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Stock exhaust diameter

MyExpWork

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1991 4-door 4X4 XLT
Anyone know the stock exhaust diameter on a 91 4door 4X4 Explorer?

I am looking to have a custom 3" exhaust installed as my current muffler, etc are falling apart.

Thanks in advance.

Brian
 



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have2goski

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i think it is 2"

you could always just measure it

3" is alot if you want to stay performance minded. from what i hear you give up a lot of back pressure which you need for the low end tourqe.

of course if you have a "built" motor in there, maybe the 3" is just what you need.

others will probably chime in
 






CarFreak146

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2" is close. It Is ACTUALLY 2.25"
 






CarFreak146

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Personally, I wouldn't go over 2.5", but that's just me.
 






Alec

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Some are 2, some are 2.25.

Either go 2.25 or 2.5, any larger and you'll start to get unwanted sound and loss of performance.
 






snocross1985

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I went with 2.25" with my flowmaster, I definitely recomend it
 






MyExpWork

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Alec,

How would i get a loss of performance by helping truck "breath" better?

I understand the low-end torque and back pressure, but what else am I looking at and how can I avoid the loss of power from getting bigger exhaust? Mod the engine to make up for the added back pressure.

Brian
 






DeRocha

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The stock engine will not benefit from any exhaust larger than 2.5". Like the others have said You will start to lose back pressure which the engine needs... No one I know runs a 3" exhaust in a stock 4.0L... I am replacing mine in a couple of weeks. I am going to put on a flowmater 50 2.25" cat back system.. Installed for $195...

When I searched on various exhaust treads I hit many that pointed out the waste of money it was to go to huge 3" pipes.
 






Alec

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It's not exactly the loss of backpressure that causes a loss in torque, but close to it. I don't feel like getting into the physics.

What to do to the motor to need bigger than 2.5" exhaust? Forced induction.
 






MyExpWork

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OK, Thanks everyone.

I guess I'll stick with 2.5 for now.

Brian
 






DeRocha

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To help ease your mind that you are doing the right thing I have attached the following from one of the racing/mod sites

Pipe Sizing

We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.

Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 ½ inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!
 






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