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 measure a greater amount of total airflow, so that the engine can be tuned accurately.
The long story – the MAF uses a heated resistor to measure the mass of air flowing past it. The more this resistor is cooled, the more voltage will be able to flow through it. The highest voltage that can flow through the resistor is 5V – once the velocity of air flowing through the MAF is great enough so that the sensor outputs 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. This will decrease the velocity of air and decrease the amount that the sensor is cooled, thus increase the limit at which the sensor will output 5V. The maximal air mass limit that a stock M4.3/M4.4 MAF sensor and housing can measure is 826kg/hr (kilograms per hour) – equivalent to about 225WHP, or about 14psi from a 18t/19t/20t/22t/K24. With a 960 MAF housing, which is .25″ larger, this limit is increased to 1107kg/hr – equivalent to about 310WHP. While it is possible to run the engine and turbo beyond the maximum measurable airflow limits of the MAF, doing so results in the inability to accurately tune the engine.
The problem of maxing the MAF is that it throws off the calculations for engine load and fueling. Engine load is effectively how much air is moving through the engine, which is roughly equivalent to engine power. The amount of fuel that the engine needs is directly calculated from engine load. Engine load is also an axis for the ignition timing, fueling and other maps. When the MAF is maxed out, the ECU doesn’t know that the actual airflow is increasing. Engine load is connected to RPM and since the 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 start decreasing the amount of fuel injected and increasing the amount of ignition advance – exactly the opposite of what is desired at high engine loads.
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, where the ECU is reading the correct amount of air entering the engine – 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″ outer diameter MAF housing from a 1998 V90/S90/960 is sufficiently large enough to allow for accurate tuning of the engine. A 960 MAF housing can be purchased for very cheap – since you’ll be reusing your factory Bosch MAF sensor inside the new housing, you can get the least expensive MAF, since all you need is the housing.
This is the 960 housing that we recommend.
Simply get this housing and put your factory 850/x70 sensor in the housing. All you need to do is enlarge the diameter of the MAF housing, slowing the air moving past the MAF sensor, increasing the maximum measurable air flow.
It is not possible to connect a 960 MAF housing to the factory pre-turbo intake tube found on 92-98 P80s. In order to adapt the larger MAF housing to your car you’ll need either a Snabb intake pipe, a DO88 or a a factory intake tube from a 99-00 P80, Volvo part number 9445351. If you already have a Snabb intake tube, a 2.75″ – > 3″ silicone right angle reducer is all you need. If you purchase a DO88 tube, make sure to get one designed for the 99-00 P80s, which has a 3″ inlet. 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, one of the custom Aluminum MAF housings that we sell is recommended.