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Old 09-08-2014
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Question about back pressure and converter

Hey everyone, I just bought a 98 XLT 2wd 5 spd ranger with a 2.5 and it has new exaust system on it but i bought it in a non emissions required county so it has no converter. My question is will that have any effect on the back pressure of the engine? It is kinda sluggish on start off. Thank for any replies.
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Old 09-08-2014
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The "myth" again.

No engine runs better with back pressure, but it is a common myth.

Tuned exhaust systems create LOWER pressure at the exhaust ports, they do this by using a smaller pipe at the exhaust port that dumps into a larger pipe(collector).
The smaller pipes size and length is what determines the RPM range where the lowest pressure and highest power occurs.
This allows headers to be designed to move the power band lower in the RPM range, or higher.
Headers don't add power, never did, but they can move the power band for your use, i.e. lower power band for quicker starts, or, higher power band for higher speed power

The size and length of the pipe at the exhaust port is important in creating the Velocity that causes the LOWER pressure, when the exhaust from one cylinder gets to the larger pipe its velocity creates a lower pressure in the other 3 pipes(4cyl), like a siphon effect, this can be tuned to create the lowest pressure at one specific RPM, that establishes the center of the power band.
The back pressure myth came from people putting on larger pipes and then losing power.
The incorrect conclusion was "engine needs back pressure", OOPS, lol.
The larger pipes at the exhaust ports INCREASED pressure, because they lost the Velocity that creates the low pressure.


Not having a Cat will not effect performance one way or the other, another myth, but if vehicle originally came with a Cat then it is a $2,500 Federal Fine for removing or not having it installed, nothing to do with State requirements, it is a Federal beef.
Will you be caught?
Not likely but not 0 either


I would see if you can ID the type of headers you have, they could be a high power band model so low-end would be less responsive.

Last edited by RonD; 09-08-2014 at 11:58 AM.
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Old 09-08-2014
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Thanks for the reply, I'm planning on having a cat installed anyway i just did not know about back pressure!
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Old 09-08-2014
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Good news on the Cat, cleaner air for all of us.

Yes, it is a popular myth, and very old.
In my younger days I took what the older guys told me as gospel, and one of those 'gems of wisdom' was "engines need some back pressure".

My first experience with engine exhaust was with 2-stroke motorcycles, early teens.
They would get better power running an Expansion Chamber exhaust, this is when "being on the pipe" meant you were in that narrow RPM range where the 2-stroke engine was generating the most power because of the tuned exhaust.
That saying now means something COMPLETELY different, lol.

2-stroke engines are a little different than 4-stroke, but creating the negative pressure at the exhaust port is what "tunes" the exhaust on both.
The Expansion part creates that lower pressure(smaller pipe into larger pipe), but on a 2-stroke the intake port and exhaust port are both open at the same time, so you also need to raise the pressure again so you don't suck the unburned air/fuel mix coming in, out the exhaust port.
That's the Chamber parts job, it reflects the last exhaust's pressure wave back at the exhaust port to raise pressure and stop the outflow of unburned air/fuel mix.
Very interesting stuff to me at that time.

This was for single cylinder engines for the most part, when you have multi-cylinder engines you can use the firing cylinders exhaust to create that lower pressure at the other cylinders exhaust ports.
For Dual exhausts they are adding "H" or "X" pipes for similar reasons, lowering pressure at exhaust valves by keeping Velocity up and balanced.

So exhaust can be used or "scavenged" for better performance.
Next time someone says, "yeh, you need bigger pipes to get free flowing exhaust", just say really? That makes as much sense as dumping a gallon of gas down the intake for more power, lol.
More gas = more power
Bigger pipe = better exhaust

Well it doesn't quite work that way

Last edited by RonD; 09-08-2014 at 10:27 PM.
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Old 09-09-2014
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Ah I love when people use the term back pressure. :)
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