Guide to PROM Tuning

1
Guide to PROM Tuning

• Todd Pepmeier
• 15-May 2004

2
Basic Tools

• Diagnostic Software
• EASE, Datamaster, Diacom, TunerPro
• EEPROMs (or Ultraviolet Eraser for stock EPROM)
• Atmel 29C256 is a direct replacement EEPROM (5
  ea)
• or use a UV eraser such as Datarase II 39
• EPROM Programmer
• Pocket Programmer II (150), Moates BURN1 (75),
  Needhams EMP-10 (200), etc.
• Binary Editor program definition file for your
  application
• TunerCat (90) .TDF files (20)
• TunerPro (free) or WinBin (free) .ecu files

3
Advanced Stuff

• Real-time EPROM Emulator
• For on-the-fly changes Xtronics Romulator
  (180)
• Multi-bin switcher
• Uses bigger EEPROM to store up to 16 different
  cals! (55-75)
• Wideband O2 w/datalogging
• For accurate air-fuel measurement FJO inc
  (600) D-I-Y (200)
• Integrated datalogging emulator EPROM
  Programmer all-in-one package
• Moates AutoProm (325)

4
How it Works

• Three parts to the GMP4 code
• Calibration data tables and constants (look-up
  data)
• Program Code 6000 lines of assembly language
  instructions
• RAM data stored input/output values used by the
  program (ex current spark advance or fuel
  correction values)
• Calibration data and Program code are stored on
  EPROM chip inside Mem-Cal
• Can be downloaded, saved as .bin (binary)
  files 32KB size for most GMP4 code
• Bin files can be read with standard Hex editors
• Get used to base-16 math
• 0-9, A, B, C, D, E, F

5
How it Works

• Most all changes are done to the calibration
  portion
• 600 items available
• Bit Flags
• Example VATS Enabled (1yes)
• Single values (constants)
• Example Minimum Coolant Temp to enable CARS
• 2-Dimensional Arrays
• Example Power Enrichment versus RPM table
• 3-Dimensional Tables
• Example Main Spark Table

6
Summary of BFXB Calibration

• 130 items which define ALDL diagnostic data
  stream
• 120 for fueling control, incl Closed Loop
  parameters
• 100 error code parameters
• 60 to control the Idle Air motor activity
• 40 for Spark advance
• 20 for Port Throttle sec Fuel pump control
• 20 for Cranking and Startup routines
• 100 for miscellaneous stuff (A/C, CCP, AIR,
  Fans, etc)
• 600 total items
• 90 of these work just fine the way GM designed
  it!

7
Program Code

• Assembly language instructions which are used to
  read input sensors, lookup calibration data, do
  calculations, and control output devices
• Essential to understanding how the various
  calibration items work
• Program executes in several loops running at
  various intervals from 6.25ms to 1 second
• Similar to other GMP4 code of late 80s / early
  90s vintage (8D mask, ANHT, from L98
  application is very similar and thoroughly
  commented in public domain)
• Can be disassembled, or translated, with
  standard Motorola 68HC11 software tools

8
Interesting Findings

• Low Octane Spark Retard
• Permanently removes 5d of advance when persistent
  knock is detected. Restart will restore lost
  advance. Not in earlier cals.
• Not the same as normal knock retard feature,
  which recovers more quickly
• Pass-by Noise Control
• Can retard ignition timing up to 5d under very
  limited circumstances. Would rarely be activated
  under normal driving.
• Upshift Light
• Disabled on service cals by using high enable
  temp
• IAC Operation/Throttle Follower
• 60 calibration items and 1000s of lines of
  program code to control the IAC motor
• Throttle follower logic opens the IAC motor with
  the throttle blades very slow decay rate
• Closed loop idle control (not fueling) tries to
  maintain idle speed within /- 50 rpm

9
Throttle Follower graph
10
Interesting Findings

• BARO corrections
• Available but not used on most cals
• DFCO (Decel Fuel Cutoff)
• Cuts fuel flow on overrun (high vacuum)
  coast-down
• Injector Flow Rate
• Stock GM cal value is 22.78 lbs/hr (actual
  injectors are 19.5?)
• Closed Throttle Spark
• Spark advance is held at a constant value when
  throttle is closed and certain other conditions
  are met
• Closed Throttle spark does not use the main spark
  table
• This constant varies on the various model years
  from 14-24 degrees
• Can be disabled by clearing a bit flag

11
Interesting Findings

• Fueling Strategy
• Basic equation
• Inj Pulse Width f (target a/f ratio, airflow,
  injector size)
• Modified by Closed Loop Correction Term (BLM,
  INT)
• Power Enrichment added for high load - 19 richer
  (121 target)
• Acceleration Enrichment added for rapid change in
  load (pump shot), then decayed out
• Can be fine tuned to optimize throttle response
• Tip-in lean condition can be seen on wideband

12
Fueling Routine

• Determine target A/F ratio
• Initially set at 14.71, richer for Power
  Enrichment, Open Loop enrichment
• Calculate airflow through the motor
• Using MAP and IAT, Volumetric Efficiency, and
  cylinder volume
• Based on airflow, determine fuel required to
  achieve target A/F ratio
• Based on injector size, calculate how long the
  pulsewidth must be
• Apply pulsewidth multipliers like Accel Enrich,
  or startup enrich, or batt voltage correction,
  etc.
• Scale final pulsewidth by 0.51 if secondary
  injectors active

13
Interesting Findings

• Snakeskinner Code
• Based on late 89, early 90 code (AUAH?)
• Open Loop (no O2 Sensors)
• Port Throttles always open (butterflies removed)
• 28 degrees WOT spark advance
• 1000 RPM idle speed
• Knock Retard completely disabled
• 7200 RPM fuel cutoff
• 2.5 lower cylinder volume constant
• Error diagnostics disabled for TPS, MAP, O2, Port
  Throttles, ESC, Valet Switch

14
Tuning Methods

• Make one change at a time, and test the results
• Take notes!
• Basic Changes
• Disable CARS (1-4 skip shift) by setting enable
  temp high
• Lower Cooling Fan temperatures
• Turn on 5-10 degrees above thermostat opening
• Change Spark Advance
• 4 tables (RPM4000, PT CLSD, PT OPEN)
• Stock usually around 25d max at WOT, can go to at
  least 28d
• Diminishing returns after around 32d on race gas
• Big dips in stock map around 3000-4000 rpm

15
Main Spark Table (PT OPEN)
Note minus 4 degree dip _at_ 4000 rpm
16
100 Kpa Spark Advance (PT OPEN)
17
75 Kpa Spark Advance (PT CLSD)
18
Tuning Methods (contd)

• Other Changes
• Changes to base fueling
• 4 tables (RPM2000, PT CLSD, PT OPEN)
• Stock is pretty good in most cases
• Increase Volumetric Efficiency to add fuel
  (decrease to subtract fuel)
• If no wideband data, shoot for BLM values of 128
  (0 corr)
• If using wideband, can disable closed loop and
  tune in Open Loop mode. Generally want
  14.5-14.71 ratio at low loads.
• Power enrichment is added around ¾ throttle, at
  which point you should shoot for around
  12.5-13.0, depending on what your car likes best

19
Stock Fuel Table (PT OPEN)
20
Custom Fuel Table (PT OPEN)
21
Tuning Methods (contd)

• Other Changes
• Reduce spark retard when intake temps get higher
• 91-95 Default to Full Power Mode
• Requires 1 byte change in program code
• Improve knock retard handling (faster recovery)
• Raise Idle RPM
• Adjust Injector constant for larger injectors
• Adjust cylinder volume for 368-415 cid
  applications
• Fine Tune Power Enrichment Accel Enrichment
  fueling
• Raise (or lower) Port Throttle Opening point
• Earlier is not always better!

22
Resources

• Websites
• www.diy-efi.org White papers and archives
• www.thirdgen.org DIY-PROM Discussion Board
• www.moates.net Autoprom, blank EPROMs and
  bin-switcher
• tunerpro.markmansur.com TunerPro software
• Mailing lists
• DIY-EFI
• GMECM
• Reading
• ANHT hac.pdf (fully commented L98 calibration)
• Turbo P4 document detailed GM document
  explaining how a P4 ecu works (available at
  diy-efi.org)
• Motorola 68HC11 instruction set (available at
  motorola.com)