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Well, the function improves if back pressure is minimized...and back pressure is related to the energy required to get the gasses out the back of the car...A car with headers through the hood produces more power than one with long pipes and a cat' and a muffler...But streat legal cars must hove those things.
Anyway, a dual exhaust can be a functional improvement if it flows better than a single. If you put two pipes that are 2" in diameter from you engine to the tail pipe instead of one, you'll reduce back pressure and let the car "breath" better...improving performance. This is rarely resulting in a large HP gain...there are lot of other things that can do that. If you have a single pipe of 2" and increase its diameter by only 40%, you can get a similar effect as doubling the pipes (this is because the cross-sectional area of the pipes is what controls the pressure drop.
Anyway, there are plenty of cars out there with fake double exhausts. (Cars with one exhaust pipe and two exhaust tips....) There are also a lot of cars out there with true double exhausts...and like someone else mentions, a cross-over is typical to balance the pressures. (A cross-over is simply a pipe or connection between the two exhaust streams to allow for uniformity of flow.
My '89 BMW 325iS (straight-6) had a dual exhaust with a Cat-cross-over (the two streams shared a large Cat). My '93 9000 Aero has a single exhaust, but it is larger than in my 325iS, and I would guess that it flows comparably....(And I'm not comparing apples to apples here, because one has a turbo and the other doesn't).
Anyway, two exhaust pipes look cool (IMHO0), but so does a nice LARGE pipe, like my Aero. You hear a lot of people talking about putting a 3" single pipe on their Aero...and that is up from the 2.5" stock size....That's almost like putting a double exhaust on it, but without the hassle of figuring out how that would work with a single turbo!
A flat 4 or flat 6, or V-6/8/10/12 engines have two headers...so a dual exhaust makes perfect sense for those.!
Now, Bugatti is making a 16-cylinder car w/ 4 turbos, and about 1000 HP. That exhaust is function...not just fashion. (check out the bugatti website, or howstuffworks.com for a link to a really cool interactive site of the new Veyron)
Keep on Saabin'
~Peter
'93 9000 Aero 5spd 57K
posted by 153.90.198...
_______________________________________
Current:
'03 9-5 Aero Wagon, 5spd, Polar/Black
Past:
'06 9-5 Combi, AT, Polar/Black
'04 9-5 Aero Wagon, AT, Nocturne/Granite
'03 9-5 Aero Wagon, AT, Steel/Charcoal
'00 9-3 Viggen, 5D, Silver/Black
'93 9000 Aero, 5MT, Cirrus/Black (Owned this one twice)
'86 900 SPG, 5MT, Edwardian/Buffalo Grey
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