Re: Dwell

Posted by Allan on November 26, 2003 at 18:09:17 from (148.63.132.164):

In Reply to: Dwell posted by Clete on November 26, 2003 at 13:25:22:

Clete,

I guess I’m the guilty one for bringing this whole silly topic up. Sometimes it is just better to keep quiet, I’m a thinkin’; but, I’d better see if I can explain myself a little better.

Here's another way of thinking of dwell.

"It, in reality, is the 'unused' portion of the distributor shaft's cam".

This unused portion (along with the ‘used’ portion) can be measured electronically with a meter by hooking between the distributor lead and ground.

Looking at the rotor end of the distributor shaft with the rotor removed, the dwell angle is created by drawing a line from the point at which the rubbing block of the point arm just comes away from the cam (during rotation) in to the center of that shaft.

Then, also from the center of that shaft, draw another line back out to the point at which the rubbing block again just touches down on the distributor shaft cam.

The angle of these two connecting lines translates into dwell angle. Or, one might say, "time", if you are thinking in terms of the rotation of that shaft.

This period of time that the cam does not actually work on that rubbing block is dwell.

So, let's say you are working on an eight-cylinder engine. The distributor shaft comprises a full 360 degrees, of course; therefore, each cylinder lobe ‘segment’ gets allotted 45 degrees of the distributor shaft's rotation.

If the points are set correctly, each lobe will be lifting the point arm for a period of 7 1/2 degrees of rotation, then lowering the arm for a period of 7 1/2 degrees of rotation (points open for a total of 15 degrees). The arm will then "rest" between the two lobes for 30 degrees of that shaft's rotation.

Meanwhile, at the same time all this has been going on, the crankshaft has actually turned 90 degrees per cylinder or a total of 720 degrees per 1 revolution of that distributor shaft, because the distributor turns at one-half crankshaft speed.

I know, clear as mud. Sorry, I'm not a good talker. :>(

And, as others have already said, for these old slow-spinnin' engines, it really isn't critical to narrow the point gap down to dwell angle.

Setting the points with a feeler gauge will get you there.

But anyway, that’s the way it was figured when I was a-bein’ learnt. :>)

Allan

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