I have seen alot of conflicting information on here about exhaust diameter so I went out and measured myself. All of these measurements were done with a digital caliper. These measurements were done on an '03 X and may differ from yours. This may be old news, but I just wanted to clear up the confusion.

The after-cat piping is 1.75" and becomes 2.25" at the "Y" all the way to the muffler inlet. The muffler outlet is 2.15" and the piping read anywhere from 2.12"-2.15" all the way to the tip. The tip is 2.5".

I don't have Photoshop at work, so I did a quick MS Paint diagram for those wanting to see.

interesting that post muffler isn't the same diameter as pre-muff

such a cute little pea-shooter.

i plan on doing a full engine-back exhaust makeover. that little 1.75" pipe seems extremely constricting.. and to change diamters that much really does nothing for performance while it's losing back pressure.. meh, i don't like it.

go w/ 2.5 from cats to tailpipe

i have a 2002 v6 and i am running staights from the cats (no muffler) and i can't tell any difference between 2.5 and staights to stock pipe and magnaflow muffer.

so i believe 2.5 is the way to go

Bigger is not always better. On a street driven car that sees 95% of its time in the lower rpms, I'd rather have a pipe that's too small than too big. People tend to think a bigger pipe is better because it "decreases backpressure" but that is only half right. Yes, backpressure in a general sense, isn't wanted for an engine to perform optimally, but what is wanted is the high velocities that create a scavenging effect in the exhaust. Scavenging is basically the vacuum created when the exhaust valve opens. To have higher exhaust velocities, pipe diameters obviously have to be kept smaller.

Knowing this, you have to look at your car and it's application and determine what the proper pipe diameter should be. To illustrate this, I'll bore you with some math.

A pipe with an outer diameter of 1.75" has a flow area of 2.23" squared. To get this, I took 1.75 and subtracted the pipe thickness (used .065") and came up with 1.685". I then divided it by 2, which gave me the radius, .8425". I then squared it and multiplied it by pi (A=piR2) and came up with 2.23" squared.

Here's a list of the pipe diameters and their flow area:

1.75"=2.23"2
2.00"=2.94"2
2.25"=3.80"2
2.50"=4.70"2

So you can see, as you increase pipe diameter, you increase flow area exponentially, but this still doesn't tell us what diameter to use for our Xterra.

The general rule for exhaust is that is a straight pipe will flow 115cfm per square inch (pipe lengths make this differ, but this is generalized). So we can conclude that an exhaust diameter of 1.75" will flow around 256cfm. We multiply this by 2 since we have 2 exhaust pipes coming from the engine, and we get 512cfm. So we have concluded that our engines can produce 512cfm of air before we need to move up in exhaust diameter. Well, that still doesn't give us any answers.

From years and years of testing, experts have concluded that you can produce 2.2cfm per HP and achieve no backpressure loss. So if we take our Xterras stock output at the crank (~200hp), we know we can produce 440cfm without any backpressure loss.

So to tie together all the loose ends, we know that our 1.75" exhaust will support 512cfm. Knowing that 2.2cfm per open-pipe HP will give you 0 backpressure loss, we can determine how much pipe our Xterra needs. If our engines can produce 440cfm in stock form with no restictions, our 512cfm 1.75" pipe is more than enough. It will take about 40 flywheel HP before we need to move up in pipe diameter.

When talking about pipe diameter after the "Y", this becomes less important due to the cooler temperatures of the exhaust gasses which means they have condensed.

In my opinion (whatever its worth) I wouldn't go with anything bigger than a 2" secondary with 2.5" after the "Y", and this is for a modded X.

My head hurts.

Dude, you made my head hurt.

I had heard that (lamens terms) "some back pressure can be a good thing" anyway. Out of curiousity, where did you get your generalizations?

I have a genius for a boyfriend

Backpressure is not a good thing. When people say they feel or don't feel backpressure, they're really feeling the lack or presence of exhaust gas velocity. Kind of like if you blow through a straw then blow through a toiletpaper roll. It's not that the straw has more backpressure, just higher air velocity. Velocity, good, backpressure, bad. Backpressure would be created by clogged cats or a poorly flowing muffler.

Depending on your desired rpm, your pipe diameter will vary. The explaination above is for a street-driven car, like I said. If you turn your X into a track-only, 10 second, iron block 454 w/ a 300 shot, then of course your desired diameter will be different.

You guys make it seem like the VG engine needs backpressure. I don't know if I agree with you on that, but I plan on making my exhaust piping 2.5" from the headers to mufflers to a 3" (or 4") single outlet (cats removed). With an intake and pulley, I'm guessing that I'll hopefully get 300 hp at the crank.

What I'm more curious to know is what some of you predict will happen if I go with this exhaust setup.

The part that's missing is the low rpm torque, which typically is increased by retaining some back pressure...so, back pressure is also good, and bad.



Depends when you get it.

It'll make a lot more noise than motion?

I'm with you though. Some day (After Warranty) I want to create a dual exhaust, but not before port matching, etc.

I have about 50,000 miles to go.

No, low-end is felt by higher exhaust gas velocity, which comes from smaller diameter exhaust. Like I said, backpressure is caused by something that hinders exhaust gas, like a muffler or cat. You wouldn't notice an increase of low-end torque by adding extra cats, would you?

I think that's too much exhaust. Keep in mind an LS1 powered car comes from the factory with 2.75" exhaust which can easily support 400hp. V8 Mustangs come from the factory with 2.25" I believe. My 475 crank hp car has 3" all the way back.

Yeah, you might have good peak numbers with the exhaust you're planning on running, but you'll notice a definite lag driving around town.

I'm exhausted reading this all

one guy from SCCX just had his '05 fitted with a gibson prototype exhaust. It was a larger diameter than stock and while offroading he reported a noticable loss of low end torque, enough that he was gonna take it back and have them redo it.