Tech discussion #1: Single/Double Fire

[mpprotomec]

The purpose of this post is to get a better understanding of a (probably) useless parameter as most of the tables I've seen have no difference.

That said, what do you think Single Fire Mode or Double Fire Mode really mean?

It seems to have been generally agreed on the its related to batch or sequential injection modes, but I really think thats wrong.

Its also been suggested that its related to spark, as in an MSD multispark operation, but while that can make sense in many ways, I have never known GM to have that capability, and it does seem to be more fuel related instead of spark.

I have investigated this through my extended sources and have had ZERO luck getting any info.

The use of 2 VE tables related to this would suggest a spark related thing. Injection change from batch to sequential would not cause a need to change VE ever IMO. VE would always be the same in both modes. On the other hand, a system that is re-lighting spark (i.e. systems with more than one plug per cylinder, MSD-like setups) could have a small change in VE between the two modes.

The tables decoded in HP do not yield any more clues.

But, I have different software for my PCMs (HP does not support my own cars) and they yield more clues, although they don't clear up much.

First, it cannot be a sequential/batch thing for my 95. My 95 PCM is not capable of sequential. It has no possible way to index cylinders (i.e. cam sensor input channel) to ever start in a sequence.
But there are still single/double references in the software. In mine, there are no VE tables for it, just injector offsets and constants.
Also, in both mine and the HP decodes, single/double is a low rpm feature only. Above a setable point, my car runs in Async Injector mode.

Here are some of the references in my car's cal:

Base Pulse Constant (no units)
Maximum Async Injector pulsewidth (msec -- stock value is 18.02)
Minimum Async Injector pulsewidth (msec -- stock value is 0)
Minimum Injector Pulse Width for Double Fire (msec -- Stock value is 3.30)
Maximum Injector Pulse Width for Single Fire (msec -- stock value is 6.00)
Minimum RPM to stay in Single Fire (rpm -- stock value is 950)
Minimum RPM to enable Single Fire (rpm -- stock value is 1000)

This leads me to believe completely that it is a fuel, not spark, component.
Beyond that, I have no accurate understanding of the concept.


What do you think???

[Chris@HPTuners]

it is a fuel mode, i believe it refers to if the injectors are fired once or twice each revolution. In every cal i have seen so far only uses single fire mode.

[EC_Tune]

Boy there's a throwback to the L98's (TPI) and earlier! Single & double fire. Thought that went away with SFI! (guess it probably has in reality)

I remember working on TBI's that had Syncronous (every DRP) and Asyncronous (Every 12.5 Msec) injector timing as well as single & double fire. Not to mention the modes to set it up so it transitioned nicely between them.
Oh the fun we had.... Ech!

[Peteej]

maybe its possbile that gm designed sfi only to work in the lower rpms as a fuel economy thing and switches back to batch fire above 4000 rpm. after all if you rev your motor past 4k you probably arent too worried about fuel economy. this might enable gm to use smaller injectors for more accuracy and a wider range of pulsewidths , while effectively doubling the available fuel flow for high rpm use. the double fire table most likely exists as a fallback when the pcm cannot use single fire due to injectors maxing out or cam position sensor fails etc. in the 95 year chevy might have only used sequential during idle to allow better control.

[BlurrSpeed]

I am going to through this out there & I could out of my head.

I seem to recall reading or seeing a video of injectors firing twice per ignition. It fired once in normal position at or just before the intake valve opened and then once just after spark. I think this was 1) to add cooler fuel and 2) to help the swirl. I dont recall the spark events, if the plug fire ones or multiple times.

Take this info with a grain of salt because I cannot find where I got this info.