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Thread: Noise control and the new exhaust project

  1. #1

    Noise control and the new exhaust project

    This story is about putting a new exhaust on my car and meeting the sound limits for autocross at Pitt Meadows.

    It was time to replace my old broken muffler and when I went to the muffler shop and discussed my goals with the tech, who had performance prep experience. I emphasized that I wanted to take out some weight and not affect power output. My car already has an aftermarket cat, possibly a free flow type, but I don't know for sure. I also suggested that I wanted to upgrade the pipe diameter from 2.25 to 2.5 inches speculating that this might improve flow a tiny bit.

    So we agreed to put a free flow muffler on the car, fabricate a new 2.5 inch pipe from cat to muffler, and delete the resonator. A Magnaflow universal fit performance muffler was chosen and it went on pretty nicely.

    Once completed and when we started the car, my first impression was "wow,that sounds really good". Very deep throated on idle, with smooth sound through the average rev range. My car has a modest sized engine, a 1.8litre 4 cylinder, normally aspirated, that is completely stock.

    My previous exhaust was already louder than stock with an annoying rasp to it, especially loud to the driver, and while I was worried about sound last year, my testing back then showed that we were barely at 87 dB. So I had learned that what you hear inside the car may not reflect what outsiders are hearing.

    Now that I've driven the car around town for two days, the new setup is obviously louder than it used to be and perhaps too loud for Pitt Meadows. Iím worried about it, so, my wife and I did a new sound level test with my Radio Shack sound meter.

    The way to do this yourself goes like this:

    1) Find a calibrated sound level meter. I use the old Radio Shack 33-2050 Sound Level Meter. It was cheap when I bought it 20 years ago and is known to be accurate enough for most casual use. Here's some proof:
    http://www.audioxpress.com/magsdirx/...a/koya2811.pdf

    2) With a helper along, find a road in an open area that mimics the kind of echo and surface that we see at Pitt Meadows. A parking lot might seem ok, but not appropriate for high rev runs in 2nd gear. I recommend finding a country road or unused bit of highway.

    3) Choose a measurement spot over which you will drive the car at speed. Measure away from that spot at 90 degrees to the path of the car, a distance of 50 feet. This is where the helper stands with the meter. It is best if the ground between the helper and measurement spot is paved to get the same ground echo as you would at Pitt Meadows. If it is grass, your measurement may be a tiny bit too optimistic (perhaps by 2 dB).

    4) the meter is set to the 90 range, the weighting switch is set to A and the response switch to SLOW.

    5) the helper must hold the meter, level, at waist height (1m off the ground), pointed to the measurement spot. Hold it steady because rustling of your fingers on the plastic will be picked up by the meter. If you have a camera tripod, you can use it to hold the meter, but we had no trouble just holding it by hand. Donít sweep it side to side, just point it to the measurement spot.

    6) Run your car through the measurement spot at wide open throttle in first and second gear. Try a few different starting points so that you measure different engine speeds as you pass through the measurement point. Just make you best guess about which is the loudest operating point and hit that point when you pass the measurement spot. The helper watches the meter and notes the highest point that the needle reaches on each pass.

    The limit is 92. There is some error in all measurements of this sort, but the amount is hard to say exactly. For us, I think we should consider that if you measure 90, then you have something to worry about, as this is marginal and you may measure over 92 on race day. Thatís how much slop there is in these kinds of measurement.

    So, we did the first measurements on the road in front of my house to get some idea of where I stand. We measured 92. For fun, we also measured with the meterís RESPONSE switch in the FAST position which gives a faster peak response as the car passes by quickly . Compared to the standard method the result will be higher. We measured 94.

    The conclusion to all this is that I need to do something to quiet the car. Thatís too bad. Iím considering a 90 degree bent tip to slide onto the end of the tailpipe. This will force much of the sound to bounce off the ground first. Others on the web report that this might improve things by 2 to 3 dB. Thatís not much.

    Plan B is to have a resonator installed in the exhaust pipe after the cat. I expect this will fix it, but will cost a few dollars thatís for sure. This would also make the car more comfortable to drive on the street too.

    Iíll post again when weíve tried some changes.

    If you are in trouble on race day, some report that they can get some short term improvement by stuffing steel wool into the tailpipe. One comment on this though is:
    ďYes, but when the burning steel wool comes flying out of your exhaust glowing red and on fire rolling across the track you tend to get in trouble with the track officials, as my friend did when he tried this..Ē

    Others use a temporary store-bought slide-on silencer at the tailpipe, which comes in various forms but sort of does the same sort of thing as the steel wool idea, only safer. A small motorcycle muffler might be used this way too, if you can fix it so it doesnít fall off.

    Worst case, you end up with something like this (attachment):
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  2. #2

    Re: Noise control and the new exhaust project

    Hi

    I'm considering taking measure like this, however it's more for my own piece & quite inside. Outside might be fine. I haven't taken any readings but irrespective of whether it's quite enough for Pitt Meadows of not, I've got to do something.

    What I plan to do first is to get a 2-1/2" pipe about 8" long (my exhaust is 3"). I will have one end of this pipe expanded to fit snugly inside my tailpipe. I will close off the smaller end and cut slots on the side. I will push this into the tailpipe, small end first so it will all be inside. I will temporarily clamp the two pipes together to test it. I will go from there, maybe installing this upstream, in the muffler outlet.
    Richard - 1989 S15 Jimmy, AWD, lowered, quick steering, C5 Corvette wheels. "If everything seems under control, you're just not going fast enough." ~ Mario Andretti

  3. #3

    Re: Noise control and the new exhaust project

    Auto Exhaust Science - I believe it is a David Vizard article. A fellow who has some reasonable knowledge on engine sort of things.
    This doesn't put food on your table, it's supposed to be fun. If you're not having fun, you're doing something wrong.
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  4. #4

    Re: Noise control and the new exhaust project

    You've got the right idea on the 90 degree exhaust tip...only point it straight up...not at the ground.

    Pointing at the ground reflects the sound waves...pointing it straight up, dissipates the sound waves.

    Back when I was racing Datsun 240z's in Conference Racing they started to introduce sound measurements. Through trial and error we found that the 90 degree turn up reduced the sound readings the most. It was surprisingly effective. 4 to 5 db was not uncommon.

  5. #5

    Re: Noise control and the new exhaust project

    Quote Originally Posted by Chickenman View Post
    You've got the right idea on the 90 degree exhaust tip...only point it straight up...not at the ground.

    Pointing at the ground reflects the sound waves...pointing it straight up, dissipates the sound waves.

    Back when I was racing Datsun 240z's in Conference Racing they started to introduce sound measurements. Through trial and error we found that the 90 degree turn up reduced the sound readings the most. It was surprisingly effective. 4 to 5 db was not uncommon.
    Your the first that I've read to suggest this. Seems that most autocrossers and road racers either want to turn it down or turn it in a direction away from the sound metering position. Either straight up or straight down both have some merit. Straight down would force the sound waves to bounce off the ground and theoretically then radiate in all directions evenly,while suffering a tiny bit of attenuation at the bounce. This would reduce the directional nature of the sound. Straight up would send the most intense waves, well, straight up, which is good. Might try it and measure the result.

    What do you recall was the length of the vertical part of the pipe after the 90 degree bend? Was it long or short? I have a feeling that is important.

  6. #6
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    Re: Noise control and the new exhaust project

    Why not just add a 6 inch or 8 inch resonator to your piping and call it a day?
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  7. #7
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    Re: Noise control and the new exhaust project

    Quote Originally Posted by SumAznGuy View Post
    Why not just add a 6 inch or 8 inch resonator to your piping and call it a day?
    ya that

    ur exchaust is new & already hitting the sound limit @ this kind of low temperature

    i don't think the 90 degree bent will able to save you once the temperature is HOT or at the end of the season (exhaust tends to get louder after awhile)
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  8. #8

    Re: Noise control and the new exhaust project

    I've now had a 12 inch resonator installed and the car is noticeably quieter and smoother sounding, but still with some attitude. Much more tolerable. A quick test with the sound meter now shows the car at about 86 to 88 dB using the meter in the FAST mode, which I found before was 2 dB conservative in my case. So it looks like I am good to go.

    Thanks to Fabrice for assisting in the measurements.

  9. #9

    Re: Noise control and the new exhaust project

    A bit I know, but this is what I did for my truck.

    2-1/2" pipe expanded at one end. I crimped the end down and cut some slots into the side. Push it into the tailpipe and drilled a hole through to lock it into place. Can't even notice it's in the tailpipe. I can remove it in seconds if needed.

    It's not high tech and certainly within the budget of all. It cost nothing but about 20 minutes of my time. It's taken the raspyness out and the resonance at 3000-3500 RPM and is much more pleasant to drive.
    Richard - 1989 S15 Jimmy, AWD, lowered, quick steering, C5 Corvette wheels. "If everything seems under control, you're just not going fast enough." ~ Mario Andretti

  10. #10

    Re: Noise control and the new exhaust project

    That's pretty neat. When I was chatting with my muffler guy about using an add-on tip, he also suggested cutting slots down the length of the pipe for an effective quick and dirty solution.

  11. #11

    Re: Noise control and the new exhaust project

    How's the Magnaflow muffler working for you? does the universal fit need any trimming?
    Last edited by Fab; December 1, 2017 at 14:46.

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