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Some differences in MAF mode vs SD Mode

3200 Views 2 Replies 2 Participants Last post by  N0DIH
Something I just tested and have some info on is how the LT1 PCM handles some of the fueling calculations with the MAF and in SD Mode.

1. MAF Mode (normal as delivered by GM) the VE tables are NOT used for primary fuel calculations. BUT, they ARE used for rapid TPS because my chief complaint of MAF usage is in rapid TPS application, the column of air is too compressable/stretchable and causes lots of inaccuracies. So the PCM actually kicks to open loop and draws directly off the VE tables and MAP sensor. So the Open Loop AFR and VE tables are cruicial for dead on accurate fueling, especially when the car is modified, and these tables are forgotten about. MAF Tables have a higher resolution than the VE tables, and this is somewhat an advantage, but it is more difficult to correlate airflow with MAF output frequency, so you can get in and tweak and update the MAF tables, and get the BLM's locked down tight. It is absolutely cruicial that the inlet and outlet of the MAF be as undisturbed as possible, smooth and have as much straight shot in and out. So the Corvette MAF location is best, the better place for the B/D MAF is probably in the middle of first base helmholz resonator. But the helmholz resonator (first base) also provides a device to cancel out the reflected standing waves that are coming back from the engine (created as the intake valves open and closing). So in many cases, removing it actually disturbs MAF flow and you need to update the MAF Translation Tables (MAF Transfer Function) to compensate. The home plate is solely for engine noise. If we could straighten out the first base it would improve performance some.

2. SD Mode/MAFLess mode the MAF tables are obviously NOT used, and all calculations are based on the MAP sensor, coolant sensor, intake air sensor, engine rpm and baro sensor. With SD/MAFLess mode intake air temp corrections are much more important, as with a MAF inherrently compensates for temp and humidity. SD Mode is unaffected by the position of most anything on the intake tract, so no issues with MAF position will disturb the intake airflow calculations.

So knowing now that the PCM drops into open loop (you can see easily, it goes to BLM 16, 17 and 18, which are reserved for open loop), and VE tables and MAP are key. So if you make mods, and you don't tune the VE Tables, you are not seeing the full potential, and possibly more issues with burst knock (if it is too lean) which would encourage the PCM to yank timing back when any agressive throttle is applied. Burst knock can be controlled and pulled out, but that isn't advisable as pinging is soooo damaging to an engine it isn't worth the risks.

Hope this sheds some lights on knowing more details on what the PCM is doing....
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Positioning of the MAF is critical, I have seen and documented 4 unique MAF calibrations on the 94-95 LT1's. Even turning the MAF in relationship to OEM location affects the airflow readings as compared to what the factory calibrated it to.

B/D Body (note uses smaller MAF too)
LT1 Z28
LT1 Trans AM Ram Air

Ram Air TA having the most different, as the incoming ram air makes a huge difference. MAF Placement cannot be understated as how much it affects air flow. So people who have a RAISS intake most likely need some serious MAF table work, unless they switched to a F/Y/Vortec MAF and then likely the Ram Air MAF should be darn close.
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