Quick Factory Boost Guage Question

[Damo Monster]

hi there..

lately i have noticed that my factory boost guage in my 32 always seems to be showing a bit of boost, even at idle, its always under 0 at idle but it sits a bit above -7... the needle goes back down when the car is off so its not a stuck needle, but was wondering if this is common or if my turbo etc. could be stuffed?

cheers

[paulr33]

its manifold vacuum, it should be -5 at idle

[Damo Monster]

its manifold vacuum, it should be -5 at idle

whats manifold vacuum? is that a fault or something that can be fixed?

[fordman351]

Manifold vaccuum is exactly that, negative pressure (vaccuum) in the inlet manifold. The engine, while idling, does not produce enough exhaust gasses to spin the turbine, therefore the engine just draws air without the compressor forcing it in. Basically like an N/A engine would.

I think i just made a fool of myself , feel free to correct me, i'm here to learn too .

[Damo Monster]

Manifold vaccuum is exactly that, negative pressure (vaccuum) in the inlet manifold. The engine, while idling, does not produce enough exhaust gasses to spin the turbine, therefore the engine just draws air without the compressor forcing it in. Basically like an N/A engine would.

I think i just made a fool of myself , feel free to correct me, i'm here to learn too .

yeah i get what your saying there.. dont know enough myself to know if thats right or not but it sounds like it could be.. what worries me though is i've had the car almost two years and never really noticed it doing that before lol...

[R31Nismoid]

www.howstuffworks.com

useful

[Damo Monster]

www.howstuffworks.com

useful

yes i started reading some of that, and from the wikipedia site.. but i was more wondering why it does it and idle, and at 100km/ph, and yet i havent ever seen it doing it much before.. i was wondering if it meant my gauge has been a bit unreliable or if i should be getting the turbo looked at... but then again, it might be completely normal and i can go back to sleeping at nights lol...

[JustinP]

yes i started reading some of that, and from the wikipedia site.. but i was more wondering why it does it and idle, and at 100km/ph, and yet i havent ever seen it doing it much before.. i was wondering if it meant my gauge has been a bit unreliable or if i should be getting the turbo looked at... but then again, it might be completely normal and i can go back to sleeping at nights lol...

I can assure you it's normal

[Damo Monster]

I can assure you it's normal

awesome.. thanks for that.. cheers for all of the quick replies...

thanks,

damo

[Link]

Check your hose to the map sensor - it could be split and you have a vacuum leak

[MANWHORE]

It's normal! Every skyline i've seen has idled at roughly that.

I'd grab my manual, but i'm not at home.

Edited December 29, 2006 by MANWHORE

[mad082]

if you have only just started noticing it, it may be that your gauge hasn't been working properly till now.

the vacuum is caused by the butterfly in the throttle body being shut, like holding your hand over the end of a vacuum cleaner, therefore it creates a vacuum in the manifold. as you open the throttle the vacuum gets less and less until you have the throttle open enough where the engine can suck in as much air as it is using (0 vacuum pressue).

that is a basic rundown on what happens.

[RANDY]

lately i have noticed that my factory boost guage in my 32 always seems to be showing a bit of boost, even at idle

As mad082 says, that's vacuum. Boost is positive pressure, so anything less than 0 is vacuum.

Also note that the guage is in millimeters of murcury mmHg, not psi or bar or cowdung. ;o)

So... if you are cruising down a big hill and not accelerating, the engine is still turning quite quickly, and because of the compression in the cylinders, air is being pushed out the exhaust, but with the throttle closed it's hard for the motor to take any air in to match what is going out... subsequently, the vacuum in the manifold increases, and you get closer to -7mmHg (roughly 14psi) in vacuum.

Same thing at idle or slow cruising, but not as exaggerated as the above scenario.

Edited January 2, 2007 by RANDY

[mad082]

just so people with aftermarket boost gauges don't get confused, most aftermarket boost gauges read either psi or bar in positive, and most that read psi positive boost don't read psi in vacuum. in vacuum they read inches vacuum or something like that.