WHAT IS "PRESSURE RATIO?" (AND HOW ALTITUDE AND RESTRICTIVE AIR INTAKES AFFECT IT)

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"The course of the flight up and down was exceedingly erratic, partly due to the irregularity of the air . . . . " -- Orville Wright

Some readers have probably wondered why turbo maps use "pressure ratio" instead of "boost" (manifold pressure).

The simple answer is that reductions in compressor inlet pressure without any change in the pressure ratio will lead to reductions in manifold pressure.

Conversely, if manifold pressure stays constant and inlet pressure decreases, the pressure ratio must increase.  formula to "prove" this is found in virtually every turbocharger matching book, from the Hugh MacInnes "classic" Turbochargers to the latest books from Miller, Hartman, and Warner.

Pressure ratio = (boost pressure (gauge pressure) + ambient air pressure)/ambient air pressure.

(See MacInnes at 16, Miller at 39, Warner at 23, A. Graham Bell at 68, Corky Bell at 26)

So how does this really work: If you're testing a car on the beach (at sea level), the average barometric pressure reading should be ~ 14.7 psi (29.92 in. Hg.) Now if the "gauge pressure" indicates 30 psi of "boost" . . .

Pressure Ratio = 30 + 14.7 / 14.7

Pressure Ratio = 44.7/14.7

And when you plug these numbers into your handy calculator or computer spreadsheet, you get 3.04. Thus,

Pressure Ratio = 3.04:1

That means you've crammed three "atmospheres" into the space of one. (You get one for free and two more from the turbo)

Now run the same calculation a little more than 2/3s of the way up Pikes Peak at 10,000 ft.

Average barometric pressure reading: 10.1 psi (20.58 in. Hg.) (A. Graham Bell summarizes you lose ~ 0.5 psi ambient air pressure for each 1,000 ft. increase in altitude.)

Pressure Ratio (10,000 ft) = 30 (gauge pressure) + 10.1/10.1

Pressure Ratio (10,000 ft) = 3.97:1

WHAT! How can that be? To produce 30 psi in the thin air of 10,000 ft., the turbo compressor had to work much harder. The pressure ratio had to increase to keep the "gauge pressure" constant.

But what if the pressure ratio hadn't increased? "Gauge pressure" would have dropped to 20.2 psi.

What should be obvious here is that a chart which equates pressure ratio to "boost" only works at a specific altitude. Most such charts assume sea level ambient air pressure.

This also illustrates a flaw in a fixed linking of compressor speed to engine r.p.m., as belt-driven superchargers do. Recalling the general relationship between pressure ratio and compressor r.p.m., a turbo can speed up to to increase its pressure ratio at altitude. The only way a supercharger can do that is either with a pulley change or variable drive system.

Of course, altitude isn't the only thing that can reduce compressor inlet pressure. A restrictive air intake, an air intake located in a low pressure zone, a restrictive mass air sensor, inadequate inlet ducting, clogged air filter or excessive intake air turbulance and heating (i.e. placing your air inlet behind the radiator) are common mistakes that reduce compressor inlet pressure in the real world.

Kenne Bell makes the following suggestion to its supercharger customers:

"If you don't have access to a [flow] bench, install a tap behind each component in the inlet track, make a dyno pull or a WOT run on the street in low or second gear and read the vacuum gauge. If it's "0" there are no losses and, therefore, upgrading [pre-compressor]components will not help. However, if there is a 4" Hg reading - that's 2 psi of lost atmospheric boost . . . ."

Although Kenne Bell's suggestion does not take into account potential pre-compressor intake air pressure increases through vehicle velocity (ram air), the basic test method is reasonable for any form of supercharger or turbo.