How much backpressure does 2.9 need?

Sworn · Jan 5, 2008

I just put a Flowmaster Muffler on my 88 4X4. I also had the cat removed as the truck has 132K on it and is 20 years old. It sounded great and ran awesome for about a day. Then I noticed that it didn't sound as good and the idle became a little rougher. I am thinking that maybe it does need to have a converter put back on.....As always, any input is appreciated.

mikhail · Jan 5, 2008

i dont think converters affect back pressure too much. the ecu just isnt used to the setup give it time to adjust, this has happened to mine before when i changed the exhuast. i have no muffler and one of the stock cats on mine and you can hardly feel it idle if you feel the shifter. it takes about a day or two maybe three depending on how much driving you do.

shadetree · Jan 6, 2008

Ideally, you do not want any backpressure in the exhaust system, but you cannot get rid of it all, just minimize it. Most do not understand the meaning of the term "backpressure." shady

reimic39 · Jan 9, 2008

Did you change the way your o2 sensor was mounted that can have a huge affect on your proformance.

Sworn · Jan 12, 2008

Did you change the way your o2 sensor was mounted that can have a huge affect on your proformance.

Nope.

AllanD · Jan 20, 2008

"backpressure" is a meanigless term because of the way it's so grotesquely misused.

What you have when you remove or "hollow out" the catylitic converter is "flow reversion"

The factory cats on a 2.9 or 4.0 don't create any meaningful "Backpressure"
However REMOVING them oincreases "flow reversion" and reduces exhaust flow.

Basically it's like running an exhaust with pipes that are too large.
the engine isn't producing enough exhaust to maintain a steady flow
so the exhaust gas pulses don't flow out of the pipe like bullets out of a machinegun barrel, but instead play "bumper cars" with each other.

Do you understand that explanation?

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Superfuss · Feb 18, 2008

How would one go about fixing flow-reversal. I got two turbo mufflers on my 86 bronco II 2.9, that's it. The O2 senser is in the same place,didn't move it. It's working good, as far as I know. From time to time the truck idlas very low(about 6-7K RPMs) then it straitens out. It does this while I'm stoped. The longer I'm stoped, the more it does this. At driving speeds I can fill some loss in power at times. Agin, it goes back and forth. The first muffler is about 13"-16" from the Y-pipe. The secend is about 20". I've stated to thank the deffence in length may have something to do with it. Someone told my about back pressere and they sounded like they know what they were talking about. If that is the case and I have scewed that up, I've thought of these to fix it. I was going to put two Flowmaster Hushpower II resonators on. Their size would let me get them up close to the Y-pipe and have them at the same length from. After that, maybe some glasspack to. Any in-put?

mhughes165 · Feb 21, 2008

ideally you will want to setup the exhuast for the kind of power you want, a smaller diameter pipe will give u more top end b/c u have better exhuast velocity, when u run a large pipe it will have better low end but drop off in the top end range, ideally the best performance would be running straight piping before the flange on the stock diameter right off the y-pipe, it has given the best overall performance in my experience and often times more power then a oversized single pipe, now i know many may bitch becuase it has no cats at that point, or argue about "back pressure" but it is what it is, if u have done it then u know it works. just my .02

86XLT · Feb 21, 2008

mhughes165 said:
ideally you will want to setup the exhuast for the kind of power you want, a smaller diameter pipe will give u more top end b/c u have better exhuast velocity, when u run a large pipe it will have better low end but drop off in the top end range, ideally the best performance would be running straight piping before the flange on the stock diameter right off the y-pipe, it has given the best overall performance in my experience and often times more power then a oversized single pipe, now i know many may bitch becuase it has no cats at that point, or argue about "back pressure" but it is what it is, if u have done it then u know it works. just my .02

+1...i cut mine before the first flange and ran pipe straight back to two thrush turbos dumped right before the rear axle. The 2.9 seemed very happy with this set up. I just had a Y pipe made and welded in going into one thrush turbo and it seems like i lost a little power. However I dont recommend cutting the cats off, I had no choice at the time as the flanges were so rusted they were unrepairable and i did not have money for a new Y pipe and a new set of cats. But it did seem to livin the motor up.

shadetree · Feb 21, 2008

mhughes165 said:
ideally you will want to setup the exhuast for the kind of power you want, a smaller diameter pipe will give u more top end b/c u have better exhuast velocity, when u run a large pipe it will have better low end but drop off in the top end range,

You have it backwards, smaller pipe more velocity better low end, larger pipe better top end. shady

mhughes165 · Feb 21, 2008

shadetree said:
You have it backwards, smaller pipe more velocity better low end, larger pipe better top end. shady

i had a brain fart earlier, think nothing of it

V8_power428@hotmail.com · Mar 16, 2008

I took the cat right out of my truck and i have a single flo pro muffler i'm running stock pipe size and my exuast dumps right out the muffler i havent had any problems with idle or power, if anything i should have gained 1 or 2 hp.

How much backpressure does 2.9 need?

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