WHAT IS "BOOST"

BY 460-BBF-Turbo-In-CC (from the legendary Car Craft Big-Cube Turbo Thread)

"Boost" can be oversimplified as resistance to air flow. Every tire on your car probably has 30+ pounds per square inch of pressure, but (hopefully) no air flow. Your shop compressor often has more than 100 psi, but at air flow volumes that are minuscule to the demands of even a low-performance automobile engine.

When we read "boost" from a tap in the intake manifold of an engine, what we're mostly reading is resistance to air flow between the compressor impeller and the cylinders. Now that's not necessarily a bad thing. Resistance to flow between the compressor impeller and the compressor housing is part of how any centrifugal compressor increases air density.

But it should be apparent that a high "boost" level doesn't necessarily mean high mass air flow into the engine. 14 psi of non-chargecooled "boost" through a Stromberg 97 on an asthmatic flathead V8 isn't going to result in as much mass air flow as 14 psi of cooled "boost" through a modern EcoBoost V6.

In other words "boost" is relative.

A great example of this is comparing the ancient Oldsmobile Jetfire V8 to the last of the turbocharged Indy V8s. Both were non-charge cooled, single turbo engines. Both engines ran less than 6 psi of turbo "boost." The Indy V8 was rules-limited to a mere 161.7 cubic inch displacement. The Oldsmobile Jetfire displaced 215 cubic inches. Yet the mass of air consumed by a turbo Indy V8 was roughly four times more than the Jetfire Olds. And it was at a lower peak boost level to boot! (4.91 psi initially and reduced to 1.96 psi by the end of the Indy turbo V8 era).

So the lesson here is that turbo Car Crafters should be focusing mass air flow instead of "boost." "Boost" is good for bragging, but it's mass air flow that wins races.Forget the old saw "there's no substitute for cubic inches." The truth is that there's no substitute for mass air flow.**

(**Okay the nitrous oxide fans may carp at this last point, but N20 injection is merely substituting part of the ordinary air mass for a higher quality (more oxygen-rich) gas supplied under high pressure. So it is is in essence a form of increasing mass flow)

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