READING TURBO COMPRESSOR MAPS, PART II

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

Turbo Tip of the Day: "Let it eat" -- Dean Skuza (Former AA/FC racer)

Moving from left to right on our old T66 turbo map, we see the dotted line labled "Surge Limit."

Pressures to the left of the surge limit are not mapped. Why? Because they are both unstable and potentially destructive to the compressor.

Surge amounts to air backing up inside the compressor and fighting to get back out through the entrance (or more properly, the "inducer bore")

To understand surge, imagine air being agitated into a mini-tornado by the compressor impeller (that's the fan blade/meat grinder thing that rotates).

Impeller

When flow out of the turbo is shut off or excessively restricted, the mini-tornado is not properly diffused into steady pressure because it has no place to go. So air being air, it takes the path of least resistance toward a lower pressure -- backing "out through the in door."

These reversals of flow fight the impeller's rotation. The exiting air molecules slam up against other air molecules that the impeller is attempting to induct (draw in). That causes inlet pressures to fluctuate and the impeller's blades to lose efficiency.

Thus, on the left side of the surge limit, the turbo compressor is not doing useful work because the exit flow is too restricted.

Simply put, surge occurs when the attempted pressure ratio is too high for the amount of air consumed by the engine. (Remember, just like with your garden hose or shop compressor, "boost" is not mass air flow. Boost without air flow creates surge)

Surge is most easily found (and heard) when a downstream throttle is slammed shut while the compressor is at speed. Surge sounds like chirping out of the compressor. Blow-off and recirculating valves are often used to combat this form of surge.

Using a turbo that is too large can also produce surge when the boost threshold is lower than an engine's abiliy to induct the compressed charge.

The simple rule is that for your turbo to live, you've got to "let it eat" by avoiding surge.

CONVERTING CFM TO MASS AIR (PART I)

By 460-BBF-Turbo-In-CC (From the legendary Car Craft big-cube turbo thread)

"Sometimes things aren't as simple as they first seem."

The problem with converting CFM to lbs/min. is that the density of air varies with changes in altitude, temperature and even moisture content.

That's causes some to just ignore the whole thing. For example, Corky Bell (Maximum Boost) avoids the whole thing during turbo selection by using the "semi-incorrect term 'CFM.'" (Pg. 27).

Mark Warner suggests that to convert CFM to lbs/min, you should divide CFM by 13.7, but points out that the conversion assumes 85 degree (F) inlet temps. (Street Turbocharging, pg. 35)

A. Graham Bell in Forced Induction Performance Tuning (2002) explains that "Most compressor maps are corrected for 85 [degrees] F/28.4 in Hg or 20 [degrees] C/98 lmB air conditions; so to convert from CFM to lbs/min, multiply by 0.07." (pg. 90)

Jay Miller concurs, stating that multiplying CFM by .069 at "standard density" yields lbs/min. (Turbo: Real World High-Performance Turbocharger Systems, pg. 40)

But other sources claim that you should multiply CFM by .076 at Standard Temperature and Pressure. (STP)


Next: What is STP and which conversion method is best?

FIRST PRINCIPLES: AIR VOLUME IS RELATIVE

By 460-BBF-Turbo-In-CC (From the legendary Car Craft Turbo Thread)

Most Car Crafters are more familiar with CFM [Cubic Feet Per Minute) than they are with measuring air in pounds/minute. After all, the time-honored measure for engine size is cubic inch displacement. And a cubic foot of air is 12 x 12 x 12 = 1,728 cubic inches. That's easy to understand.

Many of us have also read that it takes roughly 1.67 CFM for every horsepower on gasoline and ~ 1.47 CFM per pony on alcohol fuel. And some flow bench companies publish CFM charts such as this:

Flow**      Horsepower potential (V8 engine)

100 CFM   205.7 h.p.

150 CFM   308.6 h.p.

200 CFM   414.4 h.p.

250 CFM   514.3 h.p.

300 CFM   617.1 h.p.

350 CFM   720.0 h.p.

400 CFM   822.8 h.p.

450 CFM   925.7 h.p.

500 CFM   1028.6 h.p.

**cylinder head port measured at 28"

 So why do we even need to think about air in lbs/min?

Because volume of air is relative. For example, a cubic foot of air at sea level has a lot more mass than a cubic foot of air in Denver, Colorado. And a cubic foot of air at 30" hg has more mass than a cubic foot of air at 14" hg.

NEXT: Converting volume to mass.

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)