Dwell vs. Point GAp

[Bill Blue]

Busy adjusting points. I am finding a huge difference between the specified point gap and dwell reading. Book specifies .025" gap, 40 degree dwell. Currently, the settings are .022" and 32* (steady). I realize a fellow is probably not going to get them to agree 100%, but this is way off the charts. I have no way to check the dwell meter to be sure it is right; the dizzy is a rebuilt unit, so it may have been altered.

Any ideas, thought?

Bill

 

[RootesRacer]

Busy adjusting points. I am finding a huge difference between the specified point gap and dwell reading. Book specifies .025" gap, 40 degree dwell. Currently, the settings are .022" and 32* (steady). I realize a fellow is probably not going to get them to agree 100%, but this is way off the charts. I have no way to check the dwell meter to be sure it is right; the dizzy is a rebuilt unit, so it may have been altered.

Any ideas, thought?

Bill

Bill this is often the case.

The points gap method depends on the peak to valley dims of the lobes, and the actual lobe profile to be well controlled. Years of wear and possibly even sloppy machining will make the gap method not so accurate.

 

[norville]

Bill, I'm no expert. I would set the gap closer to the spec. It will wear and close the gap as you drive it. I usually let the dwell be as it is? I always did a round robin and make sure I set my time first..

May I suggest a points eliminator kit? I've had them for years in many vehicles. Most kits will allow you to convert back easily. In 20 years of use I've only had one kit (pertronics I) die on me. and it was a slow death, that I should have immediately picked up in diagnosing. The problem. they have always provided a smoother idle...

bob

 

[norville]

Bill this is often the case.

The points gap method depends on the peak to valley dims of the lobes, and the actual lobe profile to be well controlled. Years of wear and possibly even sloppy machining will make the gap method not so accurate.

another good point to convert to pointless...

bob

 

[Bill Blue]

Bob, it is fine if you mention pointless ignition, although at this time, it is somewhat pointless to change. As you know, I am planning on pulling the engine after this season, see no reason to go that route for probably 10,000 or fewer miles. I have already tried 3 different electronic ignitions of various descriptions. Its going to be points for the summer.

Bill

 

[norville]

I understand completely.

bob

 

[MikeH]

But if you do consider it, I don't think you will find anything easier to install than the Pertonix Ignitor. I've had one in a 65 Mustang for the past 14 years. When I did have a problem, it was because the coil went bad and nothing to do with the Ignitor.

 

[Rick Young]

The specs listed in Sunbeam Specialties catalog are .015 gap with a dwell angle of 60 degrees.
Rick

 

[Bill Blue]

Rick, the engine is a Pinto 2.0. I posted the question in the stock section as I figured the question had universal application and the stock guys would be more apt to have recent exposure to the problem.

But this poses a slightly different problem. How can two systems using essentially the same components, have such different specs?

Bill

 

[RootesRacer]

But this poses a slightly different problem. How can two systems using essentially the same components, have such different specs?
Bill

The peak to valley dim and shapes of the cam lobes could be different.

 

[Bill Blue]

The peak to valley dim and shapes of the cam lobes could be different.

Right, that's what I thought, but shouldn't the dwell be similar?

Bill

 

[RootesRacer]

Right, that's what I thought, but shouldn't the dwell be similar?

Bill

Dwell is just another word for duty cycle, points cams are just like valvetrain cams, the lift and duration have nothing to do with one another, but each will impact duty cycle.

More than anything it has to do with the shape of the cam ramps, and the points gap. So assuming the lobe shapes dont change after mfg, the mfgr can assure close to ideal dwell/duty cycle by specifying only the gap.

 

[norville]

So assuming the lobe shapes dont change after mfg, the mfgr can assure close to ideal dwell/duty cycle by specifying only the gap.

that's almost what I said.. Set the gap and go.


bob

 

[wipeout]

is there not a third aspect to this? Does not the centrifigal or vacuum advance play a part in charging the coil, and distributing the spark?

 

[RootesRacer]

is there not a third aspect to this? Does not the centrifigal or vacuum advance play a part in charging the coil, and distributing the spark?

This effects the timing, not the dwell.

Dwell is the controlling factor to the spark energy, it has nothi8ng to do with timing (though dwell shifts due to breaker/rubbing block wear, your timing will shift as well).

Bill and all, you really have to rid yourself of ignition points, even pertronix is better than points.

 

[Alpine Bob]

I'll have to go along with RootesRacer, with the Pertronix in two of our Alpines, that's 2 headaches I don't have. Knock on wood!QUOTE=RootesRacer;70929]This effects the timing, not the dwell.
Bill and all, you really have to rid yourself of ignition points, even pertronix is better than points.[/QUOTE]

 

[Bill Blue]

Yes, I understand the difference between duration and lift and how they are somewhat connected, but essentially independent parameters . The different gaps do not bother me. I understand how two gaps can produce the same dwell. Why the two systems have a different dwell is my problem.

Doesn't dwell affect spark quality? Okay, for a moment I will accept that dwell measures duty cycle. After all, it controls the amount of time the system is "on". Why is it important for two systems using the same components (electrically) to have different duty cycles?

This is not about why I should go to a pointless system or the amount my points should be gapped. Just trying to understand how the system operates.

Bill

 

[norville]

Bill, this is a good read:

http://autorepair.about.com/cs/generalinfo/a/aa052502a.htm


"The dwell angle is the number of degrees of rotation of the cam/distributor during which the points are closed. During each rotation of the cam/distributor, the points must open and close once for each cylinder. The points must stay closed long enough to allow the coil primary current to reach an acceptable value, and open long enough to discharge and produce a spark"


bob

 

[wipeout]

then those mathmatics correspond (damn my spelling stinks) with the degrees of Crank rotation while the piston is at top dead center ?

to think I ran two tune up shops when I was just eighteen...its obvious I retained little...

 

[RootesRacer]

then those mathmatics correspond (damn my spelling stinks) with the degrees of Crank rotation while the piston is at top dead center ?

to think I ran two tune up shops when I was just eighteen...its obvious I retained little...

No, its the crankshaft degrees that the points are closed.

The advance shifts the timing relative to TDC, but in actuality the dwell interval starts between 90 degrees and 60 degrees BTDC and ends between 30 and 0 degrees BTDC when the spark occurs.

Fixed dwell angle is the WORST way to drive an ignition coil, it results in wasted power at low RPM and often weak ignition at high RPM.
With the advent of computerized ignition (not solid state), ignition coils
were controlled with a constant dwell time (in millaseconds) instead of degrees. This results in constant energy per spark, and equal power/efficiency per spark at any RPM.
Unfortunately very few of the common aftermarket ignitions run constant dwell time (they run constant dwell angle with coil current limiting).