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Thread: Noise control and the new exhaust project

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    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:

    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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