Straight pipe confusion.

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Extrawubbs

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So a while back i blew out the cat and took the muffler off my truck. Sounded great to my 18 year old ears but i guess it got old quick. I re attached the muffler and now the truck seems to have a little more power? I thought that less back pressure would give me more power but this is really confusing to me.
 

L31MaxExpress

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So a while back i blew out the cat and took the muffler off my truck. Sounded great to my 18 year old ears but i guess it got old quick. I re attached the muffler and now the truck seems to have a little more power? I thought that less back pressure would give me more power but this is really confusing to me.

Less backpressure does give more power.
 

Erik the Awful

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So a while back i blew out the cat and took the muffler off my truck... I re attached the muffler and now the truck seems to have a little more power?
My bunch of wild guessing is that your cat was clogged. If you want to be a good neighbor, get a $120 aftermarket replacement and weld it on.
 

Pinger

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Pipes when they contract or expand create pulses which reflect back to the engine and can affect breathing and hence power. Sometimes for the better, sometimes for the worse.
 

Schurkey

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Less backpressure does give more power.
...when the engine can make use of the lower backpressure.

It's probably possible to "tune" an engine so that lower back pressure reduces power. Wild example: Some vehicles have a backpressure-sensing EGR valve. If you changed backpressure enough to disable the EGR, the resulting "ping" would drive the knock sensor crazy, which could result in retarded spark and poor power, poor fuel economy.

But, yes, if an engine isn't sabotaged like that, backpressure reduction improves power.
 

99xcss4

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I have heard that the back pressure thing is a myth on a 4 stroke engine you do not need it now how long your exhaust pipe is do matter and can impact power and torque 2 stroke engines need back pressure
 

95burban

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kinda off topic….

Back when I had my small motor and before i switched to zoomies on my drag boat, i fine tuned my headers by cutting the collectors. I took lip stick and put a line from where the primary’s went into the collector all the way to the end. Where the lip stick stopped burning that’s where I cut them. I don’t remember it making a difference.

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Schurkey

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2 stroke engines need back pressure
Absolutely not. Two-stroke engines usually improve power with a "tuned" (resonating) exhaust--they're said to come "on the pipe" the way 4-stroke engines are said to come "on the cam".

If the pipe is tuned properly, the exhaust resonance produces a low-pressure spike at the exhaust port just before the port closes, to draw-in additional fuel/air mixture.

It's the LACK of pressure (vacuum) AT THE RIGHT TIME, caused by the resonance that improves two-stroke power. Which is also why 2-strokes tuned like this tend to be "peaky".

I took lip stick and put a line from where the primary’s went into the collector all the way to the end. Where the lip stick stopped burning that’s where I cut them. I don’t remember it making a difference.
I've heard multiple variations of that story--you say lipstick, other guys use tire crayon, some folks actually use a fancy temperature-indicating marker.

Point is, temperature is not what makes a tuned exhaust. Resonance is, so the mathematics of the speed of sound is involved, which varies with temperature, humidity, and other factors including whether you're tuning for the first peak, second peak, third peak, etc.


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I just love a flatty with a visible fin. You've just made me moist. I don't expect my '73 Sanger bubbledeck ski-boat does that often...if at all. And certainly not for the last few years--hasn't been on the water in near a decade.
 
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