Active NSX Exhaust System

[Mac Attack]

* 21 Pounds Total Weight With Catalytic Converter And Electric Valve
* Extremely Quiet And Emissions Friendly (Using Magnaflow Hi-Flow Metallic Catalytic Converter) With Absolutely No Drone In Silent Mode
* Straight Through From Headers To Atmosphere (Bypassing Cat) When In Rowdy Mode

Fine Print:
-I’ve gotten a lot of PM’s on previous iterations of this project asking me to make and sell this. While flattering, I’m sorry - There are absolutely no plans to produce and sell this. You can make your own with all the info below if interested!
-Note that I don’t have a rear sway bar or rear bumper beam. I took it off long ago because I like the handling that way, and it opens up a lot of possibilities for turbo and exhaust routing configurations. Adapting this setup to a vehicle with a rear sway bar would be more complicated, and I forget if the rear bumper beam might be in the way of having the 3” pipe dump out so high.
-Also, this uses carbon steel as I will hopefully have a custom turbo system soon. Not a big issue for me as it is cheaper and easier to cut than stainless and will still offer years of service.



Sound videos first!

Silent, Emissions-Friendly Mode:
<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=13114958&server=vimeo.com&show_title=1&show_byline=1&show_portrait=0&color=&fullscreen=1" /><embed src="http://vimeo.com/moogaloop.swf?clip_id=13114958&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/13114958">Silent Mode NSX Cutout Exhaust</a> from <a href="http://vimeo.com/user1744194">Dave</a> on <a href="http://vimeo.com">Vimeo</a>.</p>


Rowdy Mode:
<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=13115041&server=vimeo.com&show_title=1&show_byline=1&show_portrait=0&color=&fullscreen=1" /><embed src="http://vimeo.com/moogaloop.swf?clip_id=13115041&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/13115041">Rowdy Mode NSX Exhaust</a> from <a href="http://vimeo.com/user1744194">Dave</a> on <a href="http://vimeo.com">Vimeo</a>.</p>



Sound volume readings were taken with a Radio Shack dB meter, on C-weighted scale (32-10000Hz), 1ft off ground, 10ft from exhaust exit. My car is my daily-driver but lacks sound deadening and has a lexan divider window. Therefore, in-cabin comparisons aren’t really representative. This is also why I needed a quiet exhaust for 99% of my driving. I am currently making an exhaust insert to stick in the 2.25” tailpipe of my Silent Mode to make it a little bit quieter.

Silent, Emissions-Friendly Mode (dB)
Idle: 79.5
3000RPM: 96

Rowdy Mode (dB)
Idle: 86
3000RPM: 102

 

[Mac Attack]

Here’s what it looks like under the car:

A nice feature of this design is that it has approximately equal flow losses for each exhaust bank, as opposed to most systems out there that restrict the rear exhaust bank more than the front. I have ‘97+ “headers” that I cut the flanges off of. Saves weight and then I can use stainless steel band clamps that allow some flexibility in mounting to go from my headers to the exhaust piping. They are quick and easy to disconnect, yet seal well. The OD of the double-walled ’97 headers is approximately 2”. I then used two short “reducers” to step up from the 2” to 2.25” stainless steel flex sections for each exhaust bank. The two 2.25” flex sections from each exhaust bank then meet up and are welded to the Flowmaster collector. The 3” outlet of the collector then makes a 180 degree turn, shoots back to the center of the car, makes another 90 degree turn and goes out the back to the electric valve. The valve is strategically positioned where it is in relation to the 2.25” piping takeoff (around 39” +/- an inch to give you some valve mounting flexibility) in order to cancel out the 105Hz frequency that causes exhaust “drone” in the 2000-2500RPM range. When the valve is closed, the 3” pipe acts as a tuned quarter-wave resonator to reflect and cancel out that specific problem frequency. That allows you to still have a relatively-free flowing Silent Mode setup that is quiet and isn’t annoying when driving around town.

To make the Silent Mode part when the electric valve is closed, you’ll need your choice of catalytic converter, piping, etc. I used a 2.25” cat. You could easily get by with 2” piping and 2” cat or whatever. In Silent Mode a single 2” pipe will still offer reasonable performance for daily driving. On my picture, I used a 2.25” pipe that had a 45degree bend to it that was welded on the inside bend of my 3” piping. That was to give me a little more distance (like 1.5 inches) between the 2.25” takeoff to the closed valve when in Silent Mode to cancel out the annoying in-cabin 105Hz drone. From there, I welded in a stainless steel 2.25” Magnaflow metallic cat. The honeycomb cores will attenuate some broad-band sound, and just the expansion and contraction of the cat housings will attenuate exhaust frequencies in the 750Hz range. Then, it makes a 90 degree turn to go out the back and stay in-line with the 3” piping leaving the center of the car.



Some drawings of what the exhaust is doing when in Silent Mode or Rowdy Mode:

 

[Mac Attack]

I did this myself over a few month time period. Please no comments on the welds - it was my first time welding :redface:

I think this parts list is pretty accurate if anyone else is interested. Do your own checking/measuring though (especially if you have a rear sway bar), and make sure you read the SUGGESTED IMPROVEMENTS below:

Summit Racing Online Parts (www.summitracing.com):

SUM-692200 CLAMP BAND SS LAP 2" $7.95 2 $15.90

WLK-41953 REDUCER 225ODX2ID 45 $3.59 2 $7.18
IMM-52015 2.25” ELBOW $20.35 2 $40.70
JEX-FLX21408B 2.25” FLEX TUBE $21.95 2 $43.90
RRP-EC300SREV1 3” ELECTRIC CUTOUT $199.99 1 $199.99
SUM-G4761 WELD ON COLLECTOR RINGS 3" $6.95 1 $6.95
IMM-52021 3” ELBOW $19.67 1 $19.67
SUM-622004 U-BEND 3" $26.95 1 $26.95
DTC-76-006302 ROUND MUFFLER $97.88 1 $97.88

Flowmaster collector (two 2.25” to a single 3”), $40:
http://www.summitracing.com/parts/FLO-Y214300/

I bought two of these cats for another $200 (but only ended up using one):
http://www.summitracing.com/parts/MPE-59955/

You may need some straight 2.25” piping (you probably won’t need it, but it’s only $11):
http://www.summitracing.com/parts/SUM-640022/

I re-used the rubber OEM muffler hangers in non-OEM locations. The system is 21lbs, so you don’t really need that much support.




Cost:
Total in parts will be ~$750. Not shown is all the other mandrel-bent piping, 3” and 2.25” SS band clamps, Borla muffler, Flowmaster muffler, etc. I bought for previous iterations of this setup. I’m easily in for $1200 in parts for my one-off setup. Throw in a nice Hobart MIG welder (with cover gas, helmet, safety stuff, cart, etc), sound-processing equipment/software, and all the time I’ve got in this and you can see the expense that went towards this setup. I didn’t go this route to save money – I went this route because I knew I could design a better system than any other product available that fit my needs. Your needs may be different, but this is the lightest, freest-flowing, quietest, emissions-friendly exhaust available for the NSX. It may not sound the neatest to you, but sound is very subjective. When the valve is open, it should sound very similar to the Taitec S-Tai setup (GT009) shown here:
http://www.youtube.com/watch?v=VA9TToq4iMc&NR=1
http://www.youtube.com/watch?v=eA75m_7VrQQ&feature=channel
The benefit of my design is that after the novelty of a loud setup wears off, you can flip of a switch and don’t have to constantly live with it!

OK – maybe mine doesn’t quite sound like those, but taking into account all the differences in video (sound) recorders, how they processed it, etc, I have to imagine these sound similar. I have four video cameras, and while none of them accurately capture the sound, my Canon 780IS point-n-shoot does the best job surprisingly. The Taitec looks like it has slightly smaller piping (making it a little more restrictive but quieter), and is a little shorter overall (making it’s resonance frequency a little higher). I don’t know what the Taitec canister looks like, but believe it’s straight through.


It would take a good shop around a day to fit up this system and do the welding. I’m guessing that’s about $400-500. Then you need to run some wires and mount the electric cutout switch. Also, you may need a custom diffuser (that’s next on my list). So, figure ~$1300 for this. Not bad at all.

 

[Mac Attack]

Suggested Improvements:
-I used one cat for my Silent Mode, but adding a second cat in-line to the first cat will make the exhaust cleaner and quieter, and just add ~1.5lbs to the weight.
-You may not want the 3” Dynatech muffler either. I just used it, again, since it was sitting around and happened to fit. This will save you another $100 and shorten the piping sticking out the back (it doesn’t matter for me as my diffuser will stick out 1” past the bumper and I’ll cover it up when I’m done). Deleting this will remove another 1.5lbs of weight, but add a little more “rasp” when WOT and the valve is open.
-Use thinner-walled SS piping to make it lighter and longer-lasting.




Finally, these mufflers have recently come out and look promising. If they had come out back in March when I started this, my outcome may have been totally different. I would like to see someone experiment with these, either in a single configuration like I’ve done, or maybe just a muffler for each exhaust bank:
http://www.dynomax.com/mufflers.php?muffler=vt




For even more reading if still interested, these were my previous iterations (the first link shows the sound analyzer and the calculations involved) and what led to the current configuration:
http://www.nsxprime.com/forums/showthread.php?t=134710
http://www.nsxprime.com/forums/showthread.php?t=137126




I hate to sound like Mr. Griot, but Have Fun In Your Garage hacking up and making your own exhaust!

Dave

 

[NetViper]

Oh Man! This is exactly what I want. You are the man.

Someone please produce something like this! Angus, STMPO, SOS!

 

[Mac Attack]

Oh yeah, the rear end isn't finished yet... I'm finishing up the custom rear diffusor and flat rear undertray :smile:

 

[Mac Attack]

Oh Man! This is exactly what I want. You are the man.

Someone please produce something like this! Angus, STMPO, SOS!

Thanks!

Coming from the Corvette world, you're used to this stuff! They have all kinds of neat active exhausts for the C5's and C6's.

I've searched on Prime and couldn't find anyone else who's done something like this It just makes sense....

Dave

 

[NetViper]

Thanks!

Coming from the Corvette world, you're used to this stuff! They have all kinds of neat active exhausts for the C5's and C6's.

I've searched on Prime and couldn't find anyone else who's done something like this It just makes sense....

Dave

Exactly,

The NPP exhaust on the vette is awesome. The Z06 one is event better.

I have no idea what this isn't all over the NSX world. Quiet when you want quiet, loud when you want loud. Best of both worlds.

 

[Mac Attack]

Yeah, with the push of a switch, you can open or close the valve in less than 3 seconds.

 

[NetViper]

Yeah, with the push of a switch, you can open or close the valve in less than 3 seconds.

How hard would it be to adapt a Taitec JGTC to be able to do this? OR does it have to be building from scratch this way?

Use something like this?

http://www.westcoastcorvette.com/p-1226-corvette-mild-to-wild-exhaust-control-c6-z06.aspx

 

[Mac Attack]

How hard would it be to adapt a Taitec JGTC to be able to do this? OR does it have to be building from scratch this way?

Use something like this?

http://www.westcoastcorvette.com/p-1226-corvette-mild-to-wild-exhaust-control-c6-z06.aspx

QTP and RaceReadyPerformance also offer something similar to that for their electric cutouts:
http://racereadyperf.net/product_info.php?cPath=31&products_id=118

The problem with adapting this to a conventional setup (I'm assuming you would want to quiet a conventional setup), is that you would need two cutouts for a dual-exit setup (adds to cost and weight), and the piping necessary to quiet it down. Not saying it can't be done, but each exhaust you would want to modify would be unique and present its own challenges.

Dave

 

[RYU]

Thanks for posting your results! Good work. Very interesting

 

[StmpoRaceProducts]

Oh Man! This is exactly what I want. You are the man.

Someone please produce something like this! Angus, STMPO, SOS!

were already working on the electric by - pass valve exhaust...


http://www.nsxprime.com/forums/showthread.php?t=138074


Stay Tuned...

Regards

 

[cwood1388]

For the record: I love rowdy mode!

 

[RYU]

Dave, have you considered using rpm or vacuum to actuate the valve? For my setup I'm thinking i'd like something to open at around 4000+ rpm and be closed below that or closed during partial throttle and full open when flooring it.

 

[ChopsJazz]

Dave, the underside of your car looks incredible!

 

[angus]

Actually I have thought of building a system based on the AP-X but with no mufflers, this would incorporate a valve at the center of the "X" - not quite the same thing but similar.

 

[Mac Attack]

For the record: I love rowdy mode!

Thanks man, you'll have to stop by sometime (bring your NSX too!)

Dave, have you considered using rpm or vacuum to actuate the valve? For my setup I'm thinking i'd like something to open at around 4000+ rpm and be closed below that or closed during partial throttle and full open when flooring it.

Yes, because the valve takes three seconds or less to open, I originally wanted to tap into the fuel pump relay to start opening the valve anytime above 4000RPM (when the fuel pump goes into high speed). However, the valve wouldn't have a way to close on it's own. I thought that would keep cycling the valve too much and wear it out prematurely (automatically opening it and then manually closing it since my car sees daily use above 4000RPM in the silent mode. You could use something like this ($100) to help control the valve and not stall the electric valve motor when it's full open or closed:
http://www.quicktimeperformance.com/Controller/

Then, because I use RaceReady's cutout (made in USA and lifetime warranty), it would have to be further modified to work :redface: It could be done, but in the end, I just decided to open/close it manually :biggrin:

Now, Ferrari and GM use vacuum-actuated valves which are more reliable and built for repeated cycling. Unfortunately, aftermarket controllers that you can program for your requirements above make the cost for one of these valves around $1000. My electric cutout cost $200 :redface: and it's considered the top-of-the-line in cutouts.

Here's a vacuum-actuated valve:
http://www.hyperflow.com.au/index.php?widget=products&section=-active-exhaust-system-aes&id=hyaes-30ks3

Described in this article:
http://autospeed.com.au/cms/title_Pure-Pipe-Perfection-3-Calibrating-and-Testing-the-Secret-Weapon/A_0893/article.html

Dave, the underside of your car looks incredible!

Thanks, it's not too bad of shape for a '92 daily beater with ~120,000 miles!

Actually I have thought of building a system based on the AP-X but with no mufflers, this would incorporate a valve at the center of the "X" - not quite the same thing but similar.

Angus, your system would be too easy to modify. Keep your system as-is, but at the rear of the x-pipe, you could cut and fit like a short 2" diameter pipe or so that would go to a cutout. It could exit the center (but dump to the ground so it wouldn't interfere with OEM rear valences), or split again and have two more smaller pipes running parallel to your existing mufflers. These could tie into the existing tips, or you could have a quad-tip exhaust.

The problem is that if you don't have a somewhat restrictive setup (or something to tune out the drone around 2-2.5k RPM), then it's really annoying driving around town. Unless the valve is vacuum-actuated, electric cutouts typically have poor lifetimes, and would fail even earlier due to constant cycling. That's why I wouldn't remove your mufflers in exchange for just a valve controlling the flow. My $0.02.

Dave

 

[Mac Attack]

OK, here's some in-car sound samples....

I wasn't really pleased with my videos above showing a hint of rasp (since I can't hear it in the cabin over the road and wind noise). It must be a similar volume of rasp to the 5.1, Bel Canto, etc. since those appear to have rasp outside of the car, but the drivers don't ever seem to mention it

Anyways, first, everyone wants a "F1-sounding" exhaust. Below's a sound analysis of an F1 accelerating. The spectrogram in the lower left is the time history while it's accelerating through a gear, while the FFT on the right is a snaphot in time. Note the discreet frequencies (the fundamental and it's harmonics). I have no idea if this was a 10 or 12 cylinder, or what RPM is was at. But, neglecting timbre, etc, you can see the high pitch frequencies (about 600 and 1200 Hz) that help contribute to this exhaust note:

Next, here's a sound clip of my car in-cabin with the windows down in Rowdy Mode cruising in 3rd gear:
http://www.nsxprime.com/photopost/data/500/open_3rd.wav

Here's the corresponding sound analysis:

You can see the distinct fundamental tone of 225Hz and the 2nd harmonic at 450Hz. From my formulas posted in another thread, I was cruising at 4500RPM here.

Sure, it's not as high-pitched as the F1 cars exhaust note, but it's also spinning at less than half the RPM too. I think this clip around 4500 RPM has an awesome howling sound to it and am pretty pleased :smile:






Finally, here's another sound clip in Rowdy Mode from ~4000 to 8000RPM in 2nd gear:
http://www.nsxprime.com/photopost/data/500/open_2nd.wav

And the sound analysis:

Around VTEC crossover, there is just so much broadband noise from the engine, road, and wind that any distinct engine firing rate frequencies are washed away. It sounds like a race car from in the cabin




Just FYI for those that are interested.

Dave

 

[Mac Attack]

Even though I couldn't hear rasp inside of the car on the setup above, the fact that you can hear it outside makes me want to change it around yet again

I've started collecting data from engines/exhausts that "sound" pleasing. I posted a Ferrari F1 exhaust analysis above, and the picture below is what the Lexus LFA designers were shooting for and how they developed their system.

So, I've been researching how many and what size resonance chambers I need to accentuate our engine firing rate acoustics to maximize certain primary and secondary harmonics.

Let's just say I've placed another order to Summit Racing :wink:

My new setup will retain the old stuff up to the 3" collector. After that, it will be all different. It will also keep a valve-actuated muffler/cat bypass, yet incorporate aluminum piping in non-stressed, low-heat areas, and have three 1.75" diameter exhaust outlets tuned to the secondary harmonics I like (ala LFA)

Stay tuned - this will not only be one of the lightest and freest-flowing mufflers out there, it should also be one of the best-sounding!

Dave

 

[NetViper]

Even though I couldn't hear rasp inside of the car on the setup above, the fact that you can hear it outside makes me want to change it around yet again

I've started collecting data from engines/exhausts that "sound" pleasing. I posted a Ferrari F1 exhaust analysis above, and the picture below is what the Lexus LFA designers were shooting for and how they developed their system.

So, I've been researching how many and what size resonance chambers I need to accentuate our engine firing rate acoustics to maximize certain primary and secondary harmonics.

Let's just say I've placed another order to Summit Racing :wink:

My new setup will retain the old stuff up to the 3" collector. After that, it will be all different. It will also keep a valve-actuated muffler/cat bypass, yet incorporate aluminum piping in non-stressed, low-heat areas, and have three 1.75" diameter exhaust outlets tuned to the secondary harmonics I like (ala LFA)

Stay tuned - this will not only be one of the lightest and freest-flowing mufflers out there, it should also be one of the best-sounding!

Dave

Dave, with all this work, are you sure you dont want to sell a limited run of them? :biggrin:

 

[RYU]

this is my favorite thread on prime

 

[RacingHeart]

Just found this and i thought it might be worth a look for you.

<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/3y_XPqBMqv0&hl=sv_SE&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/3y_XPqBMqv0&hl=sv_SE&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>

Amazing sound, And its interresting to see how much difference an X-pipe makes - even on a Ferrari.

 

[Mac Attack]

Cool video RacingHeart - thanks.

Yeah, using the slightly more restrictive x-pipe only decreased peak power less than 1%, while making the car sound a whole lot better. I think that would be well worth the trade!

On our cars, it seems to help reduce the droning too.



Thanks ryu, hopefully all this will pay off with a lightweight, non-smelly, quiet, yet pleasing tone Silent Mode, and then a stinky, extremely loud, yet pleasing tone Rowdy mode... Is that too much to ask?

Dave

 

[Mac Attack]

All of the exhaust piping in cars has a certain length, usually governed due to space and layout requirements. The NSX is pretty unique because there is so much space right behind the engine to play with and manipulate the sound. Now, the length of all that piping has it's own resonance frequency and contributes to the overall tone of the exhaust leaving the tailpipes. So, all the resonance frequencies due to the lengths of piping from a collector to a cat, cat to an x-pipe, x-pipe to a muffler, muffler to the tailpipe, etc. all sum up in the end. Of course, relative sound volumes will be different due to changes in piping area, and whether or not it will have a muffler or restriction to dampen that frequency later downstream.

An earlier post by ryu in another thread had questioned if the tone of this could be changed to make it nicer sounding. I replied that I didn't think much could be done - only to dampen the unwanted frequencies to make the pleasant stuff come through better. I may have been wrong. What I'm going to try next is a small final "chamber" where all the exhaust dumps into, whether it's in quiet or rowdy mode. This chamber will then have four 1.75" diameter tailpipes, each with a different length tuned to the frequency I want to resonate at various RPM's (highlighted in the spreadsheet).

I don't know if it will work or not, or resonate that much louder than all the other stuff coming out. To keep it simple and make it easier for me, I'll have to modify my existing setup. A sketch is below on what is next.

It will be about 2.5-3 weeks though before I can weld this up and try it out. Some other stuff has come up so this will unfortunately have to wait a bit.

Theoretically, at 3000 RPM, the ~23 in long tailpipe will resonate with the engine firing rates 2nd harmonic of 300Hz. Hopefully, this will allow a higher frequency third harmonic to be heard as well over the rest of the exhaust. Likewise for the other three tailpipe lengths to emphasize the 2nd engine firing rate harmonics at 4000, 5000, and 6000 RPMs. I didn't choose anything to highlight above 6000 RPMs since the rest of the broadband engine noise will drown it out.

Sorry about the crude sketch - this is what I'm trying next. It should only weigh just a little more than my 21lbs now because of the extra 1.75" tailpipe piping. The small 6" long 3" diameter straight-through muffler will be removed.

Dave