Tuning the GM 1227747 ECM

These are expanded notes to myself, I am in no way a TBI tuning expert, I have this one car, a 1963 Rambler Classic 550 Cross Country station wagon with a 1970 232ci six in front of a 1965 M35 automatic, with a Howell TBI kit on it.

I still don't have cold-start (southern California "cold") dialed in, it's lean until warmup, and the mileage is poor (16 mpg) even though my BLMs are all within 1 or worst-case 2.

The Howell-TBI in classic page is a mess

Specific table names in quotes are from TunerPro RT

POWER ENRICHMENT ("accellerator pump")

PE mode, Power Enrichment mode: an increased demand for power (eg. press the gas pedal or a sudden steep hill) causes end of closed-loop and begins PE mode. PE mode is generally transient (eg. you reach desired speed, crest the hill), then ends, and returns to closed-loop mode. (PE is the differential of change in throttle position; the faster you change the throttle the bigger the value.)

"% TPS Threshold Vs. RPM for PE Mode"

This table sets "accellerator pump" basic behavior (in combination with other tables, below). Essentially, what percent-change of the gas pedal results in entering PE mode. If you sloooowly open the throttle you can stay in closed-loop mode. If you blip the throttle to pass, just as in a carburetor the sudden opening of the throttle causes a loss of air velocity etc, and an additional dose of fuel is needed to respond to the change.

Especially when cold, when this threshold is too high the car feels just like a carburetor car with a bad accellerator pump. Too low a threshold, well it doesn't feel like anything because most engines are quite happy with a rich mixture.

There's 9 RPM slots; my six cylinder station wagon wants a low threshold (25%) at low speeds (when air velocities are low) but is fine with 35% at 2400 rpm highway cruise when lots of air is flowing.

PE mode uses the "Power Enrichment Air Fuel Ratio Vs. RPM" table as the rich settings.

Strategy: Set by going too lean to create a bog condition, then lower the threshold until it goes away.

"Power Enrichment Air Fuel Ratio Vs. RPM"

These rich (and open-loop) air:fuel ratios are used when in PE mode, under hard accelleration. 11 is very rich, 12 might be optimum for many engines.

Strategy: set to 11 (or 10.5 if needed) initially to eliminate this as a variable (engines like being rich, even if it's a bit wasteful). Then set "% TPS Threshold Vs. RPM for PE Mode" to get rid of all the bogs. Then set the air:fuel ratios here for the highest value that has maximum power; or if you're a cheapskate set to the highest ratio that doesn't induce bogging.

"Accell Enrichment Vs. Differential MAP"

"Accell Enrichment Vs. Differential TPS"

I'm not sure precisely how to fiddle these, but I believe they determine the "initial shot" of fuel from the virtual "squirter" (accellerator pump). One for a sudden increase in manifold pressure (eg. vacuum drop) and one for throttle blip. Mnemonic: differential means rate-of-change in (MAP, TPS) value in time; move gas pedal slowly, low differential; move it quick, high differential.

Open loop -- setting up "the choke"

Who would have thought that the choke was so complicated?

Open loop operation persists until closed loop is enabled (timers, plus "Closed Loop Enable Coolant Temp" minimum coolant temperature.

"Open Loop Air Fuel Ratio Vs. Coolant Temp" $45a

"Open Loop Air Fuel Ratio Vs. MAP" $41d

This is your basic "choke" table. When in open loop (eg. before O2 sensor warm up, or other conditions) these settings are used to set the basic air:fuel ratio. Cold engines want more gas.

The engine gets the sum of these two tables (one for temp, one for manifold pressure.)

Strategy: Tedious because you can only set it on a cold (warming up) engine. I first set "% TPS Threshold Vs. RPM for PE Mode" to a high value across the board (50%) so that when cold, I was driving "on the choke". The car was unpleasant to drive during this test, flat-spots and bogs all over. Don't freak out.

I raised (enleaned) the AF numbers to something like 12 then enriched it if it stalled out. To drive the car with the "%TPS Threshold Vs. RPM for PE Mode" set so high, I had to accellerate very slowly to avoid the bogs. When the "choke" is set right, it drives well enough, albeit accellerates slowly.


This is "pump squirt" during open-loop (choke, warmup). After adjusting table $45a, with "% Threshold Vs. RPM for PE Mode" still set high, increase these to eliminate bogs and flat spots during open loop.

"Crank Air Fuel Ratio Vs. Coolant Temp"

These control the grossly rich setting while the engine is *cranking* but not yet running. Under these circumstances the motor wants to be very rich. (In ye olden dayes pressing the pedal to the floor before cranking once or twice gave the cold motor a good squirt of gas and set the choke.) I did not need to touch this.

“PW multiplier Vs. Engine Temp”, $427

During open loop, this table controls the rate that the "choke" value decays towards zero as the engine moves towards closed loop. The number here is subtracted from the current AF ratio; a smaller number here means leaner.

"Choke Air Fuel Ratio Multiplier Vs. Coolant Temp. $438"

Multiplied by $427 values? Why?