ignition timing question

[zaq123]

For those of you with larger cams and higher compression builds, what do you find the best average ignition timing for the idle is? I'm yet to put this car on the dyno to see MBT ignition numbers.. Just trying to dial in my idle as smooth as possible at the moment. does 15 degrees adv sound reasonable for 900 rpm? I adjusted ICV base in open loop to keep rpm on target ( without any corrections from PID or ignition corrections) for 15' adv. Once I introduce ignition correction of a couple of degrees (still PIDs disabled), it seems to idle ok and has 75 rpm range spread around 900 rpm target (fluctuating above/below 35-40 rpm).
So basically my idle timing is sitting withing 12-18 degrees. Does it sound reasonable for 2.9 M20 10:1 comp with ITBs and 284 cam?

[digger]

mine is set around 15 degrees

[zaq123]

ok, I cranked it up to 15 degrees and played with PIDs and ignition correction a little and got it idling super nice at 900rpm with +/- 30-40 rpm swing total around that target. Definitely helped with the engine temp too, idles at 197F all day long now.
digger, what is your compression? Also did you have any ign logs after dyno time? Not that mine will be the same but I just wondering where these M20 top out with the timing as far as MBT goes. I'm currently putting some miles on the engine to get it broken in for the dyno and doing some road tunning while doing so with no knock/detonation as my only parameter at the moment. Had some knock audible knock 2700-3200 rpm range at mid throttle. Luckily haltech has knock sensor spectrogram feature so working with that now. Just wondering where these 10:1 M20s typically top out as far as MBT goes on pump gas.

[digger]

ok, I cranked it up to 15 degrees and played with PIDs and ignition correction a little and got it idling super nice at 900rpm with +/- 30-40 rpm swing total around that target. Definitely helped with the engine temp too, idles at 197F all day long now.
digger, what is your compression? Also did you have any ign logs after dyno time? Not that mine will be the same but I just wondering where these M20 top out with the timing as far as MBT goes. I'm currently putting some miles on the engine to get it broken in for the dyno and doing some road tunning while doing so with no knock/detonation as my only parameter at the moment. Had some knock audible knock 2700-3200 rpm range at mid throttle. Luckily haltech has knock sensor spectrogram feature so working with that now. Just wondering where these 10:1 M20s typically top out as far as MBT goes on pump gas.

There are some different strategies with idle timing, for example with mine and the cam i have installed i could run it lower like 10 degrees and have the blade stop cracked a smidge more to increase airflow (10 degrees makes less power) and arguably improve off throttle response but put more heat into the exhaust. Or i could run it higher like 20 and have to back of the blade stop to reduce airflow (20 degrees makes more power) but this means the blades are almost 100% shut so a bit "sticky" when cold and there is also less usable available advance when cold. So i ended up in the middle havent touched the tuned for over a decade.

Mine is at 175-180psi with current cam timing position and my old tester (some new testers are iffy) and flogged out scored cylinder walls, when the engine was new and i had the MM rally cam and it was 155 so this shows how sensitive the compression test is to cam timing/events.

The 284/272 i measured was like 6 degrees advanced so if you did a compression test with a 10:1 engine installed in that position it would likely show high as the compression test measures the trapping efficiency and the inlet valve close IVC is the dominant event and the closer to BDC the higher the compression test value. This is the "dynamic compression ratio" effect which isn't actually dynamic at all on a fixed cam engine

To point out the obvious the compression test isn't the engine running pressure but there is some correlation (unknown proportioning scale factor) of a compression test reading to running pressure when the engine isnt yet "on the cam and the runner length hasn't come into play" which would mean that 2700-3200 rpm could be seeing higher pressure due to advanced cam timing (need to be checked). Obviously the ignition timing needs to be scrutinized and optimized for MBT and not use junk fuel.

Also note that temperature is a dominant factor not necessarily just pressure (they are not really independent though) that causes the knock (end gas auto ignition) so make sure the engines is breathing cold air and the spark plugs are not the factory heat range.

In the middle of summer here on hot days and hot engine i sometimes get light knock (not consequential to the integrity of the engine) just off idle if i don't use enough revs when letting the clutch out and the engine is effectively being lugged. I don't get this when the weather is cold or engine is only warm. Proper ducting for the filter could eliminate this.