bigger pipes = less mpgs? - TCCoA Forums
 
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post #1 of 11 (permalink) Old 08-20-2008, 11:34 AM Thread Starter
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bigger pipes = less mpgs?

I was told that bigger diameter pipes will destroy gas mileage. That doesnt make any sense to me, is this true??? -TJ
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post #2 of 11 (permalink) Old 08-20-2008, 12:43 PM
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Not necessarily. I would think that pipes that are too large will decrease milage but not always larger = worse.
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post #3 of 11 (permalink) Old 08-20-2008, 01:22 PM Thread Starter
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that would make sense to me. would the lose in torque be the reason for a lose in milage?
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post #4 of 11 (permalink) Old 08-20-2008, 05:15 PM
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It would be the loss in exhaust scavenging that makes your power and fuel economy go down. There's more to exhaust design than most people think. Too large or too small will adversely affect the engines performance.

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post #5 of 11 (permalink) Old 08-21-2008, 12:32 AM Thread Starter
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the reason i ask is becuase im thinking of upgrading either my front half of my exhuast from 2" to 2.25" or just getting all new exhuast which would prolly be 2.25"

would an .25" really hurt me than? Am i better off with the old stock set up? id like to keep the great gass mileage i get...if i can. -TJ
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post #6 of 11 (permalink) Old 08-21-2008, 12:47 AM
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2.25" is fine

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post #7 of 11 (permalink) Old 08-21-2008, 05:38 PM
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Quote:
Originally Posted by MC94XR7 View Post
isnt exhaust scavenging created with valve overlap?
Yes some. But it's not the whole equation, and factory cams generally have very mild amount of overlap anyway.

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post #8 of 11 (permalink) Old 08-21-2008, 09:18 PM
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I ran 2.5" true dual on my 96 NPI 4.6 and still got 24-25mpg with it on the highway and never felt any difference in performance.

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post #9 of 11 (permalink) Old 08-21-2008, 10:45 PM Thread Starter
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thats good news. I enjoy my MPGs too much to chance anything dumb lol
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post #10 of 11 (permalink) Old 08-21-2008, 11:31 PM
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IMO, over 2.5" duals is where you would start to lose fuel economy....i routinely get 25-27mpg on the highway with 2.5" duals from the stock manifolds and high-flow cats.....i can still manage low-16s in the city if I keep my foot out of it
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post #11 of 11 (permalink) Old 08-22-2008, 05:20 PM
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Exhaust back pressure is a crutch. If you do a good job on the exhaust system, you don't need back pressure. Most engines make more low end torque from long tube headers.

Yes, a loss in low rpm power would be the major reason for lost mpg.

Quote:
Originally Posted by 94 Thunderbird XX View Post
Yes some. But it's not the whole equation, and factory cams generally have very mild amount of overlap anyway.
the 4.6 has none. If you want to put a degree figure on it, it's -23.




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