I have an 88 5.7 TPI IROC I'm rebuilding.  Any thoughts on the pros and cons of MAF's ?  I've heard I can modify just about anything on the engine without having to re-program the computer on MAF cars.  Any truth to this ? Some say MAP's are better but then I'd need to reprogram ALL engine mods and I'm planning on making quite a few. Stroking the 350 to a 383 just for starters.

Thoughts / Suggestions welcomed.  

This is a sort of.

MAFs are capable of telling the computer directly how much air the engine is using. To some extent this allows the computer to compensate for any modifications you've done to increase volumetric flow thru the motor. However, inside the chip is a bunch of fuel and ignition maps, they don't go to infinity. You can run off the edge of their known world. That will require a new chip with maps of the new territory.

MAP systems compute air flow indirectly. They're used because they're cheaper than MAF sensors for the factory. It's a one time programming exercise for them against the recurring cost of pricy MAF sensors. The MAP system has a bunch of fuel and ignition maps just like the MAF computer, but it calculates air flow based upon sensor inputs for RPM, throttle position, engine operating temperature, inlet air temperature, atmospheric pressure and most importantly manifold pressure (what we commoners refer to vacuum). It also knows the size of the engine so displacement changes, which affect mass flow, will upset a MAP system. So, doing anything to the engine that changes the amount of air flow thru it, also, changes the relationship of sensor readings to the air flow. This throws the information the sensors send to the computer off compared to the preprogrammed relationships it's expecting. Thus, the chip can no longer calculate the proper air flow to match with the fuel and ignition maps. Also, there is a limit as to how low the manifold pressure can go at idle and have reliable sensor readings. The problem is similar to those suffered by a carburetor when a hotter cam or larger intake is used. The signal diminishes to a point where the system becomes insensitive to what's happening and the mixture becomes hard to control. In the case of MAP systems 10 inches of mercury at idle is as low as you can go and expect the sensors to track what's going on. Personally I think 12 inches is a better limit, especially if you have to pass a SMOG test. This limits cam timing to less than 220 degrees, to be safe I think 210 to 215 is really the practical limit with a 1.5 rocker. If your pushing 1.6 rockers, back off to 205 to 210 max. All of these measured from .050 inch lift. As with the MAF system, the maps don't go on forever, so not only does the new chip have to correlate the new sensor inputs to mass flow thru the engine, but it also has to have new maps that go further than the end of the factory's known world.

If the engine is running nominally, the O2 sensor will cover about a 10% mass flow change to a MAP sensed engine as that's about its authority limit. So the computer will recognize and respond to moderate changes in mass flow resulting from things like 1.6 rockers on the stock cam, pushing the cam duration to about 195/205 degrees with 1.5 rockers, or headers and duals on the exhaust. but that's about if for a MAP system without a chip. Sometimes you can trick it with an adjustable pressure regulator and just run the injector pressure up a bit. This can get you by a little further if gas mileage and passing SMOG tests isn't a concern. But this can get you into a battle with the O2 sensor which if going too rich will try to pull the mixture back to lean. Too rich is also a problem with catalytic converters and AIR exhaust systems as the exhaust and converter temps will sky rocket with too much unburnt fuel going thru them.

Bogie  

This is a sort of.

MAFs are capable of telling the computer directly how much air the engine is using. To some extent this allows the computer to compensate for any modifications you've done to increase volumetric flow thru the motor. However, inside the chip is a bunch of fuel and ignition maps, they don't go to infinity. You can run off the edge of their known world. That will require a new chip with maps of the new territory.

MAP systems compute air flow indirectly. They're used because they're cheaper than MAF sensors for the factory. It's a one time programming exercise for them against the recurring cost of pricy MAF sensors. The MAP system has a bunch of fuel and ignition maps just like the MAF computer, but it calculates air flow based upon sensor inputs for RPM, throttle position, engine operating temperature, inlet air temperature, atmospheric pressure and most importantly manifold pressure (what we commoners refer to vacuum). It also knows the size of the engine so displacement changes, which affect mass flow, will upset a MAP system. So, doing anything to the engine that changes the amount of air flow thru it, also, changes the relationship of sensor readings to the air flow. This throws the information the sensors send to the computer off compared to the preprogrammed relationships it's expecting. Thus, the chip can no longer calculate the proper air flow to match with the fuel and ignition maps. Also, there is a limit as to how low the manifold pressure can go at idle and have reliable sensor readings. The problem is similar to those suffered by a carburetor when a hotter cam or larger intake is used. The signal diminishes to a point where the system becomes insensitive to what's happening and the mixture becomes hard to control. In the case of MAP systems 10 inches of mercury at idle is as low as you can go and expect the sensors to track what's going on. Personally I think 12 inches is a better limit, especially if you have to pass a SMOG test. This limits cam timing to less than 220 degrees, to be safe I think 210 to 215 is really the practical limit with a 1.5 rocker. If your pushing 1.6 rockers, back off to 205 to 210 max. All of these measured from .050 inch lift. As with the MAF system, the maps don't go on forever, so not only does the new chip have to correlate the new sensor inputs to mass flow thru the engine, but it also has to have new maps that go further than the end of the factory's known world.

If the engine is running nominally, the O2 sensor will cover about a 10% mass flow change to a MAP sensed engine as that's about its authority limit. So the computer will recognize and respond to moderate changes in mass flow resulting from things like 1.6 rockers on the stock cam, pushing the cam duration to about 195/205 degrees with 1.5 rockers, or headers and duals on the exhaust. but that's about if for a MAP system without a chip. Sometimes you can trick it with an adjustable pressure regulator and just run the injector pressure up a bit. This can get you by a little further if gas mileage and passing SMOG tests isn't a concern. But this can get you into a battle with the O2 sensor which if going too rich will try to pull the mixture back to lean. Too rich is also a problem with catalytic converters and AIR exhaust systems as the exhaust and converter temps will sky rocket with too much unburnt fuel going thru them.

Bogie  

2 entries on one subject, truly the hand is faster than the brain.

Bogie