Experiments with increasing idle timing on B28F

[FABombjoy]

I recently ran some experiments changing timing in the idle zone. 13 degrees always struck me as late given that a stock distributor averages 30-40deg during cruise loads.

I found that 19 degrees BTDC saw a measurable decrease in MAP and measurably reduced injector PW. Idle feels less lumpy, and transitioning from cruise back to idle shows less tendency to rev hang. Idle advance correction seems to have more affect at this higher range.

Just wondering if anyone with ignition control + idle advance correction has tried something similar?

[David T]

Base timing for an older mechanical system has to be set to meet a bunch of compromises. They include;
Ease of cranking and starting
Emissions
The octane of the fuel to be consumed
The operating temperature of the motor
Acceleration and mileage

You can only do so much with a mechanical system. An electronic management system can do a lot more and is easily adjustable. You can optimize every parameter at all points of the operating envelope. The mechanical and vacuum advance can't be adjusted nearly as well as you can with an electronic system.

[FABombjoy]

So far 19deg is amazing. I have cranking at 10deg and the immediate transition to 19deg gives a satisfying startup "flare".

Idle is more stable. Engine feels smoother. Off-throttle transitions are smoother. Idle advance control has a lot more affect with base timing at 19 vs 13. Seems like 13 is so late that removing any more advance doesn't have a lot of affect on actual RPM.

Anyone with an odd-fire + ignition control should give it a shot.

[81dmc]

So far 19deg is amazing. I have cranking at 10deg and the immediate transition to 19deg gives a satisfying startup "flare".

Idle is more stable. Engine feels smoother. Off-throttle transitions are smoother. Idle advance control has a lot more affect with base timing at 19 vs 13. Seems like 13 is so late that removing any more advance doesn't have a lot of affect on actual RPM.

Anyone with an odd-fire + ignition control should give it a shot.
Just going to chime in here. Even with the 3.0 even fire with Josh S's timing map, it idled very lumpy at 14ish deg. It would also hunt sometimes no matter the idle settings.

On a whim, I set idle to 18 deg, and the engine runs amazingly! I recommend the 18-19deg idle timing to anyone with a PRV and EFI.

[jangell]

I just tried 19 degrees on my 3.0L and it seems to have pretty much fixed my rough engine at idle RPMs. I was able to drop the idle back down to 790, too (I had it at 900 to try to smooth it out).

I checked my notes, and it seems that the original table I used (as created but he MegaSquirt tool) also had ~19 degrees at idle, but I changed it to around 13 degrees for some reason, probably as part of trying to figure out other ignition issues I was having.

Nice find!

I wonder if this will help with some relatively rare stalling issues I'd sometimes have when rapidly decelerating (like pulling into a parking lot or driveway) or when quickly tapping the pedal when stopped and in gear. Unfortunately, these are so rare that it's hard to reproduce them, or I'd have tracked them down and fixed them by now.

-- Joe

[81dmc]

I just tried 19 degrees on my 3.0L and it seems to have pretty much fixed my rough engine at idle RPMs. I was able to drop the idle back down to 790, too (I had it at 900 to try to smooth it out).

I checked my notes, and it seems that the original table I used (as created but he MegaSquirt tool) also had ~19 degrees at idle, but I changed it to around 13 degrees for some reason, probably as part of trying to figure out other ignition issues I was having.

Nice find!

I wonder if this will help with some relatively rare stalling issues I'd sometimes have when rapidly decelerating (like pulling into a parking lot or driveway) or when quickly tapping the pedal when stopped and in gear. Unfortunately, these are so rare that it's hard to reproduce them, or I'd have tracked them down and fixed them by now.

-- Joe
If you ever had stalling issues when rapidly coming off throttle, then that?s dashpot adder. Also IAC initial values need to be good.