Gents,

I am doing a similar conversion on a 454 and the edelbrock chip worked OK but high end performance was bad and ran lean at high RPM's Since the original TBI to MPFI conversion I have rebuilt the motor with significant changes and could not get it to run decent at all. I then came to this forum and was directed to the memcal jumper and PCM resistor shorting solutions and I did my homework on them. As I understand them...


Memcal jumpers

he memcal changes the threshold voltage on a comparator chip within the PCM which changes the DRP to injector bank firing number. Once this memcal mod change is done, the base pulsewidth at idle and battery voltage at idle vs pulsewidth adjustments are needed to compensate for the halving of the injector firing per cycle. I was told this is the best way to use the bank fire application since it insure a better mixture at idle, and prevents the changing of the firing sequence at idle that occurs in the TBI mode.

current sense resistor shorting

The current sense resistors are tied to ground on the low voltage side of the driver switch FET in series with the injectors and the injector drivers. The jumpering of the current sense resistor adds about 600 Milliamps of drive current at 12 volts and about 750 milliamps of drive current to the injector driver circuit at 14 volts by reducing the overall circuit series resistance by about a 1/2 ohm. The low impedance peak/hold TBI injectors are about 1.5 ohms each and the bank of 4 saturated high impedance (16 ohms each with 4 in parallel) injectors presents about 4 ohms to the driver circuit. The ability of the saturated injectors to turn on consistently and rapidly is a function of the drive current and the shorting of the sense resistor adds additional current to insure a rapid and repeatable turn on time of the saturated injectors coil. The current sense resistor shorted enables the injector coils to transition to fully open with less voltage drop/heat developed across the injector coil. The rapid saturation and shorter/crisper/faster switching time of the coil alters the actual pulsewidth (vs. commanded Pulsewidth) of the injector fuel pulse which requires initial tuning to compensate. I used an oscilloscope and a clip on current probe to analyze the current waveforms through the injectors with and without the sense resistors shorted and the shorted resistors were firing repeatably with a consistent rise time versus voltage. With the current sense resistors installed n(stock condition), the leading edge of the current pulse to the 4 injectors was slow and inconsistent, and varying pulse to pulse. The varying leading edge and "tearing" of the current injector pulse waveform means the coils were not opening reliably and repeat-ably.

I endorse the shorted current resistors and the associated retune required after viewing the current pulses on the O-scope.

I am returning to tuning this engine now and will keep you all up to date on the results. 1995 GMC 454 (30/100ths over (460 ci)), 7427 PCM with $31 mask, edelbrock MPFI with 36 lb pico injectors, edelbrock performer RPM heads, Thorley tri-wye headers and 3 inch single exhaust.

john