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On my truck I used to have a magnaflow muffler on the stock piping and I got 325 out of a full tank on a road trip. Now that I have my one chamber flowmaster with 3" piping from the o2 sensor back and I took the same road trip and i barley saw 280 out of a full tank. Would me changing the exhaust effect my mileage?
 

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stucktaco said:
somebody told me it has something to do with back pressure?
yes, but probaly youre thinking the smaller pipe had more backpressure? going to 3" exhaust allows the exhaust gases to expand and cool much faster than it did in the smaller tubing. this creates "head front" which makes it harder for the gases to travel out.

talk to Vince at Burns Stainless in CA, he'll give you the skinny. or search google for "exhaust bigger is not always better". if you're not pushing 300+ norm.asp. hp, and your not running a sc or turbo, then 3" pipe is way too much.
 

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I would say it could, but not for the reasons listed above. If my numbers are right you lost about 3 mpg or so. The mystical "backpressure" thing wouldn't have that kind of impact. You say you started your pipe at the 02 sensor? It is possible that it is causing a lean reading which means your ECU is running a rich mix killing mileage.
 

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EMPIRE said:
yes, but probaly youre thinking the smaller pipe had more backpressure? going to 3" exhaust allows the exhaust gases to expand and cool much faster than it did in the smaller tubing. this creates "head front" which makes it harder for the gases to travel out.

talk to Vince at Burns Stainless in CA, he'll give you the skinny. or search google for "exhaust bigger is not always better". if you're not pushing 300+ norm.asp. hp, and your not running a sc or turbo, then 3" pipe is way too much.
remember this...

 

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yep, i sure do. some folks just dont get it, and when they think they understand they're still lost.

a 3" exhaust on a na sub-300 hp motor is silly, simply not worth the $$$ and does absolutely nothing to help the engine breathe easier, and will actually cause the motor to scavenge less, thereby hurting performance. perhaps in this case the exhaust is not the exact cause, but more likely as noted perhaps caused the o2 sensor run the mix a tad more rich.

nothing new here folks. a proper exhaust is one that is sized correctly.

Kamr0n said:
remember this...

 

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No other changes beside the exhuast al all, right? Even your highway cruise speed the same or lower? Cruise speeds and rpm have a big effect on fuel efficiency.

I also have to second what tcmaboy said about the lean reading. Just to give a example, my prior pretaco had a dead oxygen sensor which gave the cpu a lean reading resulting in a exess of fuel being pumped. I gained 2mpg on average after replacing it.

Was the exhaust tubing welded near the sensor? If so that may have screwed it up, I guess. I would expect a CE light though.
 

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I put a 2.5" magnaflow and it didn't affect my mileage, but I did notice that it pulls the boat a SQUISH bit better.
 

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NorcalPR said:
I put a 2.5" magnaflow and it didn't affect my mileage, but I did notice that it pulls the boat a SQUISH bit better.
again, probably perception. do you have before/after dyno tests? its really the only way to know for sure if you got noticeable torq increase....
 

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You dont know what your talking about, ever seen that stock muffler? The magna flow is WAYYYY more aerodynamic, thats why it pulls better. ;)
 

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Kyota said:
You dont know what your talking about, ever seen that stock muffler? The magna flow is WAYYYY more aerodynamic, thats why it pulls better. ;)

ok, let me explain something:

an EXHAUST is not the same as a MUFFLER !!

sizing of the exhaust system correctly will properly scavenge the exhaust ports (which is typically tuned to the rpm range of the cam).

blindly choosing a 3" exhaust or even a 2.5" exhaust is not likely to produce any gains, and many times will cause performance decreases.

its all old-hat info folks, just do some googling or go talk with folks who know exhaust systems (like Vince at Burns Stainless).

but then again, prove me wrong, post your dyno results.
 

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Yeah, cuz we ALLLLLLLLL have access to a dyne machine.......

I pull my boat and my truck struggles a bit. Up the same hill it would very very slowly slow down when floored. With the exhaust, same conditions, same boat, same wieght, it would keep an even speed...

Do I need a dyno for that?
 

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NorcalPR said:
Yeah, cuz we ALLLLLLLLL have access to a dyne machine.......

I pull my boat and my truck struggles a bit. Up the same hill it would very very slowly slow down when floored. With the exhaust, same conditions, same boat, same wieght, it would keep an even speed...

Do I need a dyno for that?
maybe during the times you had "even speed" the air was more dense?? ;)

all i'm saying is, going larger does not always help things. 2.25" exhaust is plenty big. i'm sure everyone is happy that you experience better performance, but others should be aware of the misomer.
 

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3" is too much pipe for the 3.4. 2.25 or 2.5 at the most is what should be used and flowmasters sound good but are shit.
 

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Supramann93 said:
3" is too much pipe for the 3.4. 2.25 or 2.5 at the most is what should be used and flowmasters sound good but are shit.
i've wanted to try a spintech, maybe with my turbo setup i will. the walker quiet-flow (more flow, less noise) also looks ok to swap-out the oem muffler.

"if ya aint racin it, then keep that shiat quiet", heard that once at a track down in FL.
 
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