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Vacuum is known. Most cars will idle around 4.5 psi, WOT around 14.7 psi and engine brake at about 2psi of MAP. What I'm trying to figure is how these pressures translate into drag at the wheels, and how much I could hope to gain by eliminating or reducing manifold vacuum. The engine should exert a greater braking force with the throttle plate shut than with it open but I'm trying to gauge how significant the effect is.
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