Injection timing on '88 SMEC V6 ???

[RoadWarrior222]

Hi folks,

Was reading that the injection timing is "derived" from optical slots in the dizzy on 3.0 V6 applications. Was just wondering if the SMEC applies any advance or retard to this or whether it's more or less what directly sets the timing with the SMEC only varying the pulse width or what.

Also curious as to why there would be 6 slots when '88 is meant to be batch fire in pairs, not sequentially.

I'm curious because I want to screw with it, and try it earlier and later in the induction cycle to see if I can set up a condition that favours leaning out the mixture for economy. (i.e. whether slightly earlier or later firing can land the mixture closer to the plug)

So anyone have a good idea how this works? Certainly seems interesting now I know this that advancing the dizzy gives a good power boost on these motors, considering that that would appear to advance injector timing also.

thanks for any useful tidbits,

RW222

[ShelGame]

It does batch fire them in pairs. It also fires them twice every cycle (once every revolution). So, the injectors will fire once to an open valve (possibly), and once to a closed valve. I don't think timing the injection is really going to make much difference.

[RoadWarrior222]

I figure it makes a difference whether a cloud of fuel ends up next to the plug or not, if you can figure it so it does, you can lean it back a bit, because it will still fire off good.

[risen]

It's my understanding that even in a full sequential system, the injectors only catch an open valve at idle and part throttle cruise. @ WOT or under load the injector pulsewidth ends up being longer than the intake valve is actually open, unless you have *HUGE* injectors (in which case your idle is probably crap anyways).

[RoadWarrior222]

Actually, looking at some timing diagrams, it appears they are usually set up to fire at a closed valve because the heat then evaporates the fuel, firing when the valve is open causes droplets to persist into the combustion cycle... apparently and may result in unburned fuel...

So I'm thinking the other way now...

Hmmm you confused me there though.... each pair of injectors fire once per revolution? So each individual injector fires twice before intake? I guess I forgot that the dizzy spins at half the crank speed.

Edit: BTW I was thinking they were paired 1&2, 3&4, 5&6... or are they paired 1&4, 2&5, 3&6 or something??? I guess they couldn't really be paired 1&2 etc or 15% duty cycle or so would be the max fuel that would go in on the first trigger, before it overlapped with the trigger for the following cylinder... which pretty much means that 50% duty cycle is the same as 100% duty cycle when taken together, but I guess the SMEC knows this....

However it's kind of confusing that later SEFI motors didn't get too much of a change in injector size...

[ShelGame]

Also, it would really only matter at low throttle openings or low load. At hi load, the injectors are running up to 80% duty cycle. That means they are open for 80% of the cycle or 576 degrees. The intake valve is only open for ~200 degrees or so. I still don't see how injection timing is going to affect power at all. It would only have an effect where the DC is much less than the intake open time - less than 28% DC.

[RoadWarrior222]

Also, it would really only matter at low throttle openings or low load. ...

Yep, that's where I want it to matter, figured I might get it a tad leaner or more efficient at cruise. The ultimate plan is a 14s, 40mpg minivan.

Anyway, thanks for helping me think a bit clearer. Realised there's not really anything I can play with there. Might go with a higher pressure regulator for finer spray and string diodes in series with the injectors to fudge the offset a bit to give the SMEC a hand with the extra fuel.

[ShelGame]

If it was really important to you, I think it could be done. Currently, the injection calculations are begun when the HEP triggers. Obviously, it takes a small amount ofd time to do the actual fuel and spark calculations. But, you could also add in a delay to the firing of the injectors to try and control when they fire, rather than just when the calcs are done. That shouldn't really be too difficult.