisters of different sizes in-line to make the exhaust notes super super high pitch but very tinny.

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HKS for 350Z

If someone would tune a prototype version so the NSX can sound like an F1 car that would be interesting. I'm not sure how much turbulence this creates within the exhaust system though. It's the same concept as having those multiple bypass chambers in our Honda intake tubes.

Great effort and way to go on applying the concept.

 

[95ttht]

^^^

LOL at the peacock car!

 

[Mac Attack]

That's pretty wild - They sound like two stroke engines almost. I had heard of this movement about 5 years ago but never followed up on it. Thanks for the link.

This is a good technique to control the tone of the exhaust. Thanks for proving something i've wanted to experiment on for a while. You did a lot of hw there! HKS uses this method on their 350Z exhaust also.

HKS for 350Z

If someone would tune a prototype version so the NSX can sound like an F1 car that would be interesting. I'm not sure how much turbulence this creates within the exhaust system though. It's the same concept as having those multiple bypass chambers in our Honda intake tubes.

The HKS picture above and the cat bypass pipes I posted above use little Helmholtz resonators. The benefit of these are a greater volume reduction in unwanted frequencies, but the range of frequencies they'll attenuate is smaller than a quarter-wave resonator. Another benefit of the Helmholtz resonators is the packaging - you can make them into almost any shape. What's important is the neck area, neck length, and total resonator volume.

However, when you hook up a bunch of these branch resonators, you need to have them spaced properly or they won't work effectively.

Also, another problem on cars with dual exhausts is that you need resonators on both exhaust pipes. That adds weight, and is why I combined mine to a single exit in the middle.

Dave

 

[EdinKali]

Here is an exhaust that is even cheaper and lighter:

http://www.summitracing.com/parts/SUM-G4825/

Only $45. :biggrin: Probably would make the NSX sound like an 8K RPM tractor but both tips probably weigh about a pound total.

 

[RYU]

but those would drone like crazy :wink:

Here is an exhaust that is even cheaper and lighter:

http://www.summitracing.com/parts/SUM-G4825/

Only $45. :biggrin: Probably would make the NSX sound like an 8K RPM tractor but both tips probably weigh about a pound total.

 

[EdinKali]

but those would drone like crazy :wink:

Got a cheap fix for that too. Foam earplugs.

 

[Mac Attack]

Here is an exhaust that is even cheaper and lighter:

http://www.summitracing.com/parts/SUM-G4825/

Only $45. :biggrin: Probably would make the NSX sound like an 8K RPM tractor but both tips probably weigh about a pound total.

I predict that will soon be the Taitec FN-20 when all the pretzel hype dies down :biggrin:

 

[EdinKali]

I predict that will soon be the Taitec FN-20 when all the pretzel hype dies down :biggrin:

Haha :biggrin:

 

[new2club]

where did you buy the materials? I want to try this too! But no muffler?

 

[cwood1388]

All I have to say Dave is wow!

Hats off to you and all the homework. I thought you were just making an exhaust. LOL!! The sound down low in the RPM range is nasty! Cant wait to get a ride in this thing!

 

[RYU]

Dave, to theorize a bit, and setting aside weight reduction priorities, do you think smaller diameter piping or perhaps a tighter Y-pipe can get rid of that low RPM rumble? Actually the rumble at low rpms kinda sounds like an open header. What do you think would need to be done to get rid of the low octave completely?

Perhaps another pipe, but as you said neck placement and length will probably be the tuning touch points.

 

[A.S. Motorsport]

sounds great!
if it where to have twin end tips it could look very cool. (in a custom valance or with diffusers)

 

[CL65 Captain]

When are you guys going to stop using the term "F1 sound"? Unless you can get the NSX to turn 23,000 rpms it isn't going to sound like a F1 engine no matter what you do to the exhaust!

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[A.S. Motorsport]

When are you guys going to stop using the term "F1 sound"? Unless you can get the NSX to turn 23,000 rpms it isn't going to sound like a F1 engine no matter what you do to the exhaust!

+1 thats very true a stock or bolt-on NSX won't sound like a F1 car only with high end engine tuning are you going to get it (ending up with GT500 Sound) F1 is still a bit different.

maybe idea to combine with STMPO's F430 rear end

 

[slebidia]

where did you buy the materials? I want to try this too! But no muffler?

I have this setup for autox and track days. I'll try and take a pic later.