Boost Guages when car is turned OFF

[adam 32]

what does everyones guage sit at when the car is turned off? mine doesnt seem to return to 0. it sits on slight vacuum... i replaced the guage cos i thought it might be a fault with that, but even now the brand new one is doing it... is it a problem with my t-piece/plumbing of the vacuum hose? or do alot of guages do this? what effect does this have on their accuracy? should you assume theyre reading low...?

thanks

adam

[ix9]

on a 'line i was taking for a test drive i noticed the boost gauge sat WAY in the vacuum area while switched off.... when the engine was idling, it stayed at 0 the whole time...

I thought it looked a bit strange too...

[adam 32]

haha yeh at idle it should read well into vacuum... im talking about when the car is switched off as there should be no vacuum or pressure and therefore the guage should sit on 0

[XCUZME]

Is it an autometer one? they never sit at the same place on idle

when i bought mine there was 10 in the shop and everyone lined up near zero differently. give it a bit of a shake and it'll move.

weird.

[Jay95R33]

Did you take any notice of where it sat before you installed it??

If it was reading on zero when the gauge is sitting in your hand and not connected to anything, it means that you still have a vacum left in your system when you switch the motor off.

BTW: This is VERY unlikly !! I'd say the gauge isn't spot on. Make sure you take that into account when winding the boost up.

J

[adam 32]

yeh it used to sit on 0. but wobble around a little...

maybe there is some vacuum?! ill give it 1psi inaccuracy just to be sure i guess. its just anoying that 2 guages now dont read 100%, im kindah on edge with the turbo being too big for an internally std motor so want to keep an eye on any spikes etc

[Merli]

Originally posted by Jay95R33

it means that you still have a vacum left in your system when you switch the motor off.

This is 100% impossible. When the engine is switched off, there WILL be 0-vacuum/0-pressure in your intake manifold.

Why? Because there is a free path for air to flow through your air filter, and intake manifold/piping pressure will stabilise to atmosphere, causing 0-vacuum.

If you gauge isn't sitting on zero when it should be... I dunno what that means, but I put forward this as a reason why Japanese Electronic gauges read zero at vacuum, and mechanical Autometer gauges are known "crap gauges".

[adam 32]

ive heard autometers are pretty accurate...

it seems to read alright... i duno. its pissing me off though!!

it reads positive boost as the turbo spools up, and reads 14psi when the dyno set it to 14psi from their guage... its just the way its sitting when the cars off thats a bit worrying

[lou15x]

this is with mechanical gauges? what about electrical ones? where does the needle point when it's not in operation?

[2rismo]

I don't know about boost guages but autometer tachos never evre sit back at zero. They autometer website has a response in their faq about it saying THEY ALL DO IT DONT WORRY!

Mechanical boost guage though?!?! Wouldv't thought this would zero perfectly...

Just my 2.2c

[Guest INASNT]

Originally posted by adam 32

ive heard autometers are pretty accurate...

it seems to read alright... i duno. its pissing me off though!!

it reads positive boost as the turbo spools up, and reads 14psi when the dyno set it to 14psi from their guage... its just the way its sitting when the cars off thats a bit worrying

i think u heard wrong!

most of us know autometer gauges r crap and inaccurate

[Jay95R33]

Originally posted by Merli

Because there is a free path for air to flow through your air filter, and intake manifold/piping pressure will stabilise to atmosphere, causing 0-vacuum.

Unless you've got a throttle butterfly in the throttle body like the Skylines do. LOL

Naa, just joking. If there's vacume left in the system that means you've got yourself a VERY gummed up throttle butterfly AND your valves are stuffed !!!! Cause out of 6 cylinders I'd say that at least one inlet one will be open at some stage.

J

[JH32]

Don't worry Adam ... at least you know that yours isn't as bad as my shitty Splitfire guage

[RadiuM]

Originally posted by JiMiH

Don't worry Adam ... at least you know that yours isn't as bad as my shitty Splitfire guage

either your gauge is a quality gauge... or his is a shitty gauge as they come from the same factory.

Shaun

[adam 32]

however autometer are known as being of higher quality than splitfire. its nothing to really worry about. its reading approx 0.5-1in mercury vacuum

maybe i should just shutup

[Merli]

Originally posted by Jay95R33

Unless you've got a throttle butterfly in the throttle body like the Skylines do.  LOL  

Naa, just joking.  If there's vacume left in the system that means you've got yourself a VERY gummed up throttle butterfly AND your valves are stuffed !!!!  Cause out of 6 cylinders I'd say that at least one inlet one will be open at some stage.

No.

Even if your throttle was stuck 100% open, you'd still have 0 vacuum because pressure will still stabilse through the air filter.

As long as there is access to atmosphere anywhere in the path of the intake manifold/piping, pressure in the aforementioned manifold/piping will stabilse to atmosphere/0 vacuum.

You will *NEVER* have vacuum "left over" in the "system". Period. Unless you're talking about nanoseconds after you turn the ignition off.

I repeat. As long as there is access to atmosphere anywhere in the path of the intake manifold/piping (through the air filter), pressure in the aforementioned manifold/piping will stabilse to atmosphere/0 vacuum.

[Gradenko]

Originally posted by adam 32

mine doesnt seem to return to 0. it sits on slight vacuum...

From what I understand, mechanical gauges that read vacum and boost are actually two gauges in one. Theres two measuring mechanisms in each guage, one to measure vacum and one for positive pressure (seems a single device can't do both jobs). So when the gauge is reading atmosphere (engine off), the needle gets stuck between the crossover points of the two senors because 1 atm isn't enough to trigger the positve pressure sensor.

My needle always rests before 0 too. A couple of taps on the gauge glass always free's it. It doesn't affect the (in)accuracy of the guage, its just a presentation thing.

[Jay95R33]

Originally posted by Merli

Even if your throttle was stuck 100% open,

He he he, don't you mean 100% closed.. ie. Off throttle.

Anyway, I agree. it's impossible to have vacume left in the system.

J

[adam 32]

the guage has a dead area around 0. and it sits in there... i rechecked the plumbing today and i cant see anything wrong wit it. just forget about it i guess!