The short story – upgrading your MAF housing isn’t about reducing a restriction in the intake system – it is done to allow the ECU to register a greater amount of total airflow so that the car can be tuned accurately.
The long story – the MAF uses a heated resistor to measure the amount of air flowing past it. The more the resistor is cooled the more voltage will flow through the resistor. The most voltage that can flow through the resistor is 5V, so once the velocity of the incoming air reaches a point at which the resistor is cooled enough to flow 5V, the ECU can no longer register a greater amount of air entering the engine. To overcome this limit you can increase the size of the MAF housing, thus decreasing the velocity of the airflow and increasing the limit at which the resistor flows 5v. On a stock 850 housing this limit is 826kg/hr, or about 14-15psi on a 18t/19t/20t/22t or k24. With a 960 MAF housing, which is .25″ larger, this limit is 1107kg/hr. It is possible to run the engine and turbo beyond the maximum measurable airflow limits of the MAF, but doing so results in an inability to accurately tune the car.
The problem of maxing the MAF is that it throws off the calculations for engine load and fueling. The calculation for load is mostly dependent on airflow, and load is directly converted to injector opening time and is also used as the main map axis for ignition timing, fueling and other maps. When you max out the MAF, the ECU ‘thinks’ that airflow is not increasing. However, since RPM is increasing, the ECU starts to calculate a decrease in load, due a decrease in volumetric efficiency. Due to the decrease in load, the ECU will actually start decreasing the amount of fuel injected and increasing the ignition advance, exactly the opposite of what is desired. With some tricky tuning, one can increase the fueling and decrease the timing so that the engine at wide open throttle (WOT) will be OK. However, this causes a problem when the engine is operated at high load and not at WOT, when you’re at the same RPM and near the limit of the MAF, but not maxing it out. At this point the computer will reference the cells that were artificially tuned rich to account for the maxed MAF, and the car will run very rich and with conservative ignition timing.
Here is a visual of what it looks like in tuning. The red line represents the curve that the ECU will calculate with a maxed MAF. You can see at 4200 the MAF becomes maxed and the load starts to decrease. The green line represents a non-clipped MAF – the load increases and then stays constant. The blue line is a pull in which the MAF hasn’t been maxed and as you can see it traces some of the same cells as the red, clipped MAF pull.
For any turbo with a TD04HL hot side (or K24), a 3″ OD V90/S90/960 MAF housing is suffciently large to allow for accurate tuning of the engine. A 960 MAF housing can be had very cheap – you can get the least expensive 960 MAF, since all you need is the housing – you don’t care about the quality of the MAF sensor. This is the housing that is recommended. Simply get this housing and put the factory 850/x70 sensor in the housing. All you need to do is enlarge the diameter of the MAF housing, thus slowing the air moving past the MAF, increasing the maximum measurable air flow.
Since the 960 MAF housing is .25″ larger than stock you’ll need an upgraded pre-turbo intake tube to fit it. If you already have a Snabb intake tube, a 2.75″ – > 3″ silicone right angle reducer is all you need. If you are running the factory airbox then an airbox lid from a 1999 or 2000 turbo 5 cylinder car will allow you to adapt the 960 MAF housing with minimal work.
It is not recommended to upgrade your MAF housing to a size larger than necessary. Using too large of a MAF housing can result in a decrease of measurable airflow resolution, and can result in idle quality degradation. If you’re shooting for more than 350WHP, then a 3.25″ MAF housing is recommended. A BMW 540i MAF housing has been recommended in the past, but unfortunately, this MAF housing does not properly center the Volvo MAF element. This makes accurate tuning very difficult and sometimes impossible, therefore, this custom MAF housing is recommended.