Electronic Solutions for Motorsport

BOV Assist – The MAF CAL Function Explained

3 min read
Photo of MAF sensor (Mass airflow sensor)

MAF CAL, our MAF sensor calibration product has a function called BOV Assist. This is an experimental feature which we don’t recommend using. This article covers the ins and outs.

The goal of BOV Assist is to reduce over fuelling between gearchanges on cars where the BOV (blow off valve/diverter valve) is venting to atmosphere on cars equipped with a MAF sensor.


There are a few things to have in mind before using this function.

This function may or may not work on your engine setup. We’ll explain what the function does, user decides if it is appropriate.

BOV should not be venting at idle.

BOV should be activated by inlet manifold vacuum only. It must not be activated electronically.


The function activates according to engine vacuum and when the vacuum threshold is met, a fixed MAF signal voltage is sent to the engine ECU.

User sets their vacuum/load limit, below which the function is activated.

Set threshold to just below idle vacuum when engine is fully up to temperature is a good place to start. Do not set load to more than idle load when engine is fully up to temperature.

Fixed MAF voltage, I’d set it at idle MAF voltage or higher. Experiment with what works for you.

Our Experience

We’ve run BOV Assist on a VAG 1.8T 20V engine with a big turbo. Factory BOV/diverter valve activation hose was plugged. BOV (Greddy Type-S) signal was plumbed directly to inlet manifold.

BOV was venting at idle.

What we observed.

Stalling at idle/when coming to a stand still. With the BOV venting at idle it was impossible to make the engine ECU happy and the longer we ran the car in this configuration the less happy it got (the more it stalled).

Solution – Stop BOV from venting at idle. Increasing the resistance on the valve inside the BOV until it is sealed at idle.

Problems with this solution – Tightening the valve in the BOV makes it a bit harder for the BOV to vent boost and makes the dumping process a bit slower. This is less than ideal.

The turbo may be pumping air at idle. If so, not opening the valve could cause back pressure in the exhaust. This may or may not cause problems.

We chose not seal the BOV at idle because of the potential problems listed above.

Other Issues We Experienced

Hesitancy when getting back on the power between gearchanges. This may have been due to the fixed voltage being a bit low.

Do We Recommend Using the “BOV Assist” Function?

In our experience, no, at least on MAF equipped engines with a BOV venting to atmosphere.

For the problems listed above and because there are good reasons to recirculate the boost back into the intake (post-MAF, on a MAF sensor equipped car) and no downsides.

The only possible plus point of venting to atmosphere is the noise. But we can get some of that noise if we mount our air filter on the MAF sensor and vent back into the intake close to the MAF sensor.

Why Did We Include The Function?

Why have we included the function if we don’t recommend using it? There may be circumstances where this feature will help someone for reasons we have not thought of. This function is there for them. For the rest of us, we can leave it disabled.

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