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Old 01-11-2011, 11:11 AM   #1
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Back Pressure Question

So I've been reading a few threads on exhausts and people keep saying things about backpressure. It seems that people think that you need more backpressure, but I thought you only needed backpressure, and a small amount at that, if you had a turbo. So I looked it up and found this website: http://www.thumpertalk.com/forum/sho...d.php?t=659727 which explains that you want zero backpressure with the highest output velocity of exhaust gases. Sooo... According to this website, a cut out with the right diameter pipe would be the best choice for performance as to have close to zero backpressure and with the highest output velocity of exhaust gases. So in theory this increase in efficiency should also increase your miles per gallon right?

Does everyone who complains about loosing back pressure actually mean loosing the velocity of output? Because you don't want back pressure right?

Am I wrong or does this make sense?

Edit: According to further reading, you do not want back pressure on turbos either
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Old 01-11-2011, 02:57 PM   #2
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Quote:
Originally Posted by Drunknsloth View Post
So I've been reading a few threads on exhausts and people keep saying things about backpressure. It seems that people think that you need more backpressure, but I thought you only needed backpressure, and a small amount at that, if you had a turbo. So I looked it up and found this website: http://www.thumpertalk.com/forum/sho...d.php?t=659727 which explains that you want zero backpressure with the highest output velocity of exhaust gases. Sooo... According to this website, a cut out with the right diameter pipe would be the best choice for performance as to have close to zero backpressure and with the highest output velocity of exhaust gases. So in theory this increase in efficiency should also increase your miles per gallon right?

Does everyone who complains about loosing back pressure actually mean loosing the velocity of output? Because you don't want back pressure right?

Am I wrong or does this make sense?

Edit: According to further reading, you do not want back pressure on turbos either
You do not want ANY BACKPRESSURE in any forced induction application, even in NA vehicles you want the proper scavenging effect, which seems to get lost in translation...
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Old 01-11-2011, 03:00 PM   #3
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no backpressure on turbos is fine but on a NA you want some backpressure..
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Old 01-11-2011, 03:42 PM   #4
With sufficient thrust, pigs fly just fine.
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Quote:
Originally Posted by Speed Freek View Post
You do not want ANY BACKPRESSURE in any forced induction application, even in NA vehicles you want the proper scavenging effect, which seems to get lost in translation...
Bingo!
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Old 01-11-2011, 03:46 PM   #5
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What if you're not boosted though?
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Old 01-11-2011, 03:47 PM   #6
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On turbo vehicles backpressure really isn't needed since the momentum of the turbo will suck out exhaust gasses from the cylinder. Any backpressure in the system just reduces the turbo's efficiency.
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Old 01-11-2011, 03:48 PM   #7
With sufficient thrust, pigs fly just fine.
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Quote:
Originally Posted by ToucanV13 View Post
What if you're not boosted though?
Back pressure is bad regardless. Scavenging is what is used to get the most HP/TQ out of an NA engine. Scavenging doesnt play as much of a role when FI.
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Old 01-11-2011, 03:53 PM   #8
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So a cut out would be beneficial even in a stock engine application...
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Old 01-11-2011, 03:54 PM   #9
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Quote:
Originally Posted by ToucanV13 View Post
So a cut out would be beneficial even in a stock engine application...
At high RPM only.
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Old 01-11-2011, 04:29 PM   #10
With sufficient thrust, pigs fly just fine.
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Quote:
Originally Posted by ToucanV13 View Post
So a cut out would be beneficial even in a stock engine application...
Quote:
Originally Posted by whippersnapper02 View Post
At high RPM only.
This ^^^.....The exhaust is tuned to work as efficeintly as it can from the factory.More info on this:
http://www.tacomaworld.com/forum/tec...ine-basic.html
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Old 01-11-2011, 04:31 PM   #11
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Good read
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Old 01-12-2011, 11:55 AM   #13
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So how would you benefit more from a 3" as opposed to a 2.5" or is it marginal and only a racer/high performance car could notice a difference. I understand a bigger pipe allows more air flow but I want to know if it's marginal
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Old 01-13-2011, 09:16 AM   #14
Indffrnce will be the fall of manknd but who cares
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Quote:
Originally Posted by ToucanV13 View Post
So how would you benefit more from a 3" as opposed to a 2.5" or is it marginal and only a racer/high performance car could notice a difference. I understand a bigger pipe allows more air flow but I want to know if it's marginal
From what I learned from the website it depends on what RPM range you want more power at. If you go for a 3" the velocity of the exhaust will be slower at low RPM range and faster at a higher RPM range. If you go with a 2.5" the increase in power would be at a lower RPM range then the 3"
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Old 01-13-2011, 09:37 AM   #15
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Quote:
Originally Posted by Drunknsloth View Post
But according to the website, the where in the RPM range you get more power depends on the size of the pipe...
Yup. The larger the pipe the small amout of backpressure so the less amount of exhaust scavenging. Your gain top end but lose low end.
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