Ignition curve discussion

[GRTfast]

How did we get ten posts in without someone saying, "Just lock 'er in at 34 degrees and forget about all that initial and advance stuff."

I tried a 38 everywhere setup, kinda bucked at low rpm steady speed. The map in it right now runs really well and is the smoothest driving setup I've had since the car has been on the road. Mike's cam has something to do with that as well.

[novadude]

Is it the case that the engine would like more and more advance as the revs build, and the reason for not adding more advance after 3000 rpm or so is because you just run up against detonation? Just curious what you all think.

My understanding--from listening to other folks, not from my own independent research--is that there's no need for additional advance after a certain RPM. In fact, high-RPM use may need a few degrees of "high-speed ignition retard" because the in-cylinder turbulence becomes so much greater as the RPM builds. The turbulence speeds combustion.

I've heard that, but I've also seen smart guys make the case for a curve that gradually increases a few degrees above "max" advance as RPMs climb to counter the effects of slew rate in the module / coil.

[hoffman900]

To be fair, it's all guess work without instrumentation.

[David Redszus]

Is it the case that the engine would like more and more advance as the revs build, and the reason for not adding more advance after 3000 rpm or so is because you just run up against detonation? Just curious what you all think.

My understanding--from listening to other folks, not from my own independent research--is that there's no need for additional advance after a certain RPM. In fact, high-RPM use may need a few degrees of "high-speed ignition retard" because the in-cylinder turbulence becomes so much greater as the RPM builds. The turbulence speeds combustion.

I've heard that, but I've also seen smart guys make the case for a curve that gradually increases a few degrees above "max" advance as RPMs climb to counter the effects of slew rate in the module / coil.

Squish velocity, the main component of combustion turbulence, does increase proportionally to rpm.

Ignition delay, the time period from ignition to self-sustaining spark, is a function of time not degrees. Therefore we need more degrees as rpm increases.

The fastest flame speed usually is found at or very near the torque peak; we need slightly less timing.

Higher engine speeds allow less time for fuel evaporation, reducing mixture quality while requiring an increase in spark advance to maintain the same peak pressure angle.

The peak pressure angle will vary with engine speed and will need an adjustment in spark timing to optimize combustion timing.

When you out it all together, the ideal ignition curve becomes complex; somewhat like the Rocky Mountain Range.
Which is why ECU ignition maps provide the construction of complex curves, often different for each cylinder.

[GRTfast]

My understanding--from listening to other folks, not from my own independent research--is that there's no need for additional advance after a certain RPM. In fact, high-RPM use may need a few degrees of "high-speed ignition retard" because the in-cylinder turbulence becomes so much greater as the RPM builds. The turbulence speeds combustion.

I've heard that, but I've also seen smart guys make the case for a curve that gradually increases a few degrees above "max" advance as RPMs climb to counter the effects of slew rate in the module / coil.

Squish velocity, the main component of combustion turbulence, does increase proportionally to rpm.

Ignition delay, the time period from ignition to self-sustaining spark, is a function of time not degrees. Therefore we need more degrees as rpm increases.

The fastest flame speed usually is found at or very near the torque peak; we need slightly less timing.

Higher engine speeds allow less time for fuel evaporation, reducing mixture quality while requiring an increase in spark advance to maintain the same peak pressure angle.

The peak pressure angle will vary with engine speed and will need an adjustment in spark timing to optimize combustion timing.

When you out it all together, the ideal ignition curve becomes complex; somewhat like the Rocky Mountain Range.
Which is why ECU ignition maps provide the construction of complex curves, often different for each cylinder.

Makes sense. The optimal curve for our kart engines looked like a stock market chart.

[Calypso]

In our two stroke shifterkart engines, the curve for best wide open power was very advanced down low and got progressively more retarded through the rev range. Probably due to some form of the same phenomenon.

2-strokes retard at high rpm to put more heat into pipe, which changes the speed of sound to make pipe look shorter and stay in tune.

[GRTfast]

In our two stroke shifterkart engines, the curve for best wide open power was very advanced down low and got progressively more retarded through the rev range. Probably due to some form of the same phenomenon.

2-strokes retard at high rpm to put more heat into pipe, which changes the speed of sound to make pipe look shorter and stay in tune.

That is certainly part of the equation. I know each different pipe had a different optimal curve.

[David Redszus]

In our two stroke shifterkart engines, the curve for best wide open power was very advanced down low and got progressively more retarded through the rev range. Probably due to some form of the same phenomenon.

2-strokes retard at high rpm to put more heat into pipe, which changes the speed of sound to make pipe look shorter and stay in tune.

That is certainly part of the equation. I know each different pipe had a different optimal curve.

The fun part came when we found that a two stroke at constant throttle and rpm, would increase EGT with time.
Now it was time for the really sharp tuners to go have a beer.

[hoffman900]

2-strokes retard at high rpm to put more heat into pipe, which changes the speed of sound to make pipe look shorter and stay in tune.

That is certainly part of the equation. I know each different pipe had a different optimal curve.

The fun part came when we found that a two stroke at constant throttle and rpm, would increase EGT with time.
Now it was time for the really sharp tuners to go have a beer.

Well, EGT sensors are thermal sensors, so isn't that always the case?

[Calypso]

That is certainly part of the equation. I know each different pipe had a different optimal curve.

The fun part came when we found that a two stroke at constant throttle and rpm, would increase EGT with time.
Now it was time for the really sharp tuners to go have a beer.

Well, EGT sensors are thermal sensors, so isn't that always the case?

At speed or on the dyno?