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Sept 23, 2012 21:16:55 GMT
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I've spent weeks of my life googling this and asked many questions on this subject before, but I'm still stuck. On my van I've got an exhaust. It's fairly short, probably 2.5m long from cylinder head to tailpipe. It's got a pair of powerflow type stainless straight through silencers - they are as big as I could fit in. 6" diameter, both 18" long. Long story short:  See the crudely bolted on 6" long bit of 2.25" pipe on the end? I bolted that on, and it turned it from being insanely loud and doing huge great backfires on gearchanges to being almost acceptable. After spending stacks of money to stuff loads of silencers in and for it to still be mega loud, adding 6" of scrap pipe to the end and almost halving the noise has ruffled my feathers! I kind of understand that the length of a tailpipe will tune it to a certain frequency, like a port in a sub box. If I just keep making this pipe longer will the exhaust get quieter and quieter, or is it a case of hitting a certain length for ultimate quiet, and hit the wrong length and it'll be mega loud again? My Lupo had just one of these exact same silencers (which I have two of) as a center box and then one mega long pipe from that up to the back end and it wasn't a lot louder than stock. |
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Last Edit: Sept 23, 2012 21:22:06 GMT by cobblers
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sowen
Club Retro Rides Member
Posts: 2,245
Club RR Member Number: 24
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Sept 23, 2012 22:24:48 GMT
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There's lots of factors to take into account but I've found that putting some gentle 900 bends in the exhaust has a good silencing effect equivilant to putting in extra straight through silencers.
I think the best thing to do is experiment until you find something that gives the effect you want on your particular car which you have, and for some reason extending the tailpipe has worked on yours!
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Sept 24, 2012 0:20:19 GMT
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I kind of understand that the length of a tailpipe will tune it to a certain frequency, like a port in a sub box. If I just keep making this pipe longer will the exhaust get quieter and quieter, or is it a case of hitting a certain length for ultimate quiet, and hit the wrong length and it'll be mega loud again? I think there will be a sweet spot after which it will start to get louder again, but three or five times that length or any odd multiple will also be more or less as quiet. (This length may or may not include a portion of the inside of the back box.) |
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Sept 24, 2012 8:05:08 GMT
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The back box fell off my Volvo not long after I bought it and the car didn't get any louder at all. I temporarily replaced it with a bit of straight pip and the note changed, it got deeper and at 2500rpm the car was a bit boomy on the motorway.
Exhausts are not all about volume of the noise.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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Seth
South East
MorrisOxford TriumphMirald HillmanMinx BorgwardIsabellaCombi
Posts: 15,514
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Sept 24, 2012 8:19:45 GMT
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Might be to do with the relationship of the tailpipe and the rear valance. A lot of the sound might have been reflected back towards the car whereas now its getting thrown straight out the back. I have experienced this with the Hillman.
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Follow your dreams or you might as well be a vegetable. |
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Sept 24, 2012 8:22:55 GMT
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I think the answer has to do with the length of the pipe in relation to the wavelength of the exhaust note and the pulse length of the exhaust stroke. However other than having picked this up off David Vizzard or some such reputable source I have no idea how one would put this knowledge into practical use
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1937 Austin Street Rod - 1941 Wolseley Not Rod - 1956 Humber Hawk - 1957 Daimler Conquest - 1966 Buick LeSabre - 1968 Plymouth Sport Fury - 1968 Ford Galaxie - 1969 Ford Country Squire - 1969 Mercury Marquis - 1970 Morris Minor - 1970 Buick Skylark - 1970 Ford Galaxie - 1971 Ford Galaxie - 1976 Continental Mark IV - 1976 Ford Capri - 1976 Rover V8 - 1994 Ford Fiesta
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Sept 24, 2012 9:55:56 GMT
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As above, it's to do with resonance and frequencies. A complicated science and one that in this example I'm not going to bother to fully understand. But Seth also has a point, it's like if you take a short, down turned tail pipe and turn it up, the length stays the same but it will be noticeably louder in the car.
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Project Diary1975 Viva / 1988 T25 Camper / 1989 Mini / 1991 MX5 / 1992 Mini / 1994 Saab 9000 / 1997 Saab 9000 / 2008 Saab 9-5
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mikeymk
Part of things
'85 Polo Coupe S 1.6 16v
Posts: 931
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Sept 24, 2012 16:06:53 GMT
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An exhaust pipe is a musical instrument (wind). In order to make a note, just use an air pump to blow through it (engine).
No two are identical, so no two make an identical sound. Any change you make will change the sound.
Other factors are objects around it - namely huge steel speaker/microphone cones in the form of panelwork. So lenthening a consistent enclosure section where it exits a resonation scrambler box, right underneath (and not so far underneath) a boot floor panel is certainly gonna have an affect on air movement - and thus accoustic - behaviour.
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Sept 24, 2012 23:20:04 GMT
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Thanks for the replies - so it seems theres no way I can bang some numbers into a calculator and add X amount of pipe to make it all nice and quiet - Fair enough. I'll go back to my old plan of getting about 4 90 bends and send the gases round a few corners, (perhaps through another little box if I can fit one) and out the back of the van - the current pipe exits sideways out of the drivers side which I reckon makes it sound a lot louder to my - bouncing the sound off anything at the side of the road and right into my window.
Cheers lads
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bl1300
Posted a lot
Posts: 1,678
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Sept 25, 2012 16:28:30 GMT
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- the current pipe exits sideways out of the drivers side which I reckon makes it sound a lot louder to my - bouncing the sound off anything at the side of the road and right into my window. Cheers lads This I had a pinhole blow develop in the vans front pipe a couple of weeks back it wasn't really even noticeable untill you drove down a road of parked cars or by a wall, then it was bloody loud! You would also reduce the noise if you welded that extra bit of pipe on rather than bolting it on. |
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Current fleet.
1967 DAF 44
1974 VW Beetle 1303s
1975 Triumph Spitfire MkIV
1988 VW LT45 Beavertail
1998 Volvo V70 2.5
1959 Fordson Dexta
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