What is vacuum?

[colly]

Just wondering what is turbo vacuum, like when the PFC\'s reading minus boost whats actually going on and what does it mean. Whats vacuum needed for?

[antnicuk]

SUCK.

the engine is sucking air, the turbo is doing nothing,  the throttle body butterfly is closed. This means the engine is sucking against the thottle body butterfly but cant draw air through it so causes a vacume between the engine and butterfly.

Non turbo cars are always on vacume.

[nikp]

Non turbo cars are always on vacume.

Except when on full throttle....at a guess (I would presume that then they\'d be zero vacuum/pressure)

[RemarkLima]

I\'m sure N/A\'s make more vacuum at full throttle, well not at low revs buut at redline they\'re really starting to suck

[Ricco_04]

So if you put a boost guage on an N/A (some barry has probably done it! :Giggle ) what would it most likely read on idle and WOT?

[RemarkLima]

Plenty of vacuum gauge available on t\'Bay So loads of people clearly want to know what vacuum they\'re running LOL!

Anyway, AFAIK at idle you\'d get lots of vacuum as the throttle plates are closed and the engine is still sucking.

If from low revs you went straight to WOT at lower revs you wouldn\'t get much vacuum (not much air being sucked through with little restriction) and then as the revs rise you\'d get more and more vacuum as the engine spins faster and draws more air until the throttle body / filter etc become restrictive enough to create the vacuum... IYSWIM.

Stand to be corrected tho\'

[RemarkLima]

Also, added to that it\'s why if you\'re at low revs it\'s better to not just floor it as the engine bogs down on NA\'s (FI is a different matter), it\'s better to feed the throttle until there\'s enough revs to make the most of WOT.

And why throttle by wire will do all this for you, you floor it, it doesn\'t

[Rotaryguru]

Negative is sucking in air, + is forcing in air (hence turbos and superchargers are forced induction).
 
Also, once positive, the forced air creates a vortex, forcing even more air into the engine. The vortex is created by the vanes in the turbo. Thats my understanding - can anyone confirm/correct?
 
:Tongue1

[noyzboy]

I\'ve allways had trouble explaining to anyone what a vacuum tank does and what\'s in it...and then why you need it.
Had one on a Ford 100E. Rubbish Car. Vacuum wipers !!

Martin H

Age showing badly

[Martiny]

You can imagine quite a lot of energy needed to keep the engine running, to overcome this vacuum. (sometimes called "pumping loss")

[RemarkLima]

You can imagine quite a lot of energy needed to keep the engine running, to overcome this vacuum. (sometimes called "pumping loss")

Yep, quite a lot, basically engine braking is the amount of energy to overcome, so the difference in coasting in gear vs coasting in neutral.