As long as you get your dwell at 30*, does the relationship of where your Idle Mixture screws are set compaired to where the Idle Air Bleed is set really matter?

What I mean is, if I can achieve proper dwell with the Idle Mixture screws out only 3 turns, would that be any different than if I could achieve proper dwell with the mixture screws turned way out to like 7 turns?

 

Hmmm, good question.

You are adjusting the idle air bleed to get the closed loop MCS duty cycle at 50%. If the system is able to achieve that 50%, it follows that it doesn't make any difference where the screws are set.

 

Yeah, that is a good question. Here's my thinking on it.....

It doesn't matter down at curb idle where you can compensate for the mixture in 2 ways: idle mixture screws or IAB valve. However, off-idle I think it matters since you only have ONE adjustment- the IAB valve. The off-idle discharge ports are fixed and non-adjustable, and the mixture they deliver is also affected by the IAB setting exactly the same way the curb idle (idle mixture screws) are.

If you had the IAB set way on the rich or lean end of it's adjustment you could certainly compensate at curb idle with the mixture screws to bring it back to 30*, but you would probably still end up with a rich or lean spike off-idle as the (non-adjustable) idle transition slots start to have greater effect on the mixture.

 

That makes sense Damon. I figured your off-idle factored in there somehow but wasn't really sure.

So if one was getting a lean off idle stumble, you could adjust the IAB to richen it up and compensate, but then you'd have to use the mixture screws to readjust your curb idle for 30* dwell? Sound about right?

 

Yeah, that's what I'm thinking. The feedback system/ECM on those cars is rather slow and it might spike rich or lean momentarily off-idle before it figures out that the dwell needs to be adjusted quite a bit to bring the O2 readings back in line.

Of course, in closed loop when the engine isn't warmed up sufficiently (and/or other parameters haven't been met yet to allow closed loop) the ECM is just sending a fixed dwell to the M/C solenoid and not even looking at the O2 sensor yet. If it's rich or lean off-idle it will stay that way constantly in closed loop.

My usual starting point for a V8 CC-Qjet is 4 turns out on the mixture screws and 4 turns out on the IAB valve.