Originally Posted by snrusnak
I couldn't find a FI section so figured I'd post this here....
I know a little about FI but not enough to be dangerous, and thought of a question last night....
In terms of measuring boost at the intake, how is this measured? Wouldn't this constantly fluctuate depending on engine load? Or is the rated boost the max setting before any additional pressure is bypassed/blown off?
I'm just wondering if say your car runs 10psi, is it 10 psi at idle, and 10psi at WOT or is there a difference depending on engine load? I guess there'd be no boost at idle? ....my head hurts now...
The intake boost pressure is measured using a MAP (manifold air pressure) sensor just like a N/A manifold, they're just rated to a higher pressure. The MAP with a supercharger does change drastically based on engine speed, not load, where a turbo is more about load than RPM. A supercharger needs RPMs to make power, a turbo needs exhaust velocity. The turbo really needs to be spun up for it to make power, and the exhaust velocity at idle is negligable, that's why there's turbo lag; but once it's spinning, it'll keep going as long as there's flow; so even at low RPM it can still be making max velocity where a supercharger would start decreasing MAP.
The max MAP is goverend on both setups by the BOV as it'll bleed down the excess pressure in the intake system. The turbo also has a wastegate which limits the max exhaust pressure that the turbo sees. This prevents surging if you are on/off the throttle and put more pressure/volume than the turbo can handle.
Using the 10psi scenario, a supercharger would be pushing maybe 1psi at idle and a turbo won't be pushing anything. A supercharger will bring the pressure on much sooner in the RPM range, but once it reaches its max RPM, that's it, no more climb because you're limited to a max RPM based on the pulley sizing (so it's making 10psi at 3000rpm). Comparatively, the turbo comes on a little later in the RPM band (normally) but it'll keep increasing spool speed based on the exhaust velocity until it reaches max volumetric flow rate based on the turbo/intake geometry (so it may start making power at 2500rpm, and reach max flow at 3500rpm).