The goal: least advance for best torque
Tuning ignition under load follows the same pattern as fuel tuning, working through the table one load and RPM area at a time. The difference is what you're chasing. With fuel you're aiming for the right mixture. With ignition you're aiming for peak torque. Get the best torque at every engine speed and peak power takes care of itself, because power is just torque multiplied by RPM.
A dynamometer is the right tool. It lets you hold the engine at a fixed RPM and load, start from a conservative timing value, and add advance in small steps while watching the torque reading.
How we work through the map
- Start every cell on the safe side. A retarded starting point costs a little power; an advanced one can cost an engine.
- Load the engine at the cell you're tuning, note the torque, then back off.
- With the engine off load, add a small amount of advance to that area of the table.
- Load it again and compare. If torque rose, repeat. If it stayed flat, you've found the plateau.
- Move on to the next area and repeat.
Don't sit on high load and high RPM while you edit the map. Apply load briefly, back off to make your change, then load it again. It keeps heat and stress down and gives more consistent readings, because the engine isn't getting hotter with every pass.
What the map should look like
As a general trend, an engine needs less advance as load goes up and will accept more advance as RPM goes up. If your finished table fights that pattern somewhere, double check that area.
Often torque stops rising well before the advance reaches a point where knock begins, which gives you a comfortable margin. When torque is flat across a range of timing, use the lowest value in that range. Extra advance that doesn't add torque only adds heat and cylinder pressure.
Some engines, especially on pump fuel or high boost, hit knock before they reach the torque plateau. In that case, don't chase the last bit of torque. Pull timing back to leave a real safety margin. We'd always rather give up a few horsepower than a set of pistons. Listening with knock ears and setting up knock detection both help you find that line.
Related reading
If you don't have access to a dyno, see tuning with no load, and for the dyno side of things read applying load to the engine. Our ignition timing vs air-fuel ratio article explains how mixture and timing interact.
