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Old 05-07-2010, 10:51 AM   #11 (permalink)
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NA engines need back pressure?

Actually any vehicle never needs back pressure that is often a misconception made by those who go to to big of a pipe and lose performance. The reason this happens is that going to to large of a pipe exhaust gas cools and then slows. When that happens there is congestion in the pipe and then the motor has to work more to push the slower gases out.

With an appropriately sized pipe it will remove nearly all back pressure and promote the removal of gases since they will be moving faster. When the gas tries to quickly remove its self it creates a pressure wave behind it that works to suck the next exhaust pulse out of the pipe. That is what you are going for with any exhaust system.

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Old 05-07-2010, 07:12 PM   #12 (permalink)
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^Very well said. The back-pressure thing has always been a misconception.
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Old 05-07-2010, 07:57 PM   #13 (permalink)
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For NA engines, going to a diameter that is too large will reduce flow velocity and reduce power. Getting the highest-flowing cat and muffler you can find for your stock size is your best bet.

My Tempo has a 2 inch system, but the stock muffler had a 1 3/4" outlet. Changed it out and immediately noted an improvement in power on the high end.

For gas-miserly driving, you're not really going to go into the RPM range to take advantage of higher-flow exhausts.
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Old 05-08-2010, 12:09 PM   #14 (permalink)
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You know what back pressure is? Its resistance coming out of the pipe because of smaller diameter/ resenators/ cats/ muffler. Little pipe = back pressure, large pipe = free flowing. NA engines NEED back pressure. If you removed your exhaust system after your cat, you would loose performance and MPG in your na motor. On a turbo motor, an open down pipe is the way to go.

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Old 05-08-2010, 12:56 PM   #15 (permalink)
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Quote:
Originally Posted by zhillz View Post
You know what back pressure is? Its resistance coming out of the pipe because of smaller diameter/ resenators/ cats/ muffler. Little pipe = back pressure, large pipe = free flowing. NA engines NEED back pressure. If you removed your exhaust system after your cat, you would loose performance and MPG in your na motor. On a turbo motor, an open down pipe is the way to go.
Here's an article I found on the subject.
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 05-08-2010, 01:08 PM   #16 (permalink)
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i did notice my maxima has an insulated exhaust system. Maybe that is why, to preserve the heat all the way to the end.
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 05-08-2010, 05:28 PM   #17 (permalink)
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There is very little science in that article and its all to do with a/f ratio which is also determined before entering the motor.

Put it like this, if you take my stock d16y8 out of my civic and put on a header into a 3" collector and then straight back, you will LOOSE power. Dyno proven. v8 motors use X pipes to increase back pressure for low end torque.

All in all, im not sure about the effects on fuel economy.
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Old 05-08-2010, 06:25 PM   #18 (permalink)
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WHOA! science content is definitely in question throughout this thread. I could go on to explain, but it seems like this would give you enough of the information that you need.

Exhaust System Theory 101

If you read through it, you will see that back pressure, which will occur in low diameter tubes at higher rpm, is not what increases the torque, but the exhaust scavenging that happens due to the fact that your combusted air/fuel , in fact, has enough mass that at the velocity (e=m*v*v) it has significant momentum. This, coupled with overlap of the intake and exhaust lobes with each other, and the trail of that lift into the surrounding strokes, increases volumetric efficiency, which increases torque at that rpm. This is not the same as welding your 5" muffler shut to a 1", which would increase the back pressure, but not the velocities that the exhaust would be seeing in the more critical sections that effect this scavenging effect. You also should look up what an x pipe does (hint, increase cross scavenging between the two banks of cylinders) along with tri-y headers. A good rule would be that as temperature drops and density increases, diameter should as well. An exhaust system that has such a taper would be expensive to build, hence why we don't have them. This only begins to tell the story. I hope this helps.
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Old 05-08-2010, 06:33 PM   #19 (permalink)
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Also, your a/f ratio is altered during the running of the vehicle, and many times per second. With inputs such as CKPS, CPS, TPS, MAP, MAF, O2, and KS coming in at anywhere as high as 360,000 signals per minute (60tooth at 6000rpm), the computer is altering critical injection and ignition events constantly. If you change the VE, or other parameters of the engine that would effect O2 readings (rich/lean), the computer changes the long term and short term fuel trims to adjust to these conditions. Changing your exhaust doesn't change this, it just changes the efficiency at which it operates at given rpms (as some are better high than low).
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Old 05-08-2010, 08:44 PM   #20 (permalink)
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I had no idea how complicated the physics of simply getting the exhaust out of an engine were. So basically, some shade tree mechanic tinkering with his exhaust system has a 99% chance of making it worse. Especially if that car is designed with performance in mind. Such as an exhaust system that is completely encased in insulation with a heat shield on top of that.

No wonder exhaust system mods are so hotly debated. Sounds as complicated as nuclear science, yet any joe blow can claim to know something about it.

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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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