What happens to vacuum with FI?

[chrisd]

If vacuum is generated from the intake manifold, then it goes away when you pressurize the manifold, no? What am I missing? How do you work around it, hook up the vacuum lines before the turbo in the airstream?

[diegom6]

Re: What happens to vacuum with FI?

If vacuum is generated from the intake manifold, then it goes away when you pressurize the manifold, no? What am I missing? How do you work around it, hook up the vacuum lines before the turbo in the airstream?

be more specified in what do you want to do plz!


yes

[Ryan Stewart]

There is still vaccumm normally. Its created when the TB is partially closed or closed and the engine is still moving. Its attempting to suck air when it cant. The turbo only pressurizes the manifold under acceleration (when vaccumm is minimal anyway). Then if you close the TB the air the turbo has to send it somewhere because it can no longer pressurize the manifold with the TB closed so that is what the BOV is for (or in more modern cars its a redirector that puts the air behind the AFM so its still metered).

Even NA motors lose vaccumm at WOT. The engine is sucking air but since the TB is soo open that the air starts to flow smooth enough to minimize vaccumm. If you ever drive an older american car often the air shuts off because the flap that controls it relies on the vaccumm. As does your booster.

Brians S/C M3 almost always reads -11lbs pressure until he is WOT where it starts to slowly make boost and runs on the positive side. The second he closes the TB it goes right back to vaccumm....

Its hella crude, and vague, but I am not an engineer. Ive just been in on installs.

[LowR3V'in]

How about steping on the breaks while on boost?
What would happen? Still break the same?

[Z3Jonathan]

How about steping on the breaks while on boost?
What would happen? Still break the same?

When Ferrari was going for their speed-endurance record a few years back in the 550 Marenello, they switched to an electric brake booster. However, a normal street car spends little time at WOT, and besides, if you mash the brakes, you're going to probably be letting off the gas at the same time.

[LowR3V'in]

Yeah but I mean if you want to build boost while cruising at 40mph. For a street race or something..

[e30Matt]

For a street race or something..

GASP

No street racing.

lol

[Ryan Stewart]

just downshift.


Subaru has a pretty ingenious for the ProSpeed Sti. They programmed the fuel system to run rich off boost so that fuel would build up and ignite in the exhaust manifold continously keeping the turbo spooled for when they hit the throttle again.

The Brake booster has a check valve that doesnt let vaccumm leak back into the manifold and the booster always has vaccumm in it on both sides. when you press the pedal you allow a leak on one side intentionally allowing the vac stored to suck the brakes on sort of. Then once you let off the pedal the rebuilds its reserve of vaccumm.

You should have one good stab at the brakes while on WOT with vaccumm but if you hit them repeatedly the brake might not rise as quickly and when it did it woudnt have much assist.

My explanation sucks. Go to howstuffworks and look at both turbo and brake booster. it has pics that show what I am trying to say.

[808BMW]

Unless your going to floor both the gas pedal and the brake pedal at the same time, you shouldn't have a problem basically.

[chrisd]

I was under the impression that the turbo would be constantly pressurizing the manifold, but now I understand that that would be a bit of a problem...
fantastic information. Thanks everyone!

DIEGO: I was just asking out of curiosity, not trying to do anything specific.

[kylebes1]

Just to kick a dead horse, the m50 intake boot off the MAF has some Vaccum lines that, I guess, go to the idle control system. How do you keep this working properly since it is on the side of the TB that would still have some pressure on it>? or did i just answer my own question?

[Ryan Stewart]

there never is really vaccumm there unless something is in front of it like a supercharger.

I was assuming that one feeds the idle control and the other was a pressure vent that just allows oil vapors to the consumed like the M20. Those its not a matter of vac or pressure its just keeping the air clean for the IC and not allowing high pressure to build in the crankcase. Usually when people boost those engines they jsut run a remote filter for the ICV and a catchcan for the oil vapors to simplify the tubing.