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TT · 05-28-2002, 01:27 PM

[quote] The DME simply uses the RPM for reference to the fuel-tables stored in the chip<hr></blockquote>

Motronic in the 944/951 cars does two things to calculate fuel injection requirements:

1 - Determine amount of fuel (injector base pulse width or BPW) directly from the VAF signal and RPM. No other operational parameters are considered at this step.

2 - Modify the BPW based on operational parameters: O2 sensor, enrichments (accel, water temp, air temp, altitude, etc.), tuning adjustment tables (idle, part throttle, and WOT) and the values from the DME adjustment switch.

Step 1 uses the following table for the 89 951:

The resulting value y is a precalculated value that contains the VAF air flow converted to mass (at 75 degrees F), the fuel injector flow rate, and the desired A/F ratio (stoich). The BPW is then calculated in clock ticks (2 usec/tick) as:

BPW = (y/rpm) * 2000

This value is then clipped at this point. For the 89 951 the lower and upper limits are 0.85 and 11 msec. A reference value referred to as Load in later lookups is calculated at this point.

Load = BPW / 25

Step 2 is just modifications to the value calculated in step 1. These include the table lookups that you see in the editors from Protomotive and Bonneville Motorwerks.

Step 1 is what the signal massagers adapt the MAF output to fit. Step 1 is what trips up the people who go to larger injectors, it throws off the Load value which is used in step 2 (I might have a simple fix for that if anyone is interested).

The nice thing about Motronic is that step 1 results in the Load value used in step 2 that really doesn't care how the air flow is measured. It is possible to replace the current VAF based calculation in Step 1 with a MAF or MAP calculation that results in a final value that is the BPW. Then one can then use the step 2 tables to fine tune the system.

[quote] you said the volvo maf, like to one in the earlier link? that thing is monsterous,<hr></blockquote>

The MAF itself is small, I used a long tube at the input. MAF's need laminar flow across the sensor, the longer tube induces this, some designs use screens or grids. Pulsations, reverse flow and non-laminar flow are some problems you may encounter with putting a MAF on these cars. That's why the aftermarket kits put some work into the inlet designs and placement of the sensor, especially for the turbos.

05-28-2002, 01:27 PM	#18
TT Addict Rennlist Member Join Date: Jun 2001 Location: Huntingtown, MD Posts: 363	[quote] The DME simply uses the RPM for reference to the fuel-tables stored in the chip<hr></blockquote> Motronic in the 944/951 cars does two things to calculate fuel injection requirements: 1 - Determine amount of fuel (injector base pulse width or BPW) directly from the VAF signal and RPM. No other operational parameters are considered at this step. 2 - Modify the BPW based on operational parameters: O2 sensor, enrichments (accel, water temp, air temp, altitude, etc.), tuning adjustment tables (idle, part throttle, and WOT) and the values from the DME adjustment switch. Step 1 uses the following table for the 89 951: The resulting value y is a precalculated value that contains the VAF air flow converted to mass (at 75 degrees F), the fuel injector flow rate, and the desired A/F ratio (stoich). The BPW is then calculated in clock ticks (2 usec/tick) as: BPW = (y/rpm) * 2000 This value is then clipped at this point. For the 89 951 the lower and upper limits are 0.85 and 11 msec. A reference value referred to as Load in later lookups is calculated at this point. Load = BPW / 25 Step 2 is just modifications to the value calculated in step 1. These include the table lookups that you see in the editors from Protomotive and Bonneville Motorwerks. Step 1 is what the signal massagers adapt the MAF output to fit. Step 1 is what trips up the people who go to larger injectors, it throws off the Load value which is used in step 2 (I might have a simple fix for that if anyone is interested). The nice thing about Motronic is that step 1 results in the Load value used in step 2 that really doesn't care how the air flow is measured. It is possible to replace the current VAF based calculation in Step 1 with a MAF or MAP calculation that results in a final value that is the BPW. Then one can then use the step 2 tables to fine tune the system. [quote] you said the volvo maf, like to one in the earlier link? that thing is monsterous,<hr></blockquote> The MAF itself is small, I used a long tube at the input. MAF's need laminar flow across the sensor, the longer tube induces this, some designs use screens or grids. Pulsations, reverse flow and non-laminar flow are some problems you may encounter with putting a MAF on these cars. That's why the aftermarket kits put some work into the inlet designs and placement of the sensor, especially for the turbos.