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Mass Airflow Sensor to Direct Bolt-on Air Filter - Ask Marlan

Inlet Tract
I own an '85 Trans Am with an '87 350 TPI. I've seen a lot of cars in your magazine that have a TPI, but instead of a Mass Airflow Sensor they use a direct bolt-on air filter. How is this possible? Everyone I talk to says that doing this will throw the computer into a limp mode. I would like to get rid of all that plastic junk up in the front of my car, but don't know what to do.
Alfredo Corona
North Richland Hills, TX

They're right-simply remove the MAS (Mass Airflow Sensor) on a system requiring one and the engine goes into "limp" mode. There is an alternative but, as we'll see, it's no panacea. The '90-'92 production TPI engines use Speed Density air/fuel metering in place of the earlier MAS. While the MAS must be located in front of the throttle-body within the inlet tract to provide real-time measurement of the air mass inducted into the engine, Speed Density metering relies on Manifold Air Pressure (MAP) and Manifold Air Temperature (MAT) sensors that screw into the intake manifold. Whereas the MAS system directly measures airflow into the engine, Speed Density systems rely on a preprogrammed map of values that relate engine speed, vacuum, and air temperature to engine air flow.

Major performance changes to the engine require "remapping" the Speed Density computer's preprogrammed lookup table. The direct-measurement MAS is more forgiving of engine changes or modifications-at least up to the point that the MAS itself becomes a flow restriction (about 325 hp for the GM production TPI MAS unit). On the other hand, the somewhat bulky MAS is harder to package in a nonstock installation, which is why most custom TPI swaps use Speed Density; with the MAS deleted, they can bolt an air filter directly to the throttle-body. Unfortunately, for similar tight-clearance reasons, most of these swaps use those little horsepower-robbing, restrictive square air filters. And, the nonducted bolt-on air cleaner setup sucks in hot underhood air instead of cool ducted ram-air from the front grille area. That's not good because every 10-degree-F air temperature increase results in a 1-percent power decrease. For example, if the engine compartment temperature was 150 degrees F and the outside air temperature was 80 degrees F power would be down by 7 percent! Why shoot yourself in the foot?

Converting from MAS to Speed Density requires a new computer (remanufactured PN 16198262; may be labeled "1227730"), an appropriate PROM chip for your application, a different knock sensor (PN 10456549), a MAP sensor (PN 16137039; stamped "039," "460," or "466"), and a new TPI wiring harness (contact TPI Specialties or Howell Engine Developments). The existing MAT sensor works. Stock Speed Density TPI intake manifold plenums had an extra bung to mount the MAP sensor, but you can mount it with a "tee" arrangement off your existing manifold bungs. The new wiring harness won't have wires for the ninth cold-start fuel-injector, which is no longer required; just leave the extra injector in place to seal up the hole in the intake.

But why hassle with all this when there's a way, as suggested by TPI Specialties, to clean up the ugly Firebird right-angle induction tract while retaining the MAS sensor? Install the cleaner '85-'89 Corvette-style induction tract that gulped air from a large, flat air filter assembly located directly in front of the radiator. Remove the Firebird's existing top radiator plate and (if so equipped) A/C condenser support brackets to lean back the top of the radiator and condenser as far as you can without the electric fan hitting the belt drives. Make (or have a local sheetmetal shop fabricate) new top support brackets out of a 0.060-inch aluminum plate to support the repositioned radiator and condenser. The new angle provides clearance for the "over-the-top" Corvette induction tract routing. The Corvette induction tract is shown in the accompanying parts list; you may retain the Firebird MAS unit.

Howell Engine Developments

Dept. CC
6201 Industrial Way
Marine City, MI 48039-1326

TPI Specialties Inc.
Dept. CC
4255 County Rd. 10
Chaska, MN 55318-9226

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