Turbo/boost Guage question

I see many boost/turbo guages, and they read 0, 10, 20 etc. But they also go in the NEGATIVES? How is that? Why would boost become lower than 0 bar/psi?

Originally Posted by Bimmerphile

I see many boost/turbo guages, and they read 0, 10, 20 etc. But they also go in the NEGATIVES? How is that? Why would boost become lower than 0 bar/psi?

im guessing negative boost?

Originally Posted by Bimmerphile

I see many boost/turbo guages, and they read 0, 10, 20 etc. But they also go in the NEGATIVES? How is that? Why would boost become lower than 0 bar/psi?

Vacuum.

When you accelerate, the compressor spins up and builds up pressure. Hence you see the needle move from Vac to 0 to Boost. When you shift, (thus letting go of gas), the compressor is under less load, will release the pressure. Which is why, the needle bounces back and forth.

Originally Posted by Mr Paddle.Shift

Vacuum.

When you accelerate, the compressor spins up and builds up pressure. Hence you see the needle move from Vac to 0 to Boost. When you shift, (thus letting go of gas), the compressor is under less load, will release the pressure. Which is why, the needle bounces back and forth.

vince is the man

thanks for the replies, and yes, vince is the man

does this only apply to turbos or would it aply to superchargers also?

Originally Posted by jj269ci

does this only apply to turbos or would it aply to superchargers also?

This is basically what Vincent said but whatever.

Without forced induction all cars are going to run a vacuum in the intake manifold, this is how air is 'sucked' into the car. All forced induction systems have some boost lag so at low RPMS the compressor might be putting out 1 psi but it has to first overcome the vacuum that the engine is making before it equal (14.6 psi or atmospheric pressure). After that it is positive boost but if you think about it running at 0 boost is actually still a power increase as the engine doesn't have to pull against the vacuum but that is off track.

So to answer your question, YES.

Tim

Originally Posted by tim330i

This is basically what Vincent said but whatever.

Without forced induction all cars are going to run a vacuum in the intake manifold, this is how air is 'sucked' into the car. All forced induction systems have some boost lag so at low RPMS the compressor might be putting out 1 psi but it has to first overcome the vacuum that the engine is making before it equal (14.6 psi or atmospheric pressure). After that it is positive boost but if you think about it running at 0 boost is actually still a power increase as the engine doesn't have to pull against the vacuum but that is off track.

So to answer your question, YES.

Tim

A turbo car that is not running any boost will always have a slight vacuum. If you put your mouth on a boost gauge, you can suck hard enough to show a vacuum. If you try to blow and make the needle indicate boost, your head will pop with exertion and the needle won't move.

Don't ask me how I know this...