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Old 11-07-2012, 08:30 PM   #1
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Exhaust backpressure Q

Vehicle = 2011 2.7l I-4
I work at a performance shop and have access to aluminum, stainless, tig and mig welders, etc and was wondering if the modern full injected engines still need a bit of backpressure?
My plan was to order LCE headers, cut open the cat converter(s?) , gut em and re weld em on and then dump the exhaust somewhere.
But with the headers, gutted cat and no muffler, i was wondering if that would actually hurt the engine or increase the risk of blowing it up?
Like should i maintain x amount of backpressure or just free it all up?
And also will any of this help without being able to program the ECU?

Cause our ECU's arent flashable or something like that right?

I honestly am just trying to make the lil 4banger sound beefy as can be.
Any help/links/info would be greatly appreciated
Thanks
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Old 11-07-2012, 10:42 PM   #2
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I can assure you that an I4 with modifications you listed above, especially with no cats will sound horrible and raspy as hell. Not too mention opening up the exhaust like that on the I4 will lose some much needed low end power. If I were you I'd order an aftermarket muffler like a Magnaflow or Aeroturbine, dump the exhaust and be done with it.

The sooner you realize a.) the truck will never sound like a V8, b.) will never be fast; your life will be that much easier.
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Old 11-08-2012, 04:49 AM   #3
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Quote:
Originally Posted by Yamaha Dave View Post
I can assure you that an I4 with modifications you listed above, especially with no cats will sound horrible and raspy as hell. Not too mention opening up the exhaust like that on the I4 will lose some much needed low end power. If I were you I'd order an aftermarket muffler like a Magnaflow or Aeroturbine, dump the exhaust and be done with it.

The sooner you realize a.) the truck will never sound like a V8, b.) will never be fast; your life will be that much easier.
But i am a.) not trying to make it sound deep and like a V8 and b.) not trying to make it fast.

I guess i worded it wrong with the "sound beefy" in the first post but im just trying to make it sound kinda like a performance N/A 4banger.

And can you explain why it would lose low end power? Thats sorta the point of the thread, cause im guessing it involves backpressure ?
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Old 11-08-2012, 06:28 AM   #4
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chris4x4 posted a link to some info on "exhaust/backpressure" awhile back, might want to search for it.

Given the questions you're asking I would spend some time researching exhaust gas velocity vs. exhaust flow vs. performance as it sounds like you're trying to mod something you don't fully understand. Not dinging you, just going by what I see.

Reality is that backpressure is something you want to avoid. Zero backpressure is good, but people get caught up in exhaust mods that actually cause a decrease in exhaust gas velocity at low rpms. This decrease, usually caused by too big of an exhaust pipe, can show up as lost low end power and increased high end power due to the fact that low end power is affected by how fast the exhaust gases flow (faster is better here), while that same large pipe can help high end powere since it flows more gases.

You need to find the optimization point between exhaust gas velocity at low rpms and the ability for a sufficient amount of exhaust gases to flow at your high rpm point. It is a trade-off.

Do note that this explanation did not include the term "backpressure", it's a term that's greatly overused for very little actual meaning.
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Old 11-08-2012, 06:30 AM   #5
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Quote:
Originally Posted by Yamaha Dave View Post
I can assure you that an I4 with modifications you listed above, especially with no cats will sound horrible and raspy as hell. Not too mention opening up the exhaust like that on the I4 will lose some much needed low end power. If I were you I'd order an aftermarket muffler like a Magnaflow or Aeroturbine, dump the exhaust and be done with it.

The sooner you realize a.) the truck will never sound like a V8, b.) will never be fast; your life will be that much easier.
^^

im with this guy 100%.
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Old 11-08-2012, 10:07 AM   #6
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Quote:
Originally Posted by Lurkin View Post
chris4x4 posted a link to some info on "exhaust/backpressure" awhile back, might want to search for it.

Given the questions you're asking I would spend some time researching exhaust gas velocity vs. exhaust flow vs. performance as it sounds like you're trying to mod something you don't fully understand. Not dinging you, just going by what I see.

Reality is that backpressure is something you want to avoid. Zero backpressure is good, but people get caught up in exhaust mods that actually cause a decrease in exhaust gas velocity at low rpms. This decrease, usually caused by too big of an exhaust pipe, can show up as lost low end power and increased high end power due to the fact that low end power is affected by how fast the exhaust gases flow (faster is better here), while that same large pipe can help high end powere since it flows more gases.

You need to find the optimization point between exhaust gas velocity at low rpms and the ability for a sufficient amount of exhaust gases to flow at your high rpm point. It is a trade-off.

Do note that this explanation did not include the term "backpressure", it's a term that's greatly overused for very little actual meaning.
^ thanks man! Yea i know a little about exhaust systems and how too big of a pipe can hurt but then again so can too small of one but i never really learned about the velocity gases math and all that.

Im honestly not trying to gain any power from this but more like, make the truck sound like it has some sort of balls without hurting the performance any.

I will definitely search for chris4x4's thread when i get off work
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Old 11-08-2012, 02:55 PM   #7
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Old 11-08-2012, 02:57 PM   #8
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Scavenging. It's all about scavenging.
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Old 11-08-2012, 03:00 PM   #9
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/thread
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Old 11-08-2012, 03:30 PM   #10
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/thread
x2

Got some reading to do
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Old 11-08-2012, 03:41 PM   #11
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Know what the cool thing is? catalysts now-a-days now offer very little to no exhaust restriction(unless they're damaged). If you gut it you will be releasing shit loads of hydrocarbons, carbon monoxide, and NOx into the air.

Leave your cats

But to answer your questions, no. Engines don't require backpressure.
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Old 11-08-2012, 03:44 PM   #12
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Quote:
Originally Posted by Spoonman View Post
Know what the cool thing is? catalysts now-a-days now offer very little to no exhaust restriction(unless they're damaged). If you gut it you will be releasing shit loads of hydrocarbons, carbon monoxide, and NOx into the air.

Leave your cats

But to answer your questions, no. Engines don't require backpressure.

I didnt think so but i just wanted to double check
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Old 11-20-2012, 08:12 PM   #13
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Scavenging....What do ya know about it?

I know exhaust travels faster in a smaller pipe do to heat flow.

I to keep my exhaust hotter to try and make better fuel mileage.. I would like to keep everything in front of the cat stock, would adding wrap to the manifold help or hinder?

I'm thinking of putting on an aero-turbine, thinking the low pressure draw it creates would help my mpg.

Anyone know more about this subject?
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