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Boost Guages when car is turned OFF

adam 32

By adam 32,
in General Automotive Discussion

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adam 32 Grand Master

adam 32

Posted
Posted

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

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Jay95R33 Grand Master

Jay95R33

Posted
Posted

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 Grand Master

adam 32

Posted
  • Author
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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 Grand Master

Merli

Posted
Posted
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 Grand Master

adam 32

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Posted

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

2rismo Grand Master

2rismo

Posted
Posted

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

Guest INASNT

Posted
Posted
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 Grand Master

Jay95R33

Posted
Posted
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

:)

RadiuM Experienced

RadiuM

Posted
Posted
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

Merli Grand Master

Merli

Posted
Posted
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 Mentor

Gradenko

Posted
Posted
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.

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