Boost Creep
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
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
The data shown below is collected via ElanScan under the following conditions:
- 3rd Gear acceleration (allows for more datapoints than 2nd gear, and safer speeds than 4th gear)
- WOT (Wide Open Throttle)
| Wastegate | Downpipe | Pre Cat | Main Cat | Diameter | ECU | Ambient Temp | Humidity | Boost Creep | Overboost | WG Duty Cycle | User | Link |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 21 mm | OEM | PCE | NO | 2.50 in | OEM | 0 degC | 56% | 4000 RPM | NO | 0% | ccb056 | OEM ECU |
| 21 mm | OEM | PCE | NO | 2.50 in | Everest | 5 degC | 91% | NO | NO | 0% | ccb056 | Everest |
| OEM | Piper | N/A | Piper Sports | 2.25 in | Mountain | ... | ... | YES | YES | ... | paulsmithmk | specs;overboost |
| 20 mm | Piper | N/A | ... | 2.00 in | OEM | ... | ... | YES | YES | ... | Steve A | specs;overboost |
| OEM | Piper | N/A | NO | 2.25 in | OEM | ... | ... | YES | YES | ... | cliff | overboost;no cat |
| 21.5 mm | Piper | N/A | NO | 2.25 in | Everest | ... | ... | YES | 6500 RPM | ... | HJ2 | specs;port;overboost;cured with mods |
| 21 mm | Knox | N/A | NO | 2.25 | Everest | ... | ... | NO | NO | ... | steve matthews | specs |
| Ported | OEM | PCE | ... | 2.25 | Everest | ... | ... | NO | NO | ... | dapinky | specs |
| 22 mm | OEM | PCE | NO | 2.50 | ... | ... | ... | NO | NO | ... | Peter_W | specs |
| 21.5 mm | Piper | N/A | OEM | OEM | OEM | ... | ... | NO | NO | 70% | BDC Peter | specs elanscan |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
| ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... | ... |
