Cam Idle Tuning Question

[96gt4.6]

So I have a question that has recently perplexed me.

During tuning on a cam'd gen 3 setup, as I was going through and adjusting the idle spark, over and under speed spark correction to get it to not surge and idle properly I had a thought.

On a carb setup, the idle air is metered by the throttle blade set screw, so it is constant.

Fueling is constant via the idle bleed set screws.

Timing is constant as well on a distributor setup.

In my previous experience with a carb setup/large cam, there really was no issue getting it to idle after adjusting those things. So, as a test, I used the scanner to command a set AFR, Timing and IAC position, yet the RPM's like to surge/swing a LOT, which got me to thinking, why? It's technically no different than a carb setup at that point?

The car runs perfectly once I got the spark over/under set properly along with fueling and IAC counts in the right range. Just was something that I had wondered the how's and why's on.

[blindsquirrel]

Fueling is not constant with a carb, it's completely dependent on airflow and the pressure differentials applied across whatever circuit is active at the time.

[lukearmstrong1990]

if you removed the IAC and had 0% TPS the idle would be stable provided it was at the RPM you were targeting.

but if its not dead on it will use the IAC and spark correction to meet and maintain the target. if you have one or more of the idle components at a fixed value it cant maintain idle target. its a dynamic operation maintaining idle with EFI. vastly different to carb. its more pronounced with a large camshaft as idle is less stable than with a standard camshaft.

[kingtal0n]

I think it's either the scanner, or due to the complexity of the ECU overall.

I've been able to get many engines to idle correctly without iacv/spark/fuel corrections (like a carb) with stand-alone computers.

But as you have noticed, not with the scanner. I've noticed some odd behaviors with the scanner.
For example when I disable injectors #4 or #8 while the "air-fuel" ratio box is active, I get twice the a/f drop from those two cylinders.
It's like the scanner is turning off both #4 and #8 at the same time when the a/f box is operating.

very strange.

In any case. If it isn't the scanner, I would suggest that the OEM ecu is so complex and that there are so many other tables still influencing the engine's behavior that it's impossible to truly 'lock out' the dynamic conditionals as with a carb, without significant re-tuning and scouring.

[blindsquirrel]

Please don't feed the disinformation, nothing about a carb's fuel delivery is ever 'fixed'. It's completely feedback-based according to vacuum/airflow/pressure differentials.

I could talk for two hours about how amazing it is that the Holley idle circuit flows in reverse and acts as an air bleed when the main circuit is active, don't get me started...

[kingtal0n]

Please don't feed the disinformation, nothing about a carb's fuel delivery is ever 'fixed'. It's completely feedback-based according to vacuum/airflow/pressure differentials.

I could talk for two hours about how amazing it is that the Holley idle circuit flows in reverse and acts as an air bleed when the main circuit is active, don't get me started...

Yeah but air fuel ratio has almost no impact on the idle quality whatsoever for a wide range of engines, and especially with a cam.

Anywhere between 12.2 and 16.4 my 5.3 won't really care.
Next airflow is a quantity set to be fairly exact, consistent, on a carb. There are no moving parts involved with air flow. Only mixture related differential routines. There is nothing that changes or modifying the amount of airflow per unit time besides the throttle valve which is identical for EFI.

therefore the systems should run the engine remarkably similar if the timing is held constant.