Nissan XTerra Forum banner
1 - 13 of 13 Posts

·
Registered
Joined
·
38 Posts
Discussion Starter · #1 ·
I tried to search but I couldn't really find anything. Anyways, how is the boost regulated on the Supercharged X? Is it simply the RPM of the supercharger?

The reason I ask, is because I live in CO at 5000ft. and if boost is regulated by just the RPM of the supercharger, I can't be getting full(stock) boost from the supercharger.

If this is the case, then I would consider using a smaller pulley in order to achieve stock boost once again.

On a side note, we only get 91 octane fuel here. Apparently they don't understand or believe that FI cars exist at higher altitudes.
 

·
Sage Mentor/Moderator
Joined
·
12,216 Posts
I tried to search but I couldn't really find anything. Anyways, how is the boost regulated on the Supercharged X? Is it simply the RPM of the supercharger?

The reason I ask, is because I live in CO at 5000ft. and if boost is regulated by just the RPM of the supercharger, I can't be getting full(stock) boost from the supercharger.

If this is the case, then I would consider using a smaller pulley in order to achieve stock boost once again.

On a side note, we only get 91 octane fuel here. Apparently they don't understand or believe that FI cars exist at higher altitudes.

The RATIO of compression for a SC is better than for a turbo for example, so WWII fighters, etc, would use superchargers at high altitudes for that reason (Higher than you go BTW).

So, yes, engine rpm gives the SC rpm, its purely mechanical...and, yes a smaller pulley can be like shifting it into a higher gear on your 10 speed bike...a big wheel turning a smaller one (the smaller pulley), means that the smaller the wee one, the faster it rotates to keep up with the big one, and so forth.

A smaller pulley can add lots of horse pressure.

:D



The LOWER octane at higher altitudes is BECAUSE the flame kernal propagation characteristics REQUIRE lower octane at higher altitudes, to compensate for the higher altitudes.

This is WHY high altitude gas gas stations, etc, make regular a lower octane there, than they do at say sea level gas stations.

So, if you are down on power at high altitudes, using higher octane will make it WORSE.

So, if the engine is DESIGNED to use higher octane, lets say 93...in Denver, that might be the same as 91, and so forth, so that;'s what octane number they sell.

If the engine is designed to run on "regular", that might be 89 in NJ, and 87 in CO, and so forth.

>:D
 

·
Registered
Joined
·
38 Posts
Discussion Starter · #3 ·
That makes sense for a naturally aspirated engine. However, on vehicle with forced induction, running 1 bar at sea level is no different than running 1 bar at 5000ft. The turbo or supercharger is creating its own artificial atmosphere for the engine. So it goes to say that a forced induction engine that requires 93 octane, will still require 93 octane at higher altitudes, as long as boost pressure(absolute not relative) remains the same.

I guess my question was how the boost was limited on the xterra. If it's simply RPM related, then a reduced pressure at the inlet(higher altitude) would result in a lower pressure at the outlet, in which case a smaller pulley could compensate for that.

Or is it designed to have some overhead, in which case the boost would be regulated by the bypass valve and any excess would be bled off? Or is the bypass valve only used to drop boost in engine fault situations?

Sorry for any odd grammar. Posted this from a phone lol.
 

·
Registered
Joined
·
362 Posts
That makes sense for a naturally aspirated engine. However, on vehicle with forced induction, running 1 bar at sea level is no different than running 1 bar at 5000ft. The turbo or supercharger is creating its own artificial atmosphere for the engine. So it goes to say that a forced induction engine that requires 93 octane, will still require 93 octane at higher altitudes, as long as boost pressure(absolute not relative) remains the same.

I guess my question was how the boost was limited on the xterra. If it's simply RPM related, then a reduced pressure at the inlet(higher altitude) would result in a lower pressure at the outlet, in which case a smaller pulley could compensate for that.

Or is it designed to have some overhead, in which case the boost would be regulated by the bypass valve and any excess would be bled off? Or is the bypass valve only used to drop boost in engine fault situations?

Sorry for any odd grammar. Posted this from a phone lol.
Check out this thread

http://www.clubxterra.org/forums/showthread.php?t=14983

I believe there is some sort of internal wastegate to prevent system damage when you go from 7psi at WOT to closed.

A pulley would give you HP gains but too small will cause you issues and require other engine/ecu mods. I have read you can jump a size or two and not require tuning...
 

·
Registered
Joined
·
38 Posts
Discussion Starter · #5 ·
Check out this thread

http://www.clubxterra.org/forums/showthread.php?t=14983

I believe there is some sort of internal wastegate to prevent system damage when you go from 7psi at WOT to closed.

A pulley would give you HP gains but too small will cause you issues and require other engine/ecu mods. I have read you can jump a size or two and not require tuning...
This makes sense. Thanks for the info.

I'm not really trying to get any serious HP out of the engine, just get it back to 7psi. Well... I guess a little extra would be nice too :D

I suppose I could just stop being lazy and hook up a boost gauge to see what it's currently running.
 

·
Sage Mentor/Moderator
Joined
·
12,216 Posts
Just a note to clarify that an SC doesn't "make 1 bar" (or whatever bar) no matter what's outside.

Its a RATIO, so the PULLEY turns the wheels, and the wheels turn compressing the air it took in.

So, if its colder outside, the same ratio of compression yields a different final internal pressure than if it were hot outside.

Some for the octane...its really just part of your timing. Higher octane burns slower essentially.

I was simply providing info on WHY the high altitude gas stations have LOWER octane fuel than at lower elevations.

And, yes, an SC or turbo can help compensate for altitude...as the air is progressively less dense....but, the space shuttle could not count on an SC to compensate for the thinner air at ITS higher altitudes for example either....which it could have, if a supercharger made its own atmosphere.

:D

:D
 

·
Registered
Joined
·
38 Posts
Discussion Starter · #7 ·
Just a note to clarify that an SC doesn't "make 1 bar" (or whatever bar) no matter what's outside.

Its a RATIO, so the PULLEY turns the wheels, and the wheels turn compressing the air it took in.

So, if its colder outside, the same ratio of compression yields a different final internal pressure than if it were hot outside.

Some for the octane...its really just part of your timing. Higher octane burns slower essentially.

I was simply providing info on WHY the high altitude gas stations have LOWER octane fuel than at lower elevations.

And, yes, an SC or turbo can help compensate for altitude...as the air is progressively less dense....but, the space shuttle could not count on an SC to compensate for the thinner air at ITS higher altitudes for example either....which it could have, if a supercharger made its own atmosphere.

:D

:D
I'm not sure if we aren't exactly understanding each other here, or if we're just being picky about word usage.

I understand the "RATIO" point that you are trying to get across, as well as the mechanics of a super charger. And yes, this is one way to control boost, but it does little to answer the original question of how boost is regulated specifically on the Xterra. Boost can be regulated specifically by this "RATIO", or it can be regulated by the bypass valve. My question regarded which of the above methods was implemented on the Xterra.


And realistically, the space shuttle does use centrifugal turbo pumps(very similar to the concept of a centrifugal super charger or turbo charger) to pump oxygen and hydrogen(obviously stored in tanks) to the nozzle to CREATE an environment for the fuel to burn.
 

·
Sage Mentor/Moderator
Joined
·
12,216 Posts
Correct...the analogy of the shuttle was to essentially say it could not simply use an SC engine...which is why it doesn't, etc....as the SC doesn't make its own atm.

I also said the SC helps at high altitude, hence its use in WWII fighter planes, etc.

I also said the smaller pulley adds horse pressure, because it changes the ratio.

Lower octane helps with high altitude, and, changes the timing because of slower flame kernal propagation, etc...but a smaller pulley increases compression, which in turn requires an opposite change in timing to compensate...often compensated for with higher octane....so its a balancing act...and why the SC needs the knock sensor feedback loop more than the non-SC version.


In short, a smaller pulley will help a lot...and, I was just clarifying some of the other points for informational sake, as many readers will read these posts without a baseline.

:D
 

·
Registered
Joined
·
362 Posts
I was always under the impression the Higher octane was used in higher compression engines due to the higher temps required for ignition. Too low an octane creates detonation (explosion before spark fires) in the cylinder. Thats why we could never run anything less than 93 octane in my honda race mx bikes...

That being said i still dont run anything less than 88 octane in my N/A 2004.
 

·
Registered
Joined
·
38 Posts
Discussion Starter · #11 ·
I was always under the impression the Higher octane was used in higher compression engines due to the higher temps required for ignition. Too low an octane creates detonation (explosion before spark fires) in the cylinder. Thats why we could never run anything less than 93 octane in my honda race mx bikes...

That being said i still dont run anything less than 88 octane in my N/A 2004.
True. I do think the term "detonation" gets used a little loosely. In one case, the spark plug fires too early and the the pressure front rises too quickly before the piston is on its down stroke. In the other case, the amount of pressure and temperature in the cylinder rises to a point that it self ignites without any help from the spark plug, kind of like a diesel.
 

·
Registered
Joined
·
362 Posts
True. I do think the term "detonation" gets used a little loosely. In one case, the spark plug fires too early and the the pressure front rises too quickly before the piston is on its down stroke. In the other case, the amount of pressure and temperature in the cylinder rises to a point that it self ignites without any help from the spark plug, kind of like a diesel.
SO are you going to order the pulley?
 
1 - 13 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top