Draw through vs. Blow through MAF
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.Draw Through
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.
A draw through meter is a longer way 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.
On a draw through setup you need to recirculate the bypass 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 meter is more affected by pipe size and bends. This is especially true when using a draw through style MAF, i.e. with a housing in blow through. This is much less true when using an '05 style slot MAF as they are a lot more forgiving of bends. A blow through setup vents after the blower, but before the MAF, so the unmetered air can be dumped to atmosphere with a BOV, not requiring extra plumbing to bypass the air. This makes plumbing much easier.
Overall, blow through systems work well. When properly designed, they do NOT reduce the range of the MAF as internet misinformation leads some people to believe. Contrary to internet misinformation, a properly designed blow through system will be easier to tune and live with than a draw through setup. A properly designed blow through setup has the same diameter piping as the MAF’s diameter in straight sections if you are using a draw through style (factory style MAF with a housing). If the MAF is 90mm, use 4” pipe. It may not be practical to use 4” pipe for the entire inlet, but what must not happen is to use reducers right at the MAF or have bends close to the MAF. Use as long a section of the same size pipe as can fit, then do any reducing away from the MAF. 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, they have a limit of 6v. 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 make it better to do this, just adds range at the expense of 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. The MAF flange welded into the pipe and a late model pigtail. These are available from online vendors.