Trying to get pre-crank injector pulses

**Six_Shooter** · 01-05-2022

You mean like a Prime pulse? All GM ECMs do that AFAIK.

**kilohertz** · 01-05-2022

Originally Posted by Six_Shooter

You mean like a Prime pulse? All GM ECMs do that AFAIK.

Exactly! Mine doesn't, and I have tried 3 different BCCs.

My injectors won't pulse until it's cranking, and with the convoluted intake path of the Flathead, I would like to have some fuel sitting on the butterflies ready to get sucked in.

Thanks!

**In-Tech** · 01-05-2022

The '7747 is pretty archaic and I don't think there is a prime pulse, from memory. If your injector/cu in constant is correct(very critical)...then all you should have to tailor is IAC counts during crank. I realize this is a bit vague. Bottom line is it will take a bit of work since you are starting from scratch basically. It seems like you are well on your way.

**kilohertz** · 01-05-2022

Originally Posted by In-Tech

The '7747 is pretty archaic

Ha! But miles ahead of a Holley 94 carb.

Originally Posted by In-Tech

If your injector/cu in constant is correct(very critical)...then all you should have to tailor is IAC counts during crank. I realize this is a bit vague. Bottom line is it will take a bit of work since you are starting from scratch basically. It seems like you are well on your way.

Thanks Carl. Yes I have moved from 2.8L injectors to 4.3L and the math comes out to BPW of 126 for the 3.9L Flathead with BSFC of .65 and 100HP. I have played with the IAC park position but didn't realize it was that important...I will continue to fine tune it. Appreciate the info.

Cheers

**tayto** · 01-05-2022

make sure the crank wire actually has voltage while switch is in crank. also that the various ecm power feeds dont lose power while cranking. I would start with a 4.3L calibration vs a modified 2.8L bin myself.

**kilohertz** · 01-06-2022

Thanks Tayto,

I am using BCC ASDX which is '88 5.7L Sync fueling mode. Yes, there is crank voltage on the crank wire and the other main Vcc power as well. The ECM is working properly, what I am trying to do is get a few injector pulses before I hit the starter, right after key on.

Cheers