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some please help with magna flow!!!

magna flow

some please help with magna flow!!!

Postby caseycole [OP] » May 17 2014, 6:56pm

hey guys, im sure there is already a topic on here about this but im going to be a little more specific! I have a 2011 gmc sierra. I was at the muffler shop and everyone was bragging about how the magna flow muffler they put on help waken up there truck a little bit and even helped with fuel economy. I had them put one on for 150$, just did a muffler swap and left everything else stock. Well I love the sound of it and even at highway speeds the drone isn't too bad. BUT I HAVE LOST A NOTICEABLE AMOUNT OF TORQUE. I mean when I punch it I don't get pushed into my seat anymore. I have also noticed a fuel economy drop. I noticed on my little screen that everytime it changes into v4 mode within 10 seconds it switches back into v8. Im guessing all this si due to a loss of back pressure. Any help guys? some people told me that its because I didn't dump it and ran it out the back instead, some say its because its a more free flowing muffler (which I thought helped with power and fuel economy), I went under the truck and noticed that the pipe they used to fill in the gap( because the stock muffler was way longer so they had to weld extra pipe on) was a little bigger than the stock pipe. I attatched pictures of the weld and the pipe. If anyone could help It would be greatly appreciated!
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Re: some please help with magna flow!!!

Postby Silvery13 » May 18 2014, 1:12am

Bottom line: just because a muffler shop makes a muffler fit doesn't mean it will yield the desired results.
Read this thread and it should help you understand: The "Back Pressure" Myth
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Re: some please help with magna flow!!!

Postby caseycole [OP] » May 18 2014, 7:22am

Silvery13 wrote:Bottom line: just because a muffler shop makes a muffler fit doesn't mean it will yield the desired results.
Read this thread and it should help you understand: The "Back Pressure" Myth
I get what you are saying, I wasn't one of those guys that was like " oh it says if I add a performance muffler ill get 18 more hp" lol. I was just wondering if there was a way to balance this out? like maybe add some kind of rectrictor to get some back pressure back? Also I was told dumping it would take care of my problem. And someone else told me to unplug my battery for ten minutes and my computer will reset and basically "learn the best way to run" all over again. any advice?!
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Re: some please help with magna flow!!!

Postby Silvery13 » May 18 2014, 12:42pm

Unhooking the battery certainly won't hurt anything but stop thinking back pressure, it's exhaust flow/velocity. The factory exhaust is designed to give a compromise between low end and high end power. The magnaflow you installed changed the low rpm exhaust flow in such a way as to have a negative impact.
How do you fix it? I don't know the answer. Maybe another member will have some advice.
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Re: some please help with magna flow!!!

Postby caseycole [OP] » May 18 2014, 5:20pm

Silvery13 wrote:Unhooking the battery certainly won't hurt anything but stop thinking back pressure, it's exhaust flow/velocity. The factory exhaust is designed to give a compromise between low end and high end power. The magnaflow you installed changed the low rpm exhaust flow in such a way as to have a negative impact.
How do you fix it? I don't know the answer. Maybe another member will have some advice.

I got you, I knew it was called that from other posts I have seen, but I figured more people would have advice if I just called it back pressure haha! guess ill have to find someone who can fix it.
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Re: some please help with magna flow!!!

Postby ke5muj » May 19 2014, 3:24am

I'm gonna get roasted on this but I will tell you my experiences as I have been through many setups. Any straight through muffler setup I had never gave me the low end torque feel like a stock or flowmaster type muffler and mpg did suffer. The flows sound amazing plus 90% of my driving is low RPM driving. Now running wide open most the time I would choose the straight through muffler.
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Re: some please help with magna flow!!!

Postby caseycole [OP] » May 19 2014, 4:29am

ke5muj wrote:I'm gonna get roasted on this but I will tell you my experiences as I have been through many setups. Any straight through muffler setup I had never gave me the low end torque feel like a stock or flowmaster type muffler and mpg did suffer. The flows sound amazing plus 90% of my driving is low RPM driving. Now running wide open most the time I would choose the straight through muffler.

If I put a cold air intake on do you think I could make up for it? or maybe have a smalled pipe after the muffler to helpe with "back pressure"? what did you do?!
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Re: some please help with magna flow!!!

Postby RIPPEDK20 » May 19 2014, 4:56am

no and no

a Cold Air Intake will actually probably hurt you more then help, what the problem is (and i spent a good deal of time on this for my exhaust) that you need a muffler ( and the rest of your exhaust) to be properly sized for your motor. Since you went to a smaller muffler, you did indeed lose back pressure, which in this case boosted flow. This inevitable raised the point where your horsepower and torque curve come into play.. For example. in extreme cases:

you have a 3.7L V6 and you go from a well tuned 2.5" single pipe exhaust to a cat back dual 2.5" inlet to dual 3" outlets...This sounds great--tons of flow, but it effectively raised the horsepower/ torque curve 1000+rpm. Basically losing all low end

Had that person stayed with a proper sized single exhaust, they would have been better off..

My point is: People put on headers and hollow out there cats and wide open mufflers or straight pipe etc without thinking about what its doing to your engine... Count this with more flow passed the cats an 02 sensors may give false reading and or burn more or less in the cats, which changes all function


In hind-sight-> i think what happened is you got the wrong kind of flow and also added extra "time" between the muffler and the cats / Y-pipe, which now your computer sees as the same or similar to running dumps ...also cutting or shortening your exhaust is not always the right answer..Every engine is different, regardless if its a 4.8L, 5.3L etc etc... cheapest thing to do is just to pull the battery cables now for a few minutes and then re install them to reset the computer and see what happens..or get a programmer
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Re: some please help with magna flow!!!

Postby caseycole [OP] » May 19 2014, 8:47am

hey man thanks! . Does taking the battery cables off really reset the computer and will make the computer re do everything to maybe fix the problem? I feel like I pad for a performance part that lowered power and gas mileage :(. this may be a dumb question but if I ran a little smaller pipe after the muffler, would it help at all by increasing back pressure,? You suggested a programmer, the only problem is I have 60k miles left till my warranty goes out, and don't want to void it yet. A few questions :is there a different magna flow I should have went with that wouldn't cause this problem? and last question, is running it the way I have it now going to hurt anything? like damage the engine? Thanks man I really appreciate it!
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Re: some please help with magna flow!!!

Postby Silvery13 » May 19 2014, 4:21pm

Aaaarrrrgh! there's that word again. :lol:
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