Generally speaking, when modifications that increase airflow have been done, and the ECU has NOT been tuned to suit, the car will be more sensitive to the BOV adjustment. A bone-stock car, or one that has been modified AND tuned will usually be less sensitive, often allowing up to 100% atmosphere venting without issue.

This is because increasing the airflow (via increased boost, free-flowing exhaust or intake etc) without tuning the ECU to suit will usually result in rich mixtures. When an atmosphere-venting BOV is installed with such a combination, you’ll often find it’s harder to get the spring pre-load right, or that you need to run less atmosphere venting bias.

Note that is it NOT necessary to tune an ECU to suit a BOV, what you can see from the above is that if the ECU is already tuned correctly to suit the other modifications on the car, it is going to be less sensitive to the BOV settings.

Symptoms and solutions

  • Stalling or stumbling as the revs drop back to idle – increase spring pre-load one turn at a time until the engine returns to a smooth idle without stumbling or stalling. If the symptoms persist regardless of the spring pre-load, reduce the amount of atmosphere venting using the bias adjustment.
  • Backfiring on gearshift – increase spring pre-load one turn at a time until backfiring disappears. If the symptoms persist regardless of the spring pre-load, reduce the amount of atmosphere venting using the bias adjustment.
  • Fluttering sound/compressor surge at low RPM – reduce spring pre-load one turn at a time until flutter is reduced or eliminated.

On some cars, it may be difficult to find a happy medium between having the spring soft enough to prevent low RPM flutter, and hard enough to prevent backfire or stalling. In these case, it is best to reduce the amount of atmosphere venting and set the spring to solve the backfire/stalling. A small amount of flutter/compressor surge that occurs when lifting off at low RPM or less than peak boost is not detrimental to the turbo – the loads the turbo endures under full boost conditions are far greater than a small amount of low RPM flutter, so it is not of great concern if low RPM flutter cannot be completely eliminated.