Points or dwell setting for six cylinder

[billorn]

Im working with a friend on his 1930's car with a flathead 6 cylinder and the manual says .020 inch points gap. We set it there but I still dont think its quite rite because the exhaust isnt smooth. I always use a dwell meter but the manual for this car doesnt show a dwell setting. Its probly too old and was made before dwell meters. Anyway I see that other 6 cylinder cars like a Chevy from the 50's have a dwell of 31-34 degrees. Would it be a good idea to use that as a best guess or is there some downside to setting dwell without knowing the exact number? Is there a conversion for .020 gap to dwell degrees?

 

Thanks for any help......

[Bloo]

Dwell is the number of degrees the points stay closed and the coil is powered on and building a charge. It is the same thing as duty cycle, just expressed a different way.

 

For instance, a 6 cylinder engine fires every 60 distributor degrees, so a 6 cylinder dwell scale would max out at 60 degrees dwell (100% duty cycle), and 30 degrees dwell would mean that the points are closed half the time (50% duty cycle).

 

31-34 degrees dwell means that the points are closed a little more than half the time, and yes I believe that is typical. It is probably a real good guess anyhow. You could look at some other 6 cylinders to get an idea, but I think they will all be pretty close to your Chevrolet example.

 

There is no conversion from thousandths because a steeper cam lobe would cause a wider point gap at the same dwell setting. For that reason thousandths don't really mean anything when comparing one make and model to another. Dwell on the other hand is likely to be fairly close among different makes and models as long as the number of cylinders is the same.

 

P.S. Make sure your meter reads about 60 with the points closed and about 0 with the points open. Some of them don't do so well on 6 volts.

 

Edited May 28, 2023 by Bloo (see edit history)

[Rusty_OToole]

Manuals of the fifties and sixties gave both dwell and points gap settings. I have set points for years using a feeler gauge then double checked with a dwell meter and found no difference. When setting new points from scratch you need to use a feeler gauge, to recheck you use a dwell meter.

 

So, your problem is not in the points gap or dwell setting. It is something else. If the dwell reading jumps around you could have a worn distributor bushing or faulty advance. There could be a lot of things affecting spark besides dwell. We don't even know if the problem is ignition related, it could be carburetor, vacuum leak, sticky valves, lots of things.

[Rusty_OToole]

If you are curious set the points to .020 then check the dwell and use that number in future.

 

When you adjusted the points did you check the timing? It will change if you replace, file or adjust the points. You could check the advance mechanism while you are at it.

[billorn]

Thank you Bloo and Rusty. 

 

I found one of those tune up sheets on ebay and it says "cam angle 36 degrees". Is that the same as dwell?

[Grimy]

2 hours ago, Bloo said:

Dwell is the number of degrees the points stay closed and the coil is powered on and building a charge. It is the same thing as duty cycle, just expressed a different way.

 

For instance, a 6 cylinder engine fires every 60 distributor degrees, so a 6 cylinder dwell scale would max out at 60 degrees dwell (100% duty cycle), and 30 degrees dwell would mean that the points are closed half the time (50% duty cycle).

 

31-34 degrees dwell means that the points are closed a little more than half the time, and yes I believe that is typical. It is probably a real good guess anyhow. You could look at some other 6 cylinders to get an idea, but I think they will all be pretty close to your Chevrolet example.

 

There is no conversion from thousandths because a steeper cam lobe would cause a wider point gap at the same dwell setting. For that reason thousandths don't really mean anything when comparing one make and model to another. Dwell on the other hand is likely to be fairly close among different makes and models as long as the number of cylinders is the same.

 

P.S. Make sure your meter reads about 60 with the points closed and about 0 with the points open. Some of them don't do so well on 6 volts.

 

One thing to consider is that the point gap (e.g. 0.020) is just the mechanical expression of the gap needed to achieve specified DWELL **when the cam is new**.  As the lobes wear, the point gap specified no longer produces the specified DWELL/CAM ANGLE and the latter is what makes the car run well.

 

For example, about 60 years ago as a young man, I acquired a 1950 Pontiac 6 with >90,000 miles.  When I replaced the points as part of the first tuneup under my ownership I set the point gap at 0.020 (as I recall) and the engine ran like hell.  I borrowed a dwell meter (I said I was young) and the dwell was 'way off.  By trial and error, I had to reduce the point gap to about 0.014 to achieve the specified dwell, and the car ran well.  (A few years later I would have replaced the distributor but I was an impoverished student at the time.)

 

Assuming that you will not be replacing the distributor or its cam, consider adjusting the point gap narrower in 0.002 increments and check the dwell at each of those settings.  When you've found the sweet spot, make a note of it for future reference.

[Bloo]

45 minutes ago, billorn said:

Thank you Bloo and Rusty. 

 

I found one of those tune up sheets on ebay and it says "cam angle 36 degrees". Is that the same as dwell?

If that was in the tune up specs, yes.

 

 

Edited May 29, 2023 by Bloo (see edit history)

[roysboystoys]

As a side note, a match book cover is about the right thickness to use as a point gap gauge.

I used it on my 53 Packard one morning on the way to work, on the side of the road.

Useless info now , who makes match books any more.

[Marty Roth]

Matchbook thickness works well:

 

 

[Rusty_OToole]

Have heard of using a dime to set points gap. If you don't have a dime try two nickels (joke)

[maok]

I always thought about 38degrees was correct for mopar 6 cylinders engines.

[DFeeney]

Thank you everyone in this conversation.  I always wanted to know more about dwell.

[dodge28]

 At about 500 R P M use a better quality vacuum gauge at manifold port and look for flutter of the needle. It will tell you almost everything wrong with the engine. That is if ever something is wrong. If engine is OK, loosen distributor slightly  and turn right or left slowly until the highest vacuum reading . Lock the distributor at that point. You are good to go. By the way the contacts gap and the sparkplug gap are pretty standard. Overhead valve engines gaps are different. Technical issues I am not going to get into.