im sure this is a total newbie ? but why doesnt my car show boost when cruising @ say 55-60 mph on the hwy? is it because the turbo doesnt spool below a certian RPM or load?

 

the turbo only boost when you need it ie. high load heavy throttle so thats completely normal should be around 10 to 12 lbs of vacum at 55-60

 

ok thanks

 

10-12 inches of vacuum, not psi. -1 psi ~= 2.02 in/hg vacuum.

 

yeah the car will only produce boost when the engine cant breathe in the air fast enough, hence when you push the pedal to the metal the turbo starts spooling up forcing alot of air down its throat. When the engine cant inhale it quick enough, it forms a pressure know as BOOOOOOST, boost is a restriction in the engines ability to consume air.

 

It's all relative to the amount of load on the engine ....... at 55-60 mph you can see anywhere from -20 vac to actually making boost without moving the gas pedal from your cruising position ..... it depends on the terrain ...... up hill ..... down hill .... corners ect ect.

 

depends on what gradient your on as well. If you try to maintain 55mph going up a mountain, you will be in boost. If you roll down a mountain at 55mph, you will be in vacuum

 

You are correct... The engine must be under load because the extra heat and expansion of the exhaust is what spins up the turbo. If you are just cracking the throttle body open and have very light engine loads your turbo will just be free wheeling. But when you increase fuel the extra heat expands in the exhaust manifold to pressurize it and spin up the turbo. This is why turbocharged motors have no valve overlap... the pressure of the exhaust is higher than the pressure of the intake and it would cause lost efficiency.