makeing sense of intake pressure?

[wilson4]

ok while at idle my MAP sensor tells me i have around 5.4 psi in the manifold, when the car shuts off it goes up to 14.6, which would make perfect sense becuase the cylinders are not drawing air anymore and standard atmosphere is around that number, when i go full open throttle, pressure goes up to around 14.6 which again makes sense becuase throttle plate is letting tons of air in, so what happens if i were to add a turbocharger in this car? since my brake booster line runs off vaccum pressure if my turbo put too much psi into manifold then it wouldnt work anymore? how would you over come this, how close can you get to standard atmosphere before the vaccum is not effective anymore on various systems in the car that require it? also how much is too much boost psi? and how do you tell?

Thanks,

[UncleBob]

the brake booster has a one way valve, and can operate off of stored vacuum for a couple brake presses before it consumes the it and you lose the assist from the booster.

Usually you let off the gas to brake, and as soon as you let off the gas, you are supplying vacuum, boosted engine or no.

The best way to find maximum boost for an engine build is to use knock sensors to detect when it starts detonating at a given boost level. Some also use microphones to hear the detonation. Keep in mind that where you set it might not be the same situation for the future. Sometimes you can get gas from a pump that is lower than advertised, or for that matter, hot weather can sometimes cause the threshold to drop.

[wilson4]

is there a limit then to the amount of boost? like say the turbo is so powerfull that even at idle the pressure in intake is very close to standard outside, how would this affect the brake booster, etc, then?

[UncleBob]

if the throttle plate is closed, then there will be vacuum behind the throttle plate. Even if there was boost on the other side of the throttle, but really, you'll never see that situation unless your turbo was so horribly undersized it choked at 2K RPM's.

The amount of boost you can acheive is partially what turbo you have, and partially how well sized the turbo is for the engine. But thats only theoretical, most engines won't be capable of using maximum boost without a loooot of modifications and a very high octane gas.

[speedfreak]

Originally Posted by wilson4

is there a limit then to the amount of boost? like say the turbo is so powerfull that even at idle the pressure in intake is very close to standard outside, how would this affect the brake booster, etc, then?

That will never happen in a properly working system. If you can make 14+psi of boost in the manifold with the throttle 98% closed, something is seriosly wrong.