Another Perspective on Coil Tuning

[MKossor]

[Chris Barker]

I really hesitated to respond here because Mike has produced some excellent devices, is usually right and is always helpful.
BUT
His middle picture does show indeed the optimum spark timing but it relates to a datum time-to-fire of 2ms with a 12v DC supply. It does not mean that the spark occurs at TDC - and you wouldn't want it to.
In the car, a coil tuned on the ECCT will, with a normal timer lever setting, actually fire at around 18.5 degrees BTDC because that when the peak voltage from the magneto is generated.

The actual firing point will advance as rpm rises because the magneto output rises to over 20V and the time-to-fire shortens. But as Mike will agree, ECCT tuning helps you ensure that all 4 sparks have the same timing, and it's consistent.

The centre caption should really be 'Spark at Optimum Timing'. Then it's a good trio of pictures.

Sorry but....

[TXGOAT2]

In practice, the spark timing is under the driver's control, and optimum spark timing is dependent upon the driver's understanding of the engine's requirements. If time to fire is inconsistent among the coil units, the driver cannot obtain optimum spark timing in all 4 cylinders. To obtain accurate time to fire, it's also important that the rest of the ignition system be in good condition and adjustment, and that all 4 cylinders have consistent compression and volumetric efficiency characteristics. It's difficult to get an engine in perfect tune, but the closer you can get, the better.

[Erik Johnson]

I'm not an engineer or expert but I have read period automotive textbooks.

Full combustion of the air-fuel mixture takes time.

Therefore, to achieve maximum power and efficiency, spark and combustion needs to be initiated prior to top dead center in order for peak force to occur just after top dead center.

If spark occurs at top dead center, that is too late to achieve maximum power and efficiency (combustion pressure peaks too late as the cylinder volume expands during the piston downstroke).

[TXGOAT2]

I don't believe that's in dispute.

[MKossor]

Chris Barker, you are absolutely correct! Great to read substantive discussion beyond superficial acceptance and agreement. Thank you for your post.

The motivation in my posting the pictorial representation of of ECCT multi spark test results was to clarify what the multi spark charts really mean for those new or unfamiliar with interpreting charts. I did take liberties in generalizing the representations a bit too far and your keen eye caught me on that. Pat was a bit more forgiving in understanding the jest of what I had intended. Here is a more accurate depiction as recommended:

For those who wish to continue substantive dialog I offer the following diagram for further comment and discussion on the impact of 1 to 1.5ms of ignition lag on Model T engine performance:

Poorly tuned coils and/or timer related timing variation will prevent ever achieving this level of performance. Consider the crank is turning at 12degrees per millisecond (1/1000 sec) at 2000 RPM so microsecond do in fact matter if one is interested in achieving the full level of performance a Model T engine is fully capable of.