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.