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little late with a response, but i don't think the vibrant ultra quiet will fit in the mid pipe section. I considered this also with my last exhaust system but the muffler shop did not feel it would be a good fit, it would have to turn sideways so that it's longer side is facing up/down which could possibly hit speedbumps, entrance ramps, etc. i ended up fitting it near the muffler and placing a bottle resonator in the midpipe.

Once again you've probably already made your decision by now but figured i would say it anyways.
 

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Assuming a FA5 Sedan see below, for a FG2 add approximately 100-150 RPM to the below to account for the shorter exhaust length.

A 2.5" Exhaust with Race Header, 18" resonator, and a TwinLoop has a resonance of about 3000 RPM (high way drone), but is reasonably quiet at all other RPM's.

A 2.5" Exhaust with Race Header, 18" resonator and a Straight through muffler has a resonance of 3700 RPM and is fairly quiet on the highway, but much louder any load above cruising.

A straight through resonator does not affect the resonance RPM, simply the volume at the resonance frequency.

The exhaust resonance is determined simply by the length and diameter of pipe. The longer pipe will have a lower resonance, hence the twin loop virtually extending the pipe length and resonating at a lower frequency.

Drone can be notched out using a 1/4 wave length reflection resonator, or a "Helmholtz" resonator, which is essentially a pipe with an end cap cut to length of a 1/4 wave length of the frequency you are trying to eliminate (Determined by the speed of sound at ~140 Celsius which is the average exhaust temperature throughout the length). The noise is simply relocated from that drone point which is now notched out, to a point of 2x the current frequency. IE The car will be much quieter at 3000 RPM but much louder at 6000 RPM.

Steve
 

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Discussion Starter #47
I too am interested in what Steve-BV1 has to say on this. Steve's post is an epic one, one that should be read and studied by anyone contemplating buying an exhaust or changing the one they have due to sound/drone issues.
 

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More info as requested:

To change the resonance your system you will have to mount a pipe off to the side, a side branch resonator, somewhere in the piping between the resonator and the exhaust. The location is not critical but rather ideal between the resonator (if you have one) and before the exhaust. Lets go with a 2" pipe. Why 2"? The diameter of the resonator pipe will affect how much "drone" is to be cancelled. Larger is more, however 2" will remove ~70% of your noise and it will fit inside the 3" pipe. IE your welder will like you. There will be a cap on the end of the pipe.

The length of the pipe should be calculated using the speed of sound, ideally we design for a 1/4 wave length of the frequency to be cancelled (dis-regarding all the theory).

First lets calculate the speed of sound. Because the speed of sound changes with temperature you should assume an air temperature of ~100C. Why not 500C? Because your exhaust temperature at the tip (where the drone leaves) is not 500C. If it was, it would be glowing bright red. Usually its very hot to the touch (50C) or maybe burning to the touch (70C), but often not my skin bubbled and fell off (100C). So lets assume 100C to be safe, worst case scenario. The air temperature will be VERY close to the exhaust temperature. So if the exhaust tip is 70C, the air will be very close, not over 100C for a standard length exhaust system.

The formula to calculate the speed of sound vs temperature is:

v = 331m/s + 0.6m/s/C * T (331m.s is the speed of sound at 0C at sea level)

So skipping the math at 100C the speed of sound is 391 m/s. Second you need to figure out your drone frequency. So lets say at 3000 RPM it drones. Ok so 3000 revs per minute = 50 revs per second = 25 exhaust pulses per second (4 stroke so divide by 2) = 100HZ (25HZ*4 Cylinders).

So 100 sine waves (per second) spread out over 391 meters (per second) = 3.91 meters per sine wave = one wavelength.

3.91 meters / 4 (1/4 wave length)= 97 cm = 1/4 wave length.

Thats a long pipe! Well ideally you would have almost no drone if you used a 97cm side branch resonator. Reality check! Thats not gonna happen in the real world.

So be sure to use harmonics of 97cm. Like 97cm/2 or 97cm/4 (24 cm). This will not be a complete noise cancelling affect as the cancelling sine wave will be shifted left or right and not entirely cancelling the wave at 3000 RPM, but it will still make a huge difference. A 10% reduction will sound a world of difference, a 30% reduction and you won't think its the same car.

Sound/Noise is calculated in db, it is logarithmic, and every 3db is a power factor of 2. So even if you only drop the drone a few db, it will make a huge difference in over all drone.

Good luck, and theres no warranty on my math :)

Steve
 

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Discussion Starter #49
Thank you Steve. So, are we talking about a "T" type of fitment for the side branch resonator? I am trying to visualize this. After chatting with you, I searched the internet and found a thread on a Mustang forum that seems to parallel what you are saying:
Exhaust Drone is Completely Gone - Corral.net : Ford Mustang Forums

BTW, here is an exhaust installed on an Si:

 

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Thank you Steve. So, are we talking about a "T" type of fitment for the side branch resonator? I am trying to visualize this. After chatting with you, I searched the internet and found a thread on a Mustang forum that seems to parallel what you are saying:
Exhaust Drone is Completely Gone - Corral.net : Ford Mustang Forums

BTW, here is an exhaust installed on an Si:

image
I dont beleive it works the same for a mustang, not 100% sure. I say this because when I had my cobra with a dohc 281 I had longtubes, race x-pipe and magnaflow mufflers.... but never had a drone issue. but now with my gopower sutup I get drone at 3k and a little upwards while highway cruising. I think the dynamics of the exhaust is different becuase it is a V8 with a not so well designed head. but I am no pro. dont quote me :p
 

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Discussion Starter #51
^ I am just checking to see if what the guy did with the Mustang is the same concept/principle that Steve is talking about. Sounds to me that resonance can be changed by placing a pipe at a 90 degree angle to the exhaust pipe. The longer the resonator pipe, the lower the resonance.

I grasp the rationale behind what Steve is saying. My only concern is implementation. Given the very restrictive area between the resonator and muffler, I wonder where the side branch resonator could be mounted?

I must say too that Steve's post takes us from conjecture to pure science. When dealing with an issue like this, science/math wins the day over opinions. Now I wish I had paid more attention in my Algerbra, Trig and Calculus classes LOL.
 

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Steve, what exactly do you do for a living? :D

I just spent over 4 hours in the library studying for physics tonight.. that post brought me right back to it. Very interesting stuff.
 

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Help me fix my exhaust

Alright. Basic situation I've got is that I just bought a 2004 Civic SI 3 door. It came with some aftermarket parts (or maybe not, I don't have a maintenance history) of which the exhaust has become a problem.

I want to make this thing quieter, and I'm looking for help on how to do that. I was looking into silencers, but most of the ones I see promise a 4-7 decible reduction, which I doubt is anywhere near enough. I want my car to sound relatively normal, and am willing to replace the muffler with something else if necessary.

At this point I don't know what I have, I can try to figure it out if someone would like that information, or take a few pictures to post up. I'd really like to get away for a hundred bucks including installation or less, I can go higher if necessary.

I'd like brand suggestions, general information about the effects of what I'm looking at, anything you think a guy who doesn't know the muffler/silencer he wants from a hole in the ground would need to know. I saw a 90 dollar flowmaster that looked like it might be what I need, I've seen a bunch of other stuff, I just don't know what I'd be smart in picking.

And to specify what I want it to sound like, the closer to nothing at all, the better. I don't want something that's going to be a power drain, but I'm not looking to add horsepower either. If something is better on fuel efficiency though, that'll catapult it alot higher on the want list.

Specifics I'd love

- Cost of installation
- Brand or specific muffler/silencer recommendation
- Where do I get it installed if I need professional installation?
- Where should I buy whatever parts? (I don't want to arrange deals through forums and such, if at all possible I'd like to walk into a place that I can buy it from and get it installed by)
 

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buy a universal twin loop from ebay. I bought an OBX but there are others available. Cost me something like 80 bucks if I remember correctly and then 40 bucks to install at a muffler shop (meineke). No need for a silencer since its real quiet and no performance loss.
 

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buy a universal twin loop from ebay. I bought an OBX but there are others available. Cost me something like 80 bucks if I remember correctly and then 40 bucks to install at a muffler shop (meineke). No need for a silencer since its real quiet and no performance loss.
Alright. Very much appreciate the suggestion. I've read too many negative reviews of twin loops and their tendency to start getting louder/fall apart quickly to want to go that route, but it led me to alot of new information.

My current exhaust is an OBX. It's a straight through cylindrical muffler with a matte pipe running all the way back to the engine. I don't see anything that looks anything like a resonator, unless someone would attach one pretty much directly to the engine where I couldn't see it from my peeks under the car.

I'm seriously considering dealing with the noise until I can afford the Apexi WS2. I'm kind of a stickler for getting my moneys worth and even though I'm not all about tricking out cars I'm seriously thinking about saving up for and investing in that. My inability to find a stock exhaust anywhere that I'd consider a good deal is leading me to look into non-stock.

So new questions. Getting the Apexi muffler only vs getting the full exhaust system vs getting the apexi muffler and a different resonator than the one they sell with the full exhaust system. Where do the cost effective decisions lie, considering that I'll probably have to get help to install it even if it can all be bolted together?

If I got only the muffler and had it installed, would it be quiet at all, or do I absolutely need a quiet resonator? If I need the resonator, would I do better to just get their full exhaust system or to get the muffler, resonator, (do I need any other parts if I buy it a-la-carte?) and have it installed and welded?

Which would be quieter, which would be cheaper, any help appreciated.
 

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After this weekend when I made the video of my exhaust doing cruising and full throttle pulls. I noticed that my sound cruising between 3200-3700 has dramatically drawn down coming back from my girlfriends parents house to my house. It's about a 45 min. drive. Even my girlfriend noticed the change and we didn't have to rise our voices to speak. The stereo was between a comfortable 7-10 the whole way and even her cat didn't bug out as much as before the weekend (when I did the video) to the drive back.

I think it's just the whole breaking in process of the high flow cat. It's so much more comfortable to drive now. However, I still do leave the sunroof at tilt to draw the vibrations/noise out just incase.

Exhaust mods: Go Power 575" w/ 3" collector. 3" Vibrant metal core high flow cat. 3" Vibrant exhaust (ultra quiet resonator and turbo oval muffler)
 
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