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.