Draw Through vs. Blow Through Mass Air Flow
Don LaSota wrote an excellent article about Draw Through vs Blow Through MAFs:
There are two ways of configuring a mass air meter in a centrifugally supercharged or turbocharged vehicle. Here are the pluses and minuses of each setup.
A draw through meter, as MAFs come from the factory, is less affected by bends in the pipes and reducers near the MAF. Remember that the Mass air meter relies on laminar airflow to get a good signal. A draw though meter must use a bypass valve – it bleeds off boost and recirculates pressurized air from in front of the blower/turbo to behind it. It must be done this way because the air has already been metered. If a bypass is dumped to atmosphere it will result in huge rich spikes that will kill drivability.