Boost Creep

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Boost Creep

Boost creep occurs when the wastegate/turbocharger combination is improperly sized and cannot dump enough exhaust gasses. The symptom is rising boost as RPMs rise, sometimes in an uncontrolled fashion. To reduce this effect, either install a larger capacity wastegate, or an exhaust turbine with a higher A/R ratio.


Overboost occurs after boost creep, when the boost pressure reaches a level to signal fuel cutoff.

A/R Ratio

The ratio of the exhaust inducer nozzle area (A) over the exhaust inducer radius (R; measured from the center of the turbine shaft to the center of the inducer nozzle). This number (usually from about 0.4 to 1.0) gives you an indication as to the tradeoff between lag and high volume efficiency for a given configuration. When the A/R ratio is low, then the small inducer area creates high velocity flow at low volumes, thus improving spin up. A high A/R allows high volumes of gas to flow through the exhaust turbine, making the turbine more efficient at those high volumes, but it will then be sluggish to spin up.

Since A/R ratio is a significant factor in turbo lag, some manufacturers have devised means for varying the A/R ratio to reduce lag while still maintaining high-volume efficiency. Such a scheme properly carried out would eliminate the need for a wastegate, which is indeed the case with the Garrett VNT (Variable Nozzle Turbocharger). The VNT uses adjustable vanes inside the exhaust scroll to change the angle of attack of the incoming gasses as they strike the exhaust turbine.

Experimental Data

Wastegate Downpipe Pre Cat Main Cat Diameter ECU Ambient Temp Boost Creep Overboost WG Duty Cycle User Link
21 OEM PCE NO 2.5 OEM 0 degC 4000 RPM NO 0% ccb056 APR 3 2013
21 OEM PCE NO 2.5 Everest 5 degC NO NO 0% ccb056 APR 11 2013
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