Oggy249

Hi all really sorry for this post as i know it will have been answered before ,i have just fitted a boost gauge today and carnt work it out ,when i start the car on tickover it sits here


i have not been out in car yet but when i rev it it goes down then up but not that far ,when does it go to 1 bar ,sorry for eing thick

gc8-r

iTrader: (21)

Looks like you plumbed it in the right place...
You need to drive it..
Standard it will go to 0.6-0.7 unless it's been mapped or boost controller is set to a bar..
It should read in minus when sat still..

Just go for a drive and smile cause it works right

Baz82

When the car is idling it reads vaccum , it wont boost unless driving it. Also in boost 0 and above your gauge reads psi (pounds per square inch) not bar.
14.7psi = 1bar

So if the car is boosting 1 bar the needle will sit between 10 and 20psi on your gauge

JTinnovations

iTrader: (2)

First, that gauge is measuring in psi, so it won't go to 1 bar...it will go to (about) 14.7 psi instead.

Boost gauges measure pressure above and below atmospheric pressure. At idle, there is a vacuum in the inlet manifold, so it will read below "0" at idle. Only when you're "on boost" - which means wider open throttle and some load - will the needle rise above 0.

In that picture it is reading -20 psi: perfectly normal at idle

As per previous answer - go for a drive and you'll (hopefully!) see that all is well.

gc8-r

iTrader: (21)

Haha. Didn't even notice it was psi.!!!

Having a senior moment

JTinnovations

iTrader: (2)

Baz82

Its not -psi mate . Vacuum is measured in inches of mercury (inHG) and positive pressure (boost) in psi

JTinnovations

iTrader: (2)

What a confusing gauge! Psi above atmospheric and inHg below...crazy!!

Bar is much easier

JTinnovations

iTrader: (2)

And with hindsight -20 psi is a bit high for idle of course...but let's try not to confuse the OP any more

Baz82

Its like that on majority of gauges as psi and bar are positive only no minus psi or minus bar.

Baz82

Not when its in a more "readable" measurement

JTinnovations

iTrader: (2)

You can defo have negative bar, or psi, or whatever, as a boost gauge is relative to atmospheric.

Idle is -0.6 to -0.8 bar relative to atmospheric. Or 0.2-0.4 bar absolute. Or 0.6-0.8 bar of vacuum. Take your pick.

But to mix units is crazy IMHO.

Oggy249

This is why i love this you guys always know what your talking about ,thanks for the replies ,yer the gauge does read inHG bellow the 0 will take it out tomorrow see how it reads cheers

gc8-r

iTrader: (21)

After "trying" to read that... I'm off to bed...
My brain can't compute

Baz82

I reckon its because the unit for measurement of vacuum is different

JTinnovations

iTrader: (2)

Nope - the units are what you choose. No specific units for vacuum, unless by convention.

You could have a boost gauge completely using inHg if you want. Atmospheric would be (about) 29.5" of mercury, and 1 bar of boost would be (about) 59 inches. Likewise, the vacuum at idle could be 20 inches of mercury, or 0.7(ish) bar ABSOLUTE pressure, or -0.3bar vacuum.

Choose your units and stick to them!! The easiest is to measure in absolute pressure, then 0 bar is complete vacuum, 1 bar is (round about) atmospheric and a stock Subaru will boost to 1.9bar

JTinnovations

iTrader: (2)

Shall I get my (white) coat

Oggy249

Lmao

JTinnovations

iTrader: (2)

Baz82

We'll agree to disagree then

JTinnovations

iTrader: (2)

Can't resist lol...when do you say the units for vacuum is different do you mean on the OPs gauge (which it is, although I think it's daft) or "generally" (which it is not, IME - pressure is pressure)?

Baz82

I just meant the op's gauge. I know what you are saying alright and I agree the units should be kept the same. I have an old nissan track car with the factory boost gauge in mmHG
Might aswell be in units of apples.........

JTinnovations

iTrader: (2)

We agree then

Baz82

I suppose so

The Pink Ninja

iTrader: (23)

Is a higher vacuum reading a good thing or a bad thing?

Just noticed as my Defi`s are in psi that they also read in HG in negative boost or vacuum or apples etc.....I always wondered if i had a high vacuum but its about right as my calcs say that -20HG is roughly 9.7psi which is 6.6 bar??

Is this right

Baz82

No, 14.7psi is 1 bar so I dont know where you got 9.7=6.6bar from.

What does your gauge read on idle? Should be between 0.6-0.7, fair sure the defi is in kpa (similar figure to Bar)

JTinnovations

iTrader: (2)

I suspect that "6.6 bar" was a typo as -20inHg is -0.68 bar, or -9.8psi on a boost gauge. That is a perfectly respectable idle vacuum IMHO.

Baz82

Agree