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Why Have -7 And +7 On Standard Boost Gauge

Astro Bear

By Astro Bear,
in General Automotive Discussion

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Astro Bear Veteran

Astro Bear

Posted
Posted

Bit of a noob question I have for you all. I have searched for the answer on the forums but to no avail.

On my R33 gts-t, the standard boost gauge reads -7 0 +7. Fair enough, I understand that its not PSI, however, I want to know:

Why the gauge reads in minus as well (-). What is the minus in reference to?

Am in only using boost when the needle is between 0 and +7?

As such, it is more economical to drive with the needle below 0?

Is it even more economical to drive with the needle as close to -7 as possible?

I hope someone can help answer my questions. Many thanks. :P

2rismo Grand Master

2rismo

Posted
Posted
Why the gauge reads in minus as well (-). What is the minus in reference to?

It's not a boost gauge, it's a manifold pressure gauge. It shows both positive (boost) and negative (vacuum) situations.

Am in only using boost when the needle is between 0 and +7?

Yes.

As such, it is more economical to drive with the needle below 0?

Is it even more economical to drive with the needle as close to -7 as possible?

Almost always. Some cars have an Economy Gauge which is really just a manifold pressure gauge.

I hope someone can help answer my questions. Many thanks. :P

There ya go.

Baz Experienced

Baz

Posted
Posted

-value refers to manifold vacuum

+value refers to boost pressure.

Also remember that manifold vacuum will only ever be a positive value in turbocharged or supercharged cars (or in the exception of a ramming air suppy in some naturally aspirated cars travelling at a high speed) as the engine cannot created a positive pressure from the air that it is drawing in.

Hope that helps,

Cheers

simpletool Mentor

simpletool

Posted
Posted

Off the top of my head the 7 refers to 700mm/Hg , with 760mm being 1 atmosphere. So the 7 would estimate 0.92 Atm , or 13.5psi.

Mine reads just over three quarters from 0 to 7 when I run 11psi, although true 0 gives a reading just below the line on the gauge BUT each gauge has it's own error.

Astro Bear Veteran

Astro Bear

Posted
  • Author
Posted
Off the top of my head the 7 refers to 700mm/Hg , with 760mm being 1 atmosphere. So the 7 would estimate 0.92 Atm , or 13.5psi.

Mine reads just over three quarters from 0 to 7 when I run 11psi, although true 0 gives a reading just below the line on the gauge BUT each gauge has it's own error.

Yeah, one of my friends said that +7 is roughly one bar- around 14 psi.

HEKT1K Grand Master

HEKT1K

Posted
Posted
dunno about you guys but i dont think i've ever looked at the stock gauge while driving lol

i do!! helps for fuel economy etc.. that and i don't have an aftermarket boost gauge. i sometimes use it like a tacho, shifting up just before it makes boost. ah the joys of being a poor uni student.

Astro Bear Veteran

Astro Bear

Posted
  • Author
Posted
i do!! helps for fuel economy etc.. that and i don't have an aftermarket boost gauge. i sometimes use it like a tacho, shifting up just before it makes boost. ah the joys of being a poor uni student.

Lol, thats what I do mate. Most of the time I try to be really light with the pedal, so that I don't even go half way between -7 and 0. However, its also really hard to resist giving the throttle a stab every now and then haha.

GTR-N1 Grand Master

GTR-N1

Posted
Posted

Decades ago, Smiths and other gauge companies developed what they called a Vacuum Pressure Gauge that's connected to the inlet manifold.

-ve pressure will build as you decelerate = using less fuel

+ve pressure will build as you accelerate

0 pressure is maintained as you neither accelerate or decelerate on level ground ( but this is approximate)

BMWs have this gauge as standard at the base of the Speedo ( as do some other makes I'm sure)

Watching this gauge all the time would destroy the fun of driving. BUT...

yeh, I'd be watching it carefully if I was running on 'Empty' lolz

Tez

(OO)SKYLINE(OO) Grand Master

(OO)SKYLINE(OO)

Posted
Posted
Yeah, one of my friends said that +7 is roughly one bar- around 14 psi.

Hold up!

+7 Bar = 101.5266 Psi. I don't think any car would pump that much boost.

1 Bar is = +1 Bar = 14.5038 Psi.

The stock boost gauge is measured in Bars and its -0.7 / +0.7 Bar

+0.7 Bar = 10.15266 Psi

When the Pointer in the gauge is under 0 Bar, it is more economical to drive as the turbo(s) are not boosting so you do not use as much fuel, unitll you hammer the right foot, the turbo(s) start to spool then you use more fuel, hence the pointer goes up.

Cool!

2rismo Grand Master

2rismo

Posted
Posted
Hold up!

+7 Bar = 101.5266 Psi. I don't think any car would pump that much boost.

1 Bar is = +1 Bar = 14.5038 Psi.

The stock boost gauge is measured in Bars and its -0.7 / +0.7 Bar

+0.7 Bar = 10.15266 Psi

Cool!

Slow down, champ!!!

The stock gauge measures +/- 7 (x100mm) of Mercury. It's not PSI/Bar/Kpa or anything else.

B0oStEr Mentor

B0oStEr

Posted
Posted
Hold up!

+7 Bar = 101.5266 Psi. I don't think any car would pump that much boost.

1 Bar is = +1 Bar = 14.5038 Psi.

The stock boost gauge is measured in Bars and its -0.7 / +0.7 Bar

+0.7 Bar = 10.15266 Psi

When the Pointer in the gauge is under 0 Bar, it is more economical to drive as the turbo(s) are not boosting so you do not use as much fuel, unitll you hammer the right foot, the turbo(s) start to spool then you use more fuel, hence the pointer goes up.

Cool!

Except it doesn't read Bar.

It reads x100mm of Hg as per post #4

(OO)SKYLINE(OO) Grand Master

(OO)SKYLINE(OO)

Posted
Posted
Slow down, champ!!!

The stock gauge measures +/- 7 (x100mm) of Mercury. It's not PSI/Bar/Kpa or anything else.

:P

I also gota HKS boost gauge as well. I look at the Stock one and the Aftermarket one and check out the levels to see where they are at, so they give me the equivalent reading of each other.

I say the HKS one is a lot more acurate then the stock on.

trel Mentor

trel

Posted
Posted

adam! lol how is the car going man? ummm yeah as some of the others have said that gauge reads in mmhg, +7 being around 14ish psi and then the -7 being your manifold pressure. give me a call one of these days for a cruise and a chat or something dude.

Astro Bear Veteran

Astro Bear

Posted
  • Author
Posted
adam! lol how is the car going man? ummm yeah as some of the others have said that gauge reads in mmhg, +7 being around 14ish psi and then the -7 being your manifold pressure. give me a call one of these days for a cruise and a chat or something dude.

Ev! How is the new car? You must send me pictures. Sounds good to me mate.

trel Mentor

trel

Posted
Posted

car is going good mate. hope youre having fun with my baby :P lol. im going to put pics of my car up in the next few days in the gallery so check them out when you can. were probably gonna have a cruise this friday night. mate got his rx8 so were due for a nice drive! give me a ring.

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