Some basic timing questions, tuning gurus inside please
Thread Starter
Supporting Vendor
Joined: Jul 2003
Posts: 6,486
Likes: 2
From: http://www.mphparts.com 800-364-1975
Some basic timing questions, tuning gurus inside please
Inside my high and low octane tables, the cells that are around idle (e.g., low rpm and low grams/cyl) show relatively high timing... for example, 22 degrees, then the next RPM band up is 26, then 30something, etc. But my car does not actually idle at 22 or 26, the actual timing is closer to 16-17 degrees, and it fluctuates a lot (down to 12, then up to 22, and everywhere in between).
When I was datalogging another car stock car, I noticed that timing was rock solid at idle and was higher than my car is idling at.
Is this some kind of idle maintain function that has determined that my car wants to idle at timing lower than what is commanded in the high and low octane tables? Using delta spark control, I pulled out some timing and the car *seemed* happier but it was happiest when I pulled out serious timing... 10+ degrees. Can that be right? And will adjusting timing down in the high/low tables help smooth out idle timing (e.g., will the car say, "OK, this timing is good, so I'm not gonna screw around with it?")
If this is some kind of idle control that the car is automatically introducing because it thinks commanded timing in my maps is bad, how does it work? Does it learn what timing is required to keep the car at idle? Would I have to reset that somehow after changing timing?
When I was datalogging another car stock car, I noticed that timing was rock solid at idle and was higher than my car is idling at.
Is this some kind of idle maintain function that has determined that my car wants to idle at timing lower than what is commanded in the high and low octane tables? Using delta spark control, I pulled out some timing and the car *seemed* happier but it was happiest when I pulled out serious timing... 10+ degrees. Can that be right? And will adjusting timing down in the high/low tables help smooth out idle timing (e.g., will the car say, "OK, this timing is good, so I'm not gonna screw around with it?")
If this is some kind of idle control that the car is automatically introducing because it thinks commanded timing in my maps is bad, how does it work? Does it learn what timing is required to keep the car at idle? Would I have to reset that somehow after changing timing?
Melting Slicks
Joined: Mar 2003
Posts: 3,116
Likes: 10
From: Macomb MI
Oh jees..
Without writing a book
The high and low octane spark tables are only used when moving.
No wheel speed and other idle conditions are met the Idle Park/Gear tables are used.
In addition, in the idle state there is and adaptive idle spark routine (both over and under speed) that will move spark up and down to meet commanded idle rpm. Its much faster to react and control vs air flow adjustments. Bigger cams will play some havoc with this adaptive control as the crank signal will pick up the lope created by the big cam and add/remove spark to attempt to control the rpm in turn making it worse.
YOu will find that if you have added to much spark it will make the idle more sensitive to air/fuel transitions creating a surge.
Without writing a book
The high and low octane spark tables are only used when moving.
No wheel speed and other idle conditions are met the Idle Park/Gear tables are used.
In addition, in the idle state there is and adaptive idle spark routine (both over and under speed) that will move spark up and down to meet commanded idle rpm. Its much faster to react and control vs air flow adjustments. Bigger cams will play some havoc with this adaptive control as the crank signal will pick up the lope created by the big cam and add/remove spark to attempt to control the rpm in turn making it worse.
YOu will find that if you have added to much spark it will make the idle more sensitive to air/fuel transitions creating a surge.
Last edited by RoadRebel; Apr 22, 2007 at 10:01 PM.
Thanks Doc! I have some more tinkering to do 
