The Ford Barn

aussie merc · 06-07-2025, 06:03 AM

ok what's the difference between dwell angle and dwell percentage as all the material i can find from ford states dwell percentage not dwell angle

wamnram · 06-07-2025, 08:33 AM

My take:

Dwell angle is defined as the amount of time the contact breaker points are closed (current flowing through the primary ignition circuit) and it is measured in degrees of distributor shaft rotation.

Dwell Percentage (or Dwell Time) is the amount of time the points are closed (time to charge capacitor) based on 0 - 100%.

JayChicago · 06-07-2025, 11:21 AM

Agree.

Spec is 36 degree dwell (points closed), 9 degrees points open. That occurs in each 1/8 of a revolution of the distributor, or 45 degrees of revolution.

36 degree dwell = 80% of 45 degrees of revolution. Points are closed 80% of the time.

aussie merc · 06-07-2025, 06:53 PM

so if my take is correct dwell% = the portion between cam" lobes" that the points are closed and this will vary due to the number of lobes where as dwell angle does not as its a constant 360/deg regardless of the lobe count

ralfstc · 06-09-2025, 05:59 PM

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Sort of. Dwell angle will vary too, as lobes=cylinders (often). In a V-8 there are only 45 degrees per cylinder, hence the twin points in a crab to drag the dwell angle up to 36 degrees. In a four cylinder you'd have 90 degrees per cylinder to play with, so with an equally bumpy cam as an 8 you could have 81 degrees of dwell and still have your 9 degrees for firing.

In the first case the dwell % is 80 (our V-8s) in the second case the dwell % is 90%.

06-07-2025, 06:03 AM	#1
aussie merc Senior Member Join Date: Sep 2016 Location: sydney australia Posts: 1,125	dwell perctage ok what's the difference between dwell angle and dwell percentage as all the material i can find from ford states dwell percentage not dwell angle

06-07-2025, 08:33 AM	#2
wamnram Senior Member Join Date: May 2010 Location: San Tan Valley, Arizona Posts: 220	Re: dwell perctage My take: Dwell angle is defined as the amount of time the contact breaker points are closed (current flowing through the primary ignition circuit) and it is measured in degrees of distributor shaft rotation. Dwell Percentage (or Dwell Time) is the amount of time the points are closed (time to charge capacitor) based on 0 - 100%. __________________ Wayne

06-07-2025, 11:21 AM	#3
JayChicago Senior Member Join Date: Jan 2019 Location: Chicago Posts: 926	Re: dwell perctage Agree. Spec is 36 degree dwell (points closed), 9 degrees points open. That occurs in each 1/8 of a revolution of the distributor, or 45 degrees of revolution. 36 degree dwell = 80% of 45 degrees of revolution. Points are closed 80% of the time. Last edited by JayChicago; 06-07-2025 at 11:30 AM.

06-07-2025, 06:53 PM	#4
aussie merc Senior Member Join Date: Sep 2016 Location: sydney australia Posts: 1,125	Re: dwell perctage so if my take is correct dwell% = the portion between cam" lobes" that the points are closed and this will vary due to the number of lobes where as dwell angle does not as its a constant 360/deg regardless of the lobe count

06-09-2025, 05:59 PM	#5
ralfstc Junior Member Join Date: Aug 2023 Posts: 13	Re: dwell perctage Sponsored Links (Register now to hide all advertisements) Sort of. Dwell angle will vary too, as lobes=cylinders (often). In a V-8 there are only 45 degrees per cylinder, hence the twin points in a crab to drag the dwell angle up to 36 degrees. In a four cylinder you'd have 90 degrees per cylinder to play with, so with an equally bumpy cam as an 8 you could have 81 degrees of dwell and still have your 9 degrees for firing. In the first case the dwell % is 80 (our V-8s) in the second case the dwell % is 90%.