Draw through vs. Blow through MAF 1988-2010)
There are two ways of configuring a mass air meter for a centrifugally supercharged or turbocharged vehicle. Here are the pluses and minuses of each setup.
A draw through mass air meter (MAF), i.e. ahead of any blower /turbo, is more affected by bends in the pipes near the MAF. You CANNOT use 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 for a supercharger – it bleeds off boost and recirculates pressurized air from the blower and keeps metered air in the system. 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 driveability. With a turbo, you use a BOV recirculated.
A draw through meter is a long distance from the engine. Add in an intercooler and there may be an unacceptable delay in what the mass air meter reads and how the engine responds. On a turbo vehicle, the turbos natural ‘spin down’ will create backwash through the MAF that cannot be tuned around. In worst cases, you must convert to blow through to get rid of this.
As previously mentioned, on a draw through setup you need to recirculate the bypassed air when using it with a turbo or centrifugal supercharger. Designing a decent bypass can sometimes be tough - you need to route the return air line as far away from the MAF as possible, and even situate the pipe so it is blowing air at an angle away from the MAF. And maybe add an air shield inside the pipe. With an intercooler, you'd need to most likely add two bypasses, one ahead of the IC and another after the IC. This can result in a plumbing nightmare. Bypasses must be big enough to handle all the air. Bypass problems will result in horrible drivability that usually cannot be tuned around - if you suspect a bypass problem, graph a log of MAF volts where the problem occurs. If it looks like a seismograph during an earthquake - that's your problem. Naturally if you run a twin screw or roots style blower, or are NA, your only option is draw through and on those systems, they work just great.
A blow through MAF is less affected by bends. You cannot use a draw through style (tonsil) MAF in blow through. This includes factory MAFs from 1988-2004 vehicles. You need to use a 2005-2010 style slot MAF as they are a lot more forgiving of bends and much better suited for blow through. A blow through setup vents after the blower, but before the MAF, so the unmetered air can be dumped to atmosphere with a bypass for a superchager and a BOV for a turbo They do not require extra plumbing to bypass the air. Don't use a BOV with a supercharger as the springs are too heavy. Venting to atmosphere makes plumbing much easier.
Overall, blow through systems work well. When properly designed, they do NOT reduce the range of the MAF. Contrary to internet misinformation, a properly designed blow through system will be easier to tune and live with than a draw through setup.
The BEST alternative for a blow through MAF is to use an '05 Mustang style slot MAF, and in 3, 3.5 or 4" tube. The larger the tubing the more range the MAF has, but it also has less low end resolution. Using a 'performance' style MAF such as a Professional Mass Air Systems HPX MAF will increase range even further. Note: that even with a Diablosport MAF.ia the '05 style MAFs will NOT output to battery voltage. Using the HPX will negate this disadvantage as will going to larger diameter tubing for higher HP vehicles.
It's not a great idea to use an HPX MAF setup in a 4" tune on a lower HP car, it doesn't have any advantages, it just adds range at the expense of low-end resolution. Always plan your MAF setup so that you will be using 80% of the theoretical range of the MAF.
Using an '05 style MAF in an earlier vehicle will require some additional parts, a MAF flange welded into the pipe and a late model pigtail. These are available from LaSota Racing.