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Old 02-09-2010, 07:34 PM   #1 (permalink)
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smaller exaust pipe for mpg

since exaust is a bunch of pulses that line up in a row and flow nicely out the exaust system in an orderly fashon, (in fantasy land) at the perfect diameter pipe and rpm,. and we know the manufaturer has to compromise on the size to allow operation across a broad rpm range, what would be the benifit of installing a smaller pipe size from the header collector to the tailpipe, including finding a smaller size muffler and cat converter (if they exist) would you also have to get/make smaller primary tubes and collector or could you start the smaller exaust at your stock size header collector? i,ve read other data/posts that talk about restricting the exaust or creating backpressure. that is not the same thing as a smaller pipe, putting a singlar restriction in any position would only serve to create a (trubulent mess) im talking smooth sailing for the pulses, targeting around 2000-2500 rpm (where i live) for most of my driving. i dont care if my hp drops from 108 to58 ,if my mpg goes up enough,, i could be barking up the wrong tree but im interested in some of you engine guys opnions as to the possible benifit/problems.

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Old 02-09-2010, 07:51 PM   #2 (permalink)
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you want this in your tail pipe. guaranteed to produce results.
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Old 02-09-2010, 07:56 PM   #3 (permalink)
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i might "give" one of those to a fellow coworker. he deserves it!
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Old 02-09-2010, 07:56 PM   #4 (permalink)
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LOL
Despite popular belief, everything that kills performance doesn't automatically improves fuel economy.
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Old 02-09-2010, 08:01 PM   #5 (permalink)
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yeah... stick it WAY up into the pipe... and add a bumper sicker.
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Old 02-09-2010, 08:20 PM   #6 (permalink)
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Never thought about MPG for a smaller pipe but i can tell you this:
On motorcycles the exhaust length and dia. are very critical to how the bike runs. Longer/smaller pipes give better low&mid range torque while short "drag shorties" give a higher and narrower HP curve.
While most car exhausts are much longer, and many consider them to be too long to affect things, I'm not convinced they don't have some effect. I just think at the normal length they match s many 2nd and 3rd frequencies, tract tuning doesn't matter as there is still a matching wave length. Others feel at those lengths the waves have died out and turned into nearly steady flow.
Now changing the header size & length will have an affect, lowering and flattening the HP curve shouldn't change the FE but the added torque may cause you to lift off the throttle more.
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Old 02-09-2010, 10:33 PM   #7 (permalink)
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Stick your hand over the end of the pipe when it's running- that isn't a steady flow.
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Old 02-09-2010, 10:47 PM   #8 (permalink)
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Exhaust tuning can achieve specific characteristics. You can choke the engine out a bit at the higher RPMs, and get desirable lower RPM characteristics which will change your VE and BSFC maps, meaning that you'll have to change other things to fully take advantage of it.

It's not "killing performance to get better fuel economy", it's tuning for a specific characteristic.

You'll have to do some serious studying to determine the proper exhaust length/diameter configuration for the characteristics you're trying to achieve.
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Old 02-10-2010, 02:17 PM   #9 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Stick your hand over the end of the pipe when it's running- that isn't a steady flow.
Until recently, I had an '89 Celica. It got very good FE and was a great car, but its only design flaw was the muffler: the muffler would be shot in two years on average. (I had a lifetime warranty on a replacement muffler and must have hung at least six mufflers on that car over the years.)

One day I was cruising down a highway at about 65. The (then current) muffler had been getting progressively louder, but I didn't worry, because at worst, it typically would rust out and break off right where the pipe entered the muffler. This time however, I heard a loud BANG! from the rear. I looked in the rear view mirror to see pieces of muffler flying off behind me.

Yep, it exploded. Now you know what can happen from having a partially clogged exhaust. Purposely restricting the exhaust? No, thanks...
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Old 02-10-2010, 06:20 PM   #10 (permalink)
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I only have a little expirience and it's certainly not scientific. I went from a 1"7/8 to a 2"1/4 exhaust, cat back, on the ZX2 and didn't really see any noticable change in FE. Then changed my cracked cast iron log manifold to a nice SVT 4-2-1 header and didn't see any big changes in FE. During that time I was not tracking mpg numbers real close, just kinda observing miles and gallons used. However I did put in a catalytic converter where the orginal had been removed and the difference after many months of tracking FE was nil. During this whole time my FE has steadily improved from many other much cheaper mods. Like ChrisT, I believe with enough time and testing you could probably see a measurable benefit from tuning an exhaust to a certain rpm range, but the change would probably be very small unless coupled with a complete tuning of the system ie. intake tract, ports, valves, cams, and the electronic tune.

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