idle with wild cam - tuning question

[Yves]

I have the following cam in a SBC V8 on itb's : 247/249 @ .050, .600/.604 lift, 105° LSA

Idle is at 1000 -1050 rpm, I'm using 19° advance at idle. Vacuum is 69 kPa. Idle is lopey and I see the AFR varying a lot. AFR at idle is set at 13.8 which was the lowest I could go with highest vacuum.

When going to higher ignition advance (22-24) I see the AFR's become stable and lower. Haven't tried lowering the VE at that point. Is this an indication the engine needs more advance at idle than the 19° I'm giving. My impression is that volumetric efficiency is reduced, and this is not a problem of ignition timing but the engine just needing a fatter idle mixture, like 13 or so.
If so, would reducing ignition timing and at the same time richening the mixture help ?

[Rick Finsta]

I think engines are like women; best give them what they want before they feel like being insistent.

If more idle timing makes her happy, give her more timing. Then add like 5% fuel, give it a minute, subtract 10% fuel, give it a minute, and then add 5% back to baseline. If one way makes for a lower, more stable idle with better vacuum, then go that direction. If you see something that doesn't make sense (you add 5% fuel but your AFR goes LEANER, for instance) the figure out what is causing it. Then go back and try the timing again (add a few degrees, see what happens, subtract twice as much as you added, see what happens).

Since unburnt fuel is read as a lean condition, the extra timing might be causing a more complete burn (causing what you see as a "drop" in AFR); that cam is going to have a lot of overlap and lots of chance for raw fuel to be pulled right through the chamber at idle. What is the installed centerline on the cam? You might already be pretty rich - I'd pull a plug before adding too much more fuel.

[Yves]

I think engines are like women; best give them what they want before they feel like being insistent.

If more idle timing makes her happy, give her more timing. Then add like 5% fuel, give it a minute, subtract 10% fuel, give it a minute, and then add 5% back to baseline. If one way makes for a lower, more stable idle with better vacuum, then go that direction. If you see something that doesn't make sense (you add 5% fuel but your AFR goes LEANER, for instance) the figure out what is causing it. Then go back and try the timing again (add a few degrees, see what happens, subtract twice as much as you added, see what happens).

Since unburnt fuel is read as a lean condition, the extra timing might be causing a more complete burn (causing what you see as a "drop" in AFR); that cam is going to have a lot of overlap and lots of chance for raw fuel to be pulled right through the chamber at idle. What is the installed centerline on the cam? You might already be pretty rich - I'd pull a plug before adding too much more fuel.

106°

I have been thinking about a rich misfire as well. Not sure which is causing which at the moment. Will try what you told me.

Also, I have seen that when using more ignition timing at the intermediate rpms (1200-2800) that the VE values decreased when using autotune (with incorporate afr). I thought this meant that I made a wrong move, so I went back to my base setting. I was hoping that the VE's were some indication of going into the right direct, but it seems not to be the case.

[Rick Finsta]

Yeah that'll give you some overlap...

Forgive me if you've said it in another thread, but is this a sequential or batch system? If sequential you can target a closed valve for your timing to stop the fuel draw through the cylinder at low RPM but unfortunately there isn't much we can do with batch systems.

As much as I hate the voodoo of reading plugs I think gross conditions are easy to see on them. Reminds me of the "weather rock" at Boy Scout camp...

[Yves]

Yeah that'll give you some overlap...

Forgive me if you've said it in another thread, but is this a sequential or batch system? If sequential you can target a closed valve for your timing to stop the fuel draw through the cylinder at low RPM but unfortunately there isn't much we can do with batch systems.

As much as I hate the voodoo of reading plugs I think gross conditions are easy to see on them. Reminds me of the "weather rock" at Boy Scout camp...

It's an ms2 system, so I cannot fire at the closed valve.

[krisr]

Don't be scared to throw idle timing at it.

My 400 Pontiac has a 236/242 @ 0.050" w/0.580/0.590" lift cam, mild really.... but to get mine to idle @ 950rpm @ 14.2AFR, I use 22° out of gear and 35° in gear - don't forget back in the day when manifold vacuum advance was all the rage, they were using alot of timing at idle aswell. When I set my idle timing at 35° out of gear, it sounded like it was missing and popping hence why I use less timing out of gear.

I rigged up the shifter inhibitor switch as the table switching mechanism to control in and out of gear tables.

[Rick Finsta]

Agreed - I run 26* in mine and it loves it. Lower timing (15* or so IIRC) and my headers get glowing red hot FAST.

[Yves]

Hm, that's thru. When I ran on carb I had vacuum advance hooked up to manifold vacuum, so it pulled some extra timing in it. Forgot that...

Not sure I understand how you hook up an external switch as a trigger point. I've not seen any input channels on the ecu besides maybe JS4/JS5 if I'm not mistaken.

Also : how fast do you let cold advance cut in (which temps and how much advance ? )