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Old 06-18-2007, 07:10 PM   #1
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Muffler causing odd idling?

Ok, here's a good question for you all. Last night I was returning home and had to drive about 40 miles. With about 10 miles left, I was drifting through a town and I noticed my car was idling(while I was drifting) very rough. I could feel it vibrating up through the shifter and the car was extra-rattly. I came upon a redlight and I noticed the slower I got while approching the light, the harder it shook. At a near stop it was very rough but once I came to a complete stop, the idling stablized. I was hoping this would go away itself by today but it happend again go to the store.

Now, this has never happend before and I have 2 possible culprits. Recently the inlet on my muffler broke off so now my exhuast is in 2 pieces. Since the pipe still butts up to the muffler, it isnt much louder, but the metal grind is annoying. Do you think this..."irregular backpressure" is causing my shakes?

I also cleaned all my PCV hoses out yesterday but I dont see how this could make a negative effect. No I didnt accidently clog it and yes I reassembled it correctly.

What do you guys think? BTW, does lack of a muffler increase or decrease FE. I could see increase becuase itll breath easier with less resistance, but I understand engines need a certain amount of backpressure to perform properly..


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Old 06-18-2007, 08:03 PM   #2
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no on the muffler
Recheck *all* of your vaccuum lines - me thinks you have a leak

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Old 06-18-2007, 08:27 PM   #3
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Backpressure has no performance value.
Smaller exhaust piping may give you better low end power, but the gain stems from an increase in exhaust velocity and cylinder scavenging rather than an increase in backpressure. Faster moving exhaust has more momentum, so it creates a greater suction in it's wake which more completely clears the cylinder of exhaust during valve overlap.
As exhaust volume increases (such as from an increase in engine RPM), it will take more and more energy to force the increasing exhaust flow through the smaller pipe, resulting in true backpressure. Backpressure increases resistance during the engine's exhaust stroke and interferes with scavenging there after, resulting in a loss of power.
A larger pipe eliminates the high RPM backpressure issue, but reduces exhaust velocity at low RPM. Low exhaust velocity means less momentum and therefore less complete cylinder scavenging.
So, increasing the pipe size moves the engine's power band up the RPM range, decreasing it moves the band down the RPM range. As with many engine components, it's a matter of finding a happy medium... Unless you're building an engine for a specific purpose that is.

Getting back to the topic, the muffler is a complex baffled device - exhaust flow is broken up inside, so tuning for velocity isn't really possible, or maybe just futile. So long as it doesn't cause any backpressure, the muffler's presence (or not) shouldn't affect performance.
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