How do you suppose boost created by the supercharger is limited below ~3000 rpms? I would think this engine could create much more power down low right off the line. Perhaps the engineers limited boost to save the driveline. But how?

You said it yourself - lower RPMs.

To know what's going on with a SC you should have a boost or boost/vac gauge.
The Eaton SC is a belt driven, fixed displacement air pump. It will provide boost based on engine RPM, give or take a little. But it will not provide boost *until* the bypass valve is activated. I do not know exactly how nissan controls that (vacuum) valve. The ECU may restrict activation under some conditions. This is just one reason why a boost gauge is usefull.
With the instillation I did on my car, there is nothing to restrict the bypass valve. The SC will provide boost on demand just above idle. I get upwards of 4 psi at 2,000 RPM (and more later). The extra torque at low speed fills the 'hole' before the cams go to work at high RPMs.
Learn more about the Eaton SC:

http://automotive.eaton.com/product/engine_controls/supercharger.html

dave and xtoy and miatank(with the SC) since 93

The X supercharger has a bypass valve that is controlled by the throttle position, not RPM. When it is open, the extra pressure is sent back to the inlet side of the unit and the loop keeps the pressure down the manifold at normal atmospheric pressure. It stays open until "WOT" (wide open throttle) according to Nissan but that's got to be a little vague, more like 3/4 throttle or more I'd say.
The bypass valve should close under hard throttle at any time, RPMs shouldn't matter. If you stomp it from idle, the SC will go off bypass right away even at 1000 RPM.
Now, there is a tweak that will keep the bypass valve closed all the time so that you're always under boost, but that will kill your gas mileage, void your warranty, and probably shorten the life of the SC and engine.

Brent

Brent,

You are right about the boost availability at all RPM's, even off idle (if you press the pedal hard enough ).

As well if the mod is the vacuum line mod that I modified from the F150 website then it does not keep the bypass closed at all times...it only removes the bypass control solenoid from the circuit - allowing no control to the ecm to modulate (or limit boost). It is not a mechanical modification that would physically keep the bypass valve closed at all times

Pacopico -
Thanks for describing what nissan does to "fool" with the boost control. Bypassing the solenoid should make boost available on demand at any time - the way the SC on my car is set-up. It should provide excellent low speed torque. I'd like to try a SC-X one with that mod.

dave and xtoy - the car will do for now

I've noticed the limitation to be rather abrupt. I can floor it from idle and right at 3000 rpms, the boost hits like I just got hit from behind. I guess it is an ECM/vacuum controlled solenoid that closes the bypass at that engine speed. Am I right in saying a supercharger can only produce so much boost at a given rpm? And then the bypass valve opens again to limit boost? I wonder if it can be controlled manually (set a high limit with a boost controller and regulate how good the gas mileage you get with your foot)It just seems strange to me that this truck's acceleration is so characteristic of a turbo with lag.

Think of your engine as having two intake systems. A standard intake where the engine draws the air/fuel mixture in. A second intake that forces the air/fuel into the engine under pressure. This system, using the SC produces more power.
Why do you want two systems? Under easy acceleration or cruise, the standard system is fine. No need to use the SC when it is not needed.
It is not made for 100% duty cycle and would use lots of fuel.
Need power for fast acceleration or climbing a steep hill? The SC is no longer bypassed - the boost yeilds more power. This action is controlled by the 'bypass valve'. You observed that boost seems to be delayed til 3,000 RPMs. A solenoid valve controlled by the ECU blocks vacuum to the bypass valve. The SC is quite able to provide boost at much lower speed, just over idle. Check the specs:
http://www.automotive.eaton.com/product/engine_controls/M62.html

Taking the solenoid out of the picture would allow for boost at any time. Anything beyond easy acceleration should activate the SC and give boost. After reaching cruise speed, the system would revert to standard intake mode.

dave and xtoy - 2 engines in one

I must say I agree with this statement. No matter how far I put my foot in it, power comes on with a rush at 3,000 rpm. Below that it's quite sluggish...I think I need some type of gauge to see what is going on..I too am ...

Gentlemen,
(staubers and xterradon)

Please get a boost gauge. The truck does build boost (even in stock form) below 3000 rpm. The fact that is at hand are two things:

1. The supercharger is not spinning at enough rpm's to produce more than 5lbs of boost below 3000 rpm's. In stock form the boost will stay at 5 psi up to 3,000 rpms, from there it will build to it's maimum of 7 psi. After the vacuum line mod it will start at the same pressure, and build to a maximum of 9 psi at 5999 rpm. The rush you feel at 3000 rpm is partially when the supercharger has reached enough rpm to start producing more boost...Remember that the supercharger on the trucks is technically a compressor. All compressors have a rpm/efficiency rating, and therefore are not producing their maximum boost at all rpm's. To keep the compressor within it's maximum rating it will inherently have some losses at the lower end due to the wide operating range (rpm's) of the engine. The same applys for turbo's (a/r ratio), and blowers.

2. The engine is still camshaft dependent (just like many other cars), and to enhance fuel economy the cams are ground to provied good fuel economy at lower rpm's (where the sane minded people usually drive), and better power at higher rpm's. The rush you feel at 3000 rpms is the engine coming up on to the camshaft's designed torque/horsepower curve. If you would look at the horsepower/torque curve for the vg33sc you would see that at (or about) 3000 rpm's the horsepower is surpassing the torque, which means that the power is being produced at a greater rate - hence faster acceleration. If you were to load the truck (towing over 4000 lbs) you would not feel the rush, as the acceleration would then be dependent on the torque more than the horsepower.

I don't believe anyone here wants to go into the depths of torque -vs- horsepower...

That said, I have the same rush in my X at 3000 rpm's, as well as my 2002 v-6 powered F150 (company car) at the same rpm's...My mod won't help that.

Dave - I'll send you the pics for the mod if you do by an S/C X...

Ahh, so the aftermarket smaller diameter supercharger pulley allows the unit to spin faster thus creating more boost at all engine speeds? Or will the solenoid still limit the low rpm boost?

I think you are all missing the point of the super charger. If you go to your local dealer and pick up the brochure for the 2002 Xterra, you will find the following information on the fifth page under "Supercharger": More Xterra power. And lots of it. AND A BOOST THAT IS IMMEDIATE (NO TURBO SPOOL-UP LAG), CONSISTENT AND ON TAP AT ANY ALTITUDE.
Nissan advertises that there is no lag...we all know different. That is false advertising. I am trying to see if I can't get a refund or a lemon-law judgement. If not, I'm trading mine in, even if it means I lose a couple grand...this is BULLS#%T!!!

Ever try a test drive before buying? Sounds like that would have saved you some trouble and money. I love my S/C and it has plenty of boost for me when I want it. If it came on too early, I would be buying stock in Firestone!

Boo fuckin' hoo. You were expecting a hot rod?

Brent

Well that would be pretty fucking stupid, now wouldn't it...

Like Brent said - it's a 4,000 pound truck, not a freakin' sports car.