What happens to vacuum when you're boosting?

[M52 POWER!]

Yea this is prolly a noob FI question but today I was thinking about how boost effects the engine and wondered how the engine still creates a vacuum for certain accessories when you are compressing air into the engine? Is there such thing as a one way valve for the vacuum hoses or does it not matter at all if the vacuum isn't going temporarily?

[nickdrivesm3]

vacuum becomes boost because you're pressurizing the cylinder. If you were NA your vacuum would drop but wouldn't pressurize.

[M52 POWER!]

Yea I know but some devices/accessories require there to be a vacuum to operate right? So how can they operate if they go from negative pressure to positive pressure so fast?

[e30-323ti]

Most vehicles have one-way valves (even NA) to stop any damage to accessories under boost (or back-fire in a NA).

Some rally cars run hydraulic brake boosters (or no booster) so they can run a anti-lag system (mmmm, 20psi at idle).

[BlownM3]

The only thing that gets thrown off is a MAP sensor if the computer isn't supposed to see boost. The brake booster and such should have a check valve. I know some N/A cars with big cams don't pull much vaccum, than they use a vaccum tank that holds vaccum as a back up. I have not heard of any problems with F/I BMW's.

[///3oris]

It would be much easier answering your question if you told us your specific concern.

Boris

[r6jon]

Sorry to dig up this old thread, but I am pondering the same..

Will the functionality of the brake booster be affected by the presence of boost (S/C setup)?

I read about the check valve... is it necessary to verify, upgrade or maintain this check valve? Will the brake performance be inconsistent because of the occasional positive manifold pressures? For instance, aggressive heel-toe shifting and the like?

[slimjim]

my golf 1.8t had a 'vacuum reservoir', complete with solenoid controling the path of the vacuum hoses.

i had no clue how it worked, actually it tended to leak boost.

[M52 POWER!]

Wow a very old thread. I still wonder this though...

[5mall5nail5]

Sorry to dig up this old thread, but I am pondering the same..

Will the functionality of the brake booster be affected by the presence of boost (S/C setup)?

I read about the check valve... is it necessary to verify, upgrade or maintain this check valve? Will the brake performance be inconsistent because of the occasional positive manifold pressures? For instance, aggressive heel-toe shifting and the like?



my theory is, you will NEVER need power assisted brakes while on boost... think about it.... how many times do you rev over 3,000 rpm and hold the brakes? And even then, if you were to try and brakestand a launch, you don't need the power brakes, they should still function un-powered.

[DefactoM6]

ehm.....brake boosting......

[techno550]

the brake booster is vacuum assisted. It also happens to have a rather large vacuum resevoir. That resevoir is attached to the intake manifold... but also has a check valve inline. Unless that resevoir is broken (leaks), you'll never have an issue.

For that system, you only pull meaningful vacuum levels off throttle anyway. So boosted or not, it won't care.

You're off throttle when on the brakes anyway, so it goes back to full vacuum then.

[r6jon]

the brake booster is vacuum assisted. It also happens to have a rather large vacuum resevoir. That resevoir is attached to the intake manifold... but also has a check valve inline. Unless that resevoir is broken (leaks), you'll never have an issue.

For that system, you only pull meaningful vacuum levels off throttle anyway. So boosted or not, it won't care.

You're off throttle when on the brakes anyway, so it goes back to full vacuum then.

Aaah, I see..

So, for heel-toe shifting (blipping past 4k while braking hard), the vacuum "reservoir" you are referring has enough to take you from 5th to 2nd for instance?

(edit: 5th to 2nd I mean 5th -> 4th -> 3rd -> 2nd. You know, show off style :D )

[Rich V]

Aaah, I see..

So, for heel-toe shifting (blipping past 4k while braking hard), the vacuum "reservoir" you are referring has enough to take you from 5th to 2nd for instance?

(edit: 5th to 2nd I mean 5th -> 4th -> 3rd -> 2nd. You know, show off style :D )

Blipping the throttle create such a lack of vacuum for such a short period of time it really is irrelevant.

If you left foot brake on a race track I could see it comming in to play (even on an N/A car), but there is enough "stored vacuum" to keep it going long enough that you shouldn't have to worry.