WD WD45 Points Dwell

[gatz]

What is the Dwell angle for the WD and WD45 points ?

 

[David Grubb]

It is twice the value of a V8(28-32 degrees); so it will be 60 degrees.

 

[Brian G. NY]

David, that sorta sounds good but it is actually 25-34 degrees. The dwell is the number of degrees the rotor turns from the time the points open until they close. Using your example, points on a two cylinder would be open about 120 degrees. I wouldn"t have known the actual spec had it not been for a response to this question over on the Unofficial Allis site.

 

[Bob]

You got 'er bassackwards, Brian G.

ACTUALLY, dwell angle is the degrees of distributor rotation during which the points are CLOSED between firing events.

The points open, the coil "fires", points close (building up the magnetic field in the coil during the "dwell" period)m and the cycle repeats.

Quoted from "The Automobile Encyclopedia":

"The Cam angle, or dwell angle is the number of degrees through which the distributor cam rotates while the breaker points are closed. It is directly related to the breaker point gap. Decreasing the breaker point gap will increase the cam angle. If the cam angle is too small, the current will have insufficient time to pass though the primary winding of the ignition coil, and a weaker spark will result. If the cam angle is too great, the breaker points will not open far enough, they will tend to stick together, and misfiring will result. The best method of adjusting breaker points is by means of a dwell meter."

 

[Texas Denny]

This is actually an excellent question. I do not have the answer and do not know which of the answers is correct. It should be easy to verify via an experiment. Adjust the points, measure the dwell. Increase point gap, measure the dwell. Compare the readings and you will know which the dwell is measuring (time closed or time open).

As far as the spec, that would need to come from Allis Chalmers as the shape of the distributor cam will affect the dwell and the rate of point opening. I have never seen a dwell reading in any Allis Chalmers literature from before the 60's. Don't have any knowledge after the 60's.

I believe pre 60's was a simpler time. Most tractor manufacturers took the simple approach that their customers could easily understand and didn't require tools that they wouldn't buy - feeler gauge and point opening at top of cam lobe.

 

[Kentb of SWMO]

Y'all know that a dwell meter is just an ohm meter with a different scale.

Kent

 

[Bob]

HMMMM... an Ohmmeter is generally self-powered and flows a tiny current through the part under test to determine it's resistance.

A dwell meter is generally powered by the ignition circuit and measures duty-cycle, quite a different thing than Ohms!

(You ARE kidding, RIGHT?)

 

[Jeff B.]

The number of cylinders is of no consequence. The recommended setting for nearly all motors is 28-32 or 34 degrees.

 

[Bob]

The number of cylinders IS of consequence as well as the point cam profile.

In THIS example, the points are OPEN for 4 spark events per distributor revolution, 36º at a time, for a total of 144º. They are CLOSED for 54º (for coil charging) between each of 4 spark events, for a total of 216º.

The total "OPEN" degrees, 144º + the total "CLOSED" degrees, 216º, add up to a full revolution, 360º.

If you were to make it a 6-cylinder distributor cam with the same "OPEN" profile, you would have 6 X 36º "OPEN" events, for a total of 216º "OPEN" degrees, leaving 144º for the 6 "closed" events, or 24º per event, a "dwell" of 24º. This is just a hypothetical case, but not far off from the actual dwell of many 6-cylinder distributors.

<img src = "http://83.70.181.166/moodlecp/file.php/4/ign/cpdwellw.jpg">

 

[Brian Jasper co. Ia]

Bob, not all dwell meters are powered by the ignition system. I have one that belonged to Dad that uses an AA battery. While I can see Kent's comparison to an ohm meter, dwell is actually a measure of time. In the early 80's, a dwell meter was used to measure duty cycle of mixture control solenoids on GM carburetors.

 

[Bob]

"In the early 80's, a dwell meter was used to measure duty cycle of mixture control solenoids on GM carburetors."

Yep, you got that right. Been there, done that, with more Rochester feedback carburetors than I care to remember.

Going back to what I replied to, though, an Ohmmeter would make a POOR dwell meter!

 

[Anonymous-0]

In my opinion, it should be influenced by the speed of the engine and the voltage. Since you need some time to charge the condenser, you need to be closed for a longer distance on six volts or fast-running engines. For an old six volt, four cylinder tractor engine, a dwell of about 45-50 degrees would work. They usually specified a point gap. You can then measure the dwell after the fact.