I'm simply trying to sort out the difference between point gap and dwell. I understand that "dwell" is the cam angle range...I'm assuming that means the advancing range it has, but I'm not sure about all of it. I this a set right or wrong thing? Or can this be adjusted? I know this is dizzy 101 but its somthing thats never been sorted out for me. Thanks!
Stupid Question
- Thread starter maunster
- Start date
Scoutboy74
Moderator
Point gap is a physical measurement set with a feeler gauge of a specific thickness and is not as precise as a dwell reading for engine tuning purposes. The wider the point gap is, the smaller the dwell angle will be as a result. A hand-held meter or engine analyzer of some sort is required to measure dwell. The more cylinders and distributor cam lobes an engine has, the lower the resulting dwell angle will be because the points are being opened with greater frequency.
Scoutboy74
Moderator
For added clarity...using a v8 engine as an example, because that's what I know best and they seem to be what most folks are tinkering with...the acceptable dwell angle range for an optimum state of tune is between 28 and 32 degrees. Service manuals specify a narrower point gap for new points than for used, as in .016 new and .019 used for example. This is because the rubbing block gradually wears down over time as the engine logs hours. This allows the point gap to narrow which causes the dwell angle to increase. Once the angle has gotten higher than 32 degrees, the ignition system is no longer in an optimum state of tune. This will contribute to harder start ups and reduced performance/economy. So if you have a dwell meter or scope and want to measure your angle after setting your point gap, no matter if they are new or used, you should try to fine tune your point gap so that your dwell reading is towards the lower end of the range. That way, as the gap slowly narrows, you've maximized the available run time before needing to reset the gap.
In the case of breakerless or electronic ignition, there is an air gap between the pickup module and the reluctor, which May or May not be adjustable, depending upon the unit in question. This is generally a set it an forget it deal. Once that air gap is locked down, the dwell will remain consistent, provided the distributor bushings are not overly worn and sloppy. So in this instance, even if your dwell reading is right at 32 degrees, its fine. It will never go higher than that figure because there are no contacts rubbing and wearing down. Not a stupid question at all by the way.
In the case of breakerless or electronic ignition, there is an air gap between the pickup module and the reluctor, which May or May not be adjustable, depending upon the unit in question. This is generally a set it an forget it deal. Once that air gap is locked down, the dwell will remain consistent, provided the distributor bushings are not overly worn and sloppy. So in this instance, even if your dwell reading is right at 32 degrees, its fine. It will never go higher than that figure because there are no contacts rubbing and wearing down. Not a stupid question at all by the way.
Scoutboy74
Moderator
Think of the dwell as measuring the amount of time the points are closed. With eight lobes close together on a distributor cam, the points are not able to stay closed for long before the next lobe opens them up again. With a four cylinder engine, there are half as many lobes, so the points remain closed for twice as long in between each lobe, therefore the dwell angle is higher. I really can't explain it any more clearly than that.