Quote:
Originally Posted by MadisonMPG
Because it's already got a vacuum?
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I'll try to answer this, but I'm not entirely sure that I'm understanding what you're asking here.
The idea with a turbo is that even when the manifold is under vacuum, the exact same circumstances (normally) without a turbo would put the manifold under higher vacuum.
Thusly, adding a turbo to ANY engine will decrease the vacuum necessary to induce fresh air to the pistons, because as long as the engine is under
some load, the turbo is starting to spool.
To quantify this effect, remove the intake piping from the turbo and pipe it into a cap with a pressure gauge, and pre-load the engine at part throttle w/ the brakes engaged. You'll see positive boost pressure on the turbo's cold side outlet.
The reason most people don't recognize this as boost is b/c the boost gauge is still in vacuum. The reality is that the engine is just requiring more CFM than the turbo can produce under that specific load at that specific RPM, but the turbine is still moving more air than the engine could on it's own, so the result is something like:
(Engine under partial load @ -14in/hg + 2psi = -9.92 in/hg.)
Versus engine under partial load without turbo @ 14in/hg + 0 PSI = 14in/hg)
(This is the engine
with the turbo)
This is the engine without the turbo.