redbullgotwings

Hey guys,
Ive been trouble shooting my 2.7t issues for a while now. installed a boost gauge to get a more accurate idea of whats going on. I cant find an answer anywhere online or in the forums. What should my boost pressure read at idle?

I know its about 9 psi max (i am only getting 7) and some 1.8t owners say they are at -20 psi at idle. I am only reading -10 psi at idle. I am going to do a boost leak test anyways, i just wanna know what the correct reading should be.

thanks,
RedBull

SloopJohnB@mac.com

Your engine does not have boost at idle under normal circumstances. It would be very difficult to get -20psi at idle since atmospheric is only 14.7. Boost gauges often have dual scales such that boost is read in psi and vacuum in inches Hg. You have to read the gauge scale. Atmospheric pressure in in Hg is almost 30 in so chances are someone told you their boost gauge read -20 but they should have said in. Hg (inches of mercury).

Throttled engines do not have boost at idle, they have vacuum or manifold pressure less than atmospheric. Often -20 or higher inches Hg vacuum when the throttle is lifted and the wheels drive the engine.

Unthrottled engines, like diesels or perhaps a valvetronic where power is controlled by fuel delivery (diesel) or valve lift rather than a throttle plate have zero vacuum, or a slight vacuum due to air filter and intake piping pressure drop. BMW's valvetronic retains a throttle plate for emergency shutdown and some diesels have an emergency shutdown throttle plate but in general these engines do not have vacuum at idle nor do they have boost pressure.

Valvetronic..from BMW:
This highly advanced technology replaces the conventional throttle butterfly with a electrical mechanism that controls the amount of lift of the individual intake valves on each cylinder. Your engine is able to breathe freely, delivering better performance while using less fuel.
The performance of the engine is more efficient and immediate, thanks to the elimination of the pumping losses and air-flow disturbance caused by a conventional throttle butterfly. Instead, air can flow through the intake manifold freely, and Valvetronic precisely regulates the quantity of air entering the cylinders.
Valvetronic uses a stepper motor to control a secondary eccentric shaft fitted with a series of intermediate rocker arms, which in turn control the degree of valve lift. The throttle butterfly is no longer needed as a means of controlling the air supply - though for safety reasons it is still fitted as an emergency back-up.
By optimising the fuel/air mix process, Valvetronic produces fuel savings of up to 10 percent (based on the ECE driving cycle). In addition, Valvetronic improves cold start behaviour, lowers exhaust emissions and provides smoother, more immediate power.