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After constantly trying to stop people from telling others, "you need backpressure for torque," I decided to write something up on the subject (possibly to be put in the faq [IMG]i/expressions/face-icon-small-wink.gif[/IMG]) to be information I could just link to instead of trying to describe how everything works. But while I was looking on magnaflows site I noticed they have an awesome piece on it thats simple and accurate. Here's the section, and hopefully the next time someone tells you, "You'll lose power with less backpressure," you can give them an educated pimpslap!


From Magnaflow.com

BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.

EDIT: entire article here, including good information on the next page [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

 
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Thanks for clarifying, I actually was unaware of all that.[IMG]i/expressions/face-icon-small-cool.gif[/IMG]

 
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That's great avenger, I was a little puzzled about that effect, but the article explains it well. Thanks.

 
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Thanks bro'

 
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Another lesson learned thanks

 
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Nice.

 
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So everyone had the right idea, but for the wrong reason...

 
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Here we go again, Avenger. [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

The article states:

"An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque... Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. "

How is this different from "The Myth." [IMG]i/expressions/face-icon-small-confused.gif[/IMG]
A larger exhaust will produce less backpressure.
A smaller exhaust will produce more backpressure.

"Like the article says, an exhaust pipe that is TOO BIG......" will have an adverse effect.
Conversely, an exhaust pipe that is TOO SMALL would have an opposite, but also adverse effect.

So the truth is that you need an exhaust that is PROPERLY sized for the displacement and
power level of the car. Not too big...not too small...but just right! [IMG]i/expressions/face-icon-small-wink.gif[/IMG]
Nelson (can't find my sig) [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

 
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The myth is "if you lose backpressure you lose torque"

the truth is :

You always want to have the least amount of backpressure as possible, while still keeping a high exhaust velocity and proper delta pressure.


The article says "Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters"

so when you say:
"A larger exhaust will produce less backpressure.
A smaller exhaust will produce more backpressure"
it's not necessarily true. A smaller exhaust with less restriction will make less backpressure vs. a smaller exhaust with a restrictive muffler.
[IMG]i/expressions/beer_yum.gif[/IMG]

 
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Definately FAQ worthy. Adding it to the FAQ. Releasing the new FAQ soon...

 
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Fair enough. [IMG]i/expressions/beer_yum.gif[/IMG]

Perhaps the relation between backpressure and torque is not so much a "cause and effect" relation as there is a "correlation" between the two.

I don't suppose the phrase "Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters" has anything to do with the fact that Magnaflow sells "straight-through free-flowing" mufflers? [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

Nelson (can't find sig.... [IMG]i/expressions/face-icon-small-sad.gif[/IMG] )

 
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