In order to inject the proper amount of fuel, the computer needs to measure the quantity of air flowing into the engine. The device which does this is called a mass air flow sensor, or MAF. That's the aluminum device mounted on the air cleaner cover with a black box atop it. Ford designed the meter to serve dual purposes; in addition to measuring airflow, it actually makes the intake system quieter. The next time you have the air cleaner box open, look at the inside of the MAF. See where the main tube diameter gets much smaller? Ford deliberately put a discontinuity there to create a reflection coefficient; in plain English, the noise of intake valves opening and closing is partially reflected back into the intake manifold, with less of it transmitted to the outside world.
Unfortunately, the same discontinuity that makes the engine quieter also sharply restricts the maximum airflow into the engine. In order to realize significant power gains at high rpm's, the stock MAF needs to be replaced with a higher-flow unit. Several aftermarket replacements exist for the stock unit, many of which use the stock sensor electronics. Some list subscribers have also reported good results from boring the stock (55mm diameter) unit out to 60mm or so.
You can expect (hope for) gains of about 5 - 10HP at the front wheels with the aftermarket MAFs available for the SHO today. As with most of the bolt-ons available for the SHO, you never know how your SHO will react to a larger mass airflow sensor. Some SHOs just don't seem to like 'em.
WARNING: Rumors persist that you can use the 71mm MAF for a T- Bird SC in the SHO unmodified. DON'T DO IT. According to Ford Motorsport's tech guys, the calibrations are much different, and the T-Bird sensor will cause the engine to run much too lean and the timing will retard to prevent knocking. It will adversely affect the long-term life of your SHO engine.
The importance of quality
Ted Breaux's opinion:
A MAF is sensitive to the way air flows through the unit. This can be changed merely by altering the physical position of the MAF under the hood (if you have a cone filter)! If you don't believe you can change the way your car runs by turning your MAF 90 degrees in the plane of the ground, just try it!! Since a MAF is the ONLY method of determining proper fuel metering under wide open throttle, your fuel metering will only be as accurate as your air metering. Some MAFs have a more linear response curve than others, and this is usually measured via a flow bench with MAF, hotwire assembly, and air filter in place. This is why if you swap hotwire assemblies and use the factory airbox, only by sheer luck would you have the exact proper response curve. I have never believed it a good practice, swapping your factory hotwire on a different MAF sensor. In addition, you still end up with the source of error being the way you position the sensor (i.e. the way air flows through it). In my opinion, these are good reasons why some SHOs do not seem quite as 'peppy' as others. On the other hand, if you have a good, linear MAF sensor which has been properly calibrated, regardless of whether it is 55mm, 73mm, or 80mm, the car will run properly at WOT. This is more important than the static diameter of the MAF itself (within reason of course), although this variable can hurt you equally if it is too small or too large.
Also, I believe that the passages both to and from the MAF should be as turbulence free as possible.
C L 73mm $175-190 street.
Uses stock sensor electronics. The SHO sample tube will run a little lean and cause a loss of mid-range power. This can be solved buy using the Mustang 19 lb/hr sample tube in its place.
Pro-M Bullet 75mm $250 street plus core charge.
Properly calibrated for stock engines. Uses stock air box and filter.
Pro-M 77mm about $575,
properly calibrated for stock engines. Comes with a high-flow conical K N air filter. An excellent MAF for highly modified engines. Includes provisions for mounting the air intake temperature sensor (the sensor that goes into the side of the stock air box, before the air filter).
Pro-M 80mm Properly calibrated for stock engines.
Available from SP Motorsports (www.spmotorsports.com). A possible replacement for the 80mm MAF / LPM combo?
SHO Shop 80mm $325.
This is the MAF from the Lincoln Mark VIII's 4.6L DOHC V8. Requires a chip (LPM). Comes with a high-flow conical K N air filter. A budget alternative to the Pro-M 77mm if you intend to buy an LPM anyway. No provisions for mounting the air charge temperature (ACT) sensor are provided. The Can, also available from the SHO Shop, provides an ACT mounting.
Kenne-Belle 80mm (Price?)
This is the same MAF as the SHO Shop 80mm, but it doesn't come with a conical K N. Here's how to adapt it to the stock air box: The stock air box has a 4" outlet that is reduced for the stock 55mm MAF. Remove the step down ring. Remove the stock MAF from the air box lid. Plug up the bolt holes. Get strong 4 inch flex duct and connect the meter to the box. Get 3.5 inch flex duct to connect the rear of the meter to the throttle body. Make sure you keep the larger distance between the TB and the MAF, or you will haave idle problems. Get some nice wide T-bolt clamps to bolt it all together nicely.
NOTE: The stock MAF sensor is equipped with a screen, presumably to prevent the mounting bolts from being drawn into the engine should they work loose. Other units have no such screen, and Loctite 271 (the heavy duty stuff) doesn't really do much to bond the brass bolts to the aluminum sensor body.
Air Filter Box and Silencer Cone
A common trick on Mustang 5.0's which also works on the SHO is to remove the inlet resonator, which is mounted inside the left fender. This gadget exists solely to attenuate noise. Removing this device will make your car noticably louder. As it is a bit of an obstruction for a suitably modified intake, removing it may provide a bit more flow (I recall seeing something about 5-7 HP worth on the 5.0; I'd guess less than that on the SHO). The factory manual contains instructions for removal, or you can follow the link below. How to remove the air box silencer cone
Drilling holes in the stock air box (below the filter, please) is not recommended because any new air flow that is provided (which will be virtually negligible) will be offset by an increase in the air charge temperature.
The SHO Shop offers a replacement for the air box silencer called The Horn ($80). It bolts in place of the silencer with its much larger opening in a high pressure area in the left front fender, and a nice smooth bend leading to the air box.
One of the problems with the conical air filters that come with the Pro-M 77mm MAF and the SHO Shop 80mm MAF is the engine ingests hot engine compartment air. The SHO Shop offers an air box called The Can ($120) that goes over the conical air filter, allowing the engine to draw in cooler outside air. The Can also provides a mounting point for the air intake temperature sensor for users of the SHO Shop 80mm MAF.