Ignition timing, guessing game or not?

[bracketchev1221]

I am trying to find some correlation to setting ignition timing on my 540. Is cranking compression a variable you can use? The reason I am asking is that I have always just gone for the general 36-40 degrees on all Chevy's. This always seemed to be where I made most power at the track and on the dyno. My 355 CNC Dart Pro 1 heads are on the car now, and I have the timing at 40-41. I was told by the head porter that because the chamber is so efficient that it doesn't need that much timing. I agree that is a variable in timing. But I was told depending on your cranking compression dictates timing too. That is something I have never used. I checked it and I think it was 205? So I was told to open the intake valve lash up and it would increase that pressure to maybe to 210. Apparently there is some table somewhere that says if you have that much cranking compression, you should be around 35-36 timing total. I am going to try 36 the next time I go out, but just wondering if anyone has seen this mysterious cranking compression/ignition timing table.

[jmarkaudio]

Timing needs are a combination of things, the primary factor being the ability of the combustion chamber to have a homogenized mix and have an even burn through the chamber. If the fuel quality is good, one that atomizes and vaporizes well the distribution will be more even and require less timing. Using the correct heat range plug to keep them free of deposits is another. Having an ignition system with sufficient energy to make the cylinders fire and sustain the burn is also a factor. Making sure the induction evenly distributes fuel to the cylinders, and using the right size carb to get sufficient atomization and vaporization. Any of the above that is less than ideal will result in larger timing leads. My 23˚ used to run about 41˚ with 11X heads, making major tuning changes to the carb, hotter plug, better ignition, and some home porting to the intake resulted in 37˚ timing and a couple tenths in ET. So even though the chamber may be consistent, it depends on a lot of other factors you may be crutching with more timing. Go to a test session, make a couple runs, and start taking timing out until it slows.

[John_Heard]

I think at best any table you would find would only give you a starting point. Too many variables to hit it right on the head 1st shot, air quality for example.

[bracketchev1221]

I was just curious, because I have never heard of cranking compression being used as a tuning aid for anything. I've only used it to see if a cylinder was bad. I mean, I can 100% agree, that if you are trying to push the piston back down early with too much advance you are losing power. But to arbitrarily say the timing is too high, is odd to me.

[jmarkaudio]

Cranking compression might tell you something about the cam, but to arbitrarily say timing is too high because of it doesn't factor in the rest of the info. Larger heads may need you to turn up the wick a little, more RPM, more converter stall or gear to see gains.

[Craig W.]

Another consideration is fuel. When changing fuels, ignition timing will need to be addressed to find what's best with the new burn rate.