Snapshot 26-11-2021 21:24

Ignition coil dwell calibration

Why calibrate ignition coil dwell?

I am using a distributor based ignition for my EFI build and ignition coil dwell calibration will help optimise various performance aspects.  A limitation of using a distributor on a 6 cylinder is as RPMs climb the time between spark events reduces.  This reduces that available time for the ignition coil to charge and the spark burn time.

To verify if my ignition coil (trusty Bosch GT40) is being handicapped at higher RPM I investigated the effect of dwell time on primary current draw and spark burn time.  20 years ago measuring this required expensive equipment but nowadays equipment is much more cost effective.  The equipment I used is as follows (click to buy your own from eBay):

HT25 on the left and CC-65 to the right

Testing ignition coil dwell

I ran the car at idle and varied the ignition coil dwell time on the Megasquirt MS3X ECU.  Waveforms of each dwell was captured with the picoscope for a desktop study later. I also captured a datalog through the Megasquirt ECU to verify battery voltage.

The advantage of this method is the results are real world.  Undertaking this test on the bench lakes the ability to replicate in cylinder pressures which alter resistance across the spark plug gap.  This resistance changes firing voltage and spark burn time.

At the desk I used the picoscope software to review the waveforms and measure, dwell time, primary current peak, firing voltage, spark voltage and spark burn time.

Measuring various properties of the captured waveforms

The results

The results were then compiled into a table.  The saturation current appears to be around the 3.3 to 3.4 amps.  Firing voltage and spark voltage were consistent across all dwell times which was expected.  The burn time is almost linear in fashion.  Table of results below.

dwell (ms)firing (kV)Spark (kV)current (A)Burn time (ms)batt V
1.75.00.91.580.5013.1
2.25.00.92.000.6013.2
2.85.00.92.300.7513.4
3.35.00.92.450.9013.4
3.85.00.92.701.0013.4
4.45.00.92.901.1013.4
4.95.00.93.001.2013.4
5.55.00.93.101.2513.5
6.05.00.93.201.3013.5
6.45.00.93.251.4013.6
7.35.00.93.301.4713.6
7.55.00.93.361.5013.6
7.95.00.93.401.5513.6

So what does this mean to me?  I know roughly want my saturation current and dwell time is as well as burn time.  I also hav confidence that as my RPMs climb and the available time for dwell and burn time decreases, that the GT40 coil still provides sufficient spark (0.6ms burn time as a minimum).

Check out these links to other dwell related resources:

DTEC ignition coil dwell calibration

http://www.derek.com.au/ignitioncoils.html

If you’d rather watch than read, check out the below YouTube video!

Share:

Share on facebook
Facebook
Share on twitter
Twitter
Share on pinterest
Pinterest

Leave a Reply

Your email address will not be published. Required fields are marked *

On Key

Related Posts

Datalog recording of induced low load knock

Knock control tuning

Knock control tuning is important for any modified EFI engine. In this post I explain theory and practical verification of knock control tuning.

Ignition coil dwell calibration

Why calibrate ignition coil dwell? I am using a distributor based ignition for my EFI build and ignition coil dwell calibration will help optimise various

Megasquirt battery voltage calibration

These days injector behaviour at different voltages is well understood. The speed of opening and amount of fuel delivered vary with voltage. Corrections are made in fuelling calculations to account different performance for differnt voltages at the injector. If what the Megasquirt ECU understands is the voltage at the injector is different in reality, then it is likely fuelling calculations will be incorrect. This is why the battery voltage calibration needs to be correct in your Megasquirt ECU.

Manual Idle Air Control

The ITB idle air control conundrum Idle air control is generally a challenge with individual throttle bodies.  This is from the sensitivity of throttle position