How to check Dwell on a MK I 3000?

[GregW]

Pretty sure--but not positive--that the duration of the spark event will be the same no matter,

Usually yes. I have a Jacobs ignition on the Healey. For a spark event, initially it is high voltage. When the spark occurs, the computer senses that and switches to a lower voltage, but higher amperage to maintain the spark for a longer period.

the dwell--in time--remains constant (although dwell is measured in degrees of cam rotation, it's really a measure of time).

I'm not following you here. If dwell is constant with time, when you increase the RPM, the spark would need to come earlier, further into the dwell period the higher the RPM goes.

 

[Bob_Spidell]

Usually yes. I have a Jacobs ignition on the Healey. For a spark event, initially it is high voltage. When the spark occurs, the computer senses that and switches to a lower voltage, but higher amperage to maintain the spark for a longer period.



I'm not following you here. If dwell is constant with time, when you increase the RPM, the spark would need to come earlier, further into the dwell period the higher the RPM goes.

re: "If dwell is constant with time, when you increase the RPM,the spark would need to come earlier"

Note I was speaking specifically of electronic points (e.g. Pertronix, etc.). There is an ideal amount of time--measured in milliseconds or less, I suspect--that is required to fully charge the primary circuit in the coil to produce the best spark. That amount of time remains constant regardless of engine speed but, yes, the charging period will need to start earlier as engine speed increases; hence the need for advance mechanisms (both mechanical and vacuum). IOW, ideally dwell should remain a constant. With a points system, the dwell as measured in cam degrees--i.e., an angle--remains the same with engine speed, but because the engine is running faster the time decreases, and the coil will not have as much time to charge. Very high-revving engines can experience missing because of this.

Some good info here: https://spdispark.com/pages/frequently-asked-questions-dwell

and here: https://www.dtec.net.au/Ignition Coil Dwell Calibration.htm

 

[Michael Oritt]

Given all these variables it is a wonder that our cars run at all!

 

[Rob Glasgow]

Bob, your Dad was correct. I used to have a 1967 Chevolet Camero that had a little port on the side of the distributor to insert a screw driver with an Allen wrench end. By turning the Allen driver and watching the dwell meter, you could easily set the dwell. So much easier than setting the points the old way. Always wished the Healey had that feature until I installed the Petronics unit and gave up setting the points forever. :smile-new:

 

[DerekJ]

'Given all these variables it is a wonder that our cars run at all!'

Michael, part of the reason for that is that a lot of these variables are not actually that critical for the general operation of a road car. When I had a points system I never once checked or cared about what the dwell was. I just set the points gap correctly and checked it regularly. I now run the very basic Powerspark electronic ignition which operates with a constant dwell and there is no problem with sparking even at high revs.

 

[Michael Oritt]

'Given all these variables it is a wonder that our cars run at all!'

Michael,part of the reason for that is that a lot of these variables are not actually that critical for the general operation of a road car.

Hi Derek--

My comment was made about 3/4 in jest, but thinking about the gestalt of an internal combustion engine and the number of components that can fail, go out of adjustment, etc. etc. all of which are necessary to get an appropriately strong spark to a correct amount of vaporized fuel at just the right millisecond for an appropriate duration of time--over and over and over--I must pay homage to the car gods who created gasoline, electricity and oxygen and imbued man with the intelligence to manipulate these elements in such a way that I am effortlessly propelled around town, etc. etc.

Stepping down from the pulpit....

 

[DerekJ]

Michael,

I absolutely agree with you, it is amazing to think how well an 'old' combustion engine actually works with what appears to be rudimentary technology and basic equipment. I guess my point was more directed at the 'need' to try and obtain perfection in all its operations when the effort/reward ratio doesn't warrant it.

 

[Michael Oritt]

Michael,

I absolutely agree with you, it is amazing to think how well an 'old' combustion engine actually works with what appears to be rudimentary technology and basic equipment. I guess my point was more directed at the 'need' to try and obtain perfection in all its operations when the effort/reward ratio doesn't warrant it.

Agree, and besides my car always runs best after a fresh coat of wax.