Dwell & Timing

[Michael A.]

I'm enjoying the Memorial Day weekend by doing some needed maintenance on the old 66. I've finished "tuning" it up and it's running well. As someone who doesn't have much experience tuning cars, each time I do it is a chance to learn some more. This year's tune up had me thinking about dwell and timing.

To start, here's the basics - My car has it's original 327/300hp. It's a K19 car but the AIR has been disabled - belt removed and AIR tubes removed with plugs put into the exhaust manifold. The engine has over 40,000 miles on a rebuild that the original owner had done. He told me that it had a "mild" cam in it - but it's still a hydraulic cam, just with a bit of a rumble to it.

I have a fairly good understanding of timing but have always just set the dwell to 30 and went on to setting the timing. This tuneup had me thinking about this a bit more. Just what exactly is "dwell". I'm not sure I understand exactly what I'm setting when I do this.

The reason I am pondering is that the points I bought were so out of whack the car would not start when they were first installed. I removed them and compared them to the old ones I took out and saw that the allen screw was noticably further in on the old ones than the new ones. I screwed the allen screw in further on the new points to get somewhat close to the old ones and then reinstalled them. The car fired right up and actually idled really well. I checked the dwell and it was around 20 vs the spec of 28 to 32. As I adjusted the allen screw to move to the spec, the engine lost rpm. I checked the timing and it was moving from 18 degrees advance to about 5 degrees advance at 30. I decided to set the dwell at 28 and then adjusted the timing to spec at 6 degrees (idling at about 500 rpm). I then set the idle back up to 700 to improve it's drivability - I'm guessing that's because of the cam.

With all of that as background it made me think about dwell. What exactly is it? Is dwell the length of time that spark is transmitted? Why as the dwell is increased does the timing retard? And finally, at the end of the day, is dwell something to just set and forget?

Thanks in advance for helping advance (pun intended) my understanding.

Cheers
Mike

[Mike R.]

Re: Dwell & Timing

Dwell is the time (in degrees) that the coil being energized. At the end of the dwell period, the points open causing the magnetic field in the coil primary windings to collapse which induces a high voltage current in the secondary windings.

What you observed is that as you increase dwell by adjusting the points, the points open later which retards the timing. BTW dual point distributers use the dual points to further increase dwell.

[Ray G.]

Re: Dwell & Timing

" And finally, at the end of the day, is dwell something to just set and forget?"

Cheers
Mike[/QUOTE]

Hello Mike;
Try connecting your dwell meter and watch for dwell changes as RPM increases and the vacuum advance works.
We have run the dwell meter inside the car and observe while driving.
Large changes could mean worn breaker plate pivot bearing and/or distributor shaft and or bushing wear.

Hope these suggestions help.
Ray

[Michael A.]

Re: Dwell & Timing

Ray and Mike - Thanks for the replies. This helps. I appreciate understanding a bit better what is going on in those devices. In particular, Ray, your comments on the variations in dwell are interesting. I did watch the dwell meter as I took the rpm up to about 2000 rpm and it stayed rock steady at 28 so I take that as a good sign.

One other question - what kind of grease should I be putting on the wick in the distributor? Is a dielectric grease the right thing?

Thanks
Mike

[Ray G.]

Re: Dwell & Timing

Ray and Mike - Thanks for the replies. This helps. I appreciate understanding a bit better what is going on in those devices. In particular, Ray, your comments on the variations in dwell are interesting. I did watch the dwell meter as I took the rpm up to about 2000 rpm and it stayed rock steady at 28 so I take that as a good sign.

One other question - what kind of grease should I be putting on the wick in the distributor? Is a dielectric grease the right thing?

Thanks
Mike

Hello Mike;
Steady dwell through the RPM range is a very good thing.

Dielectric grease does not inhibit points contact, promotes conduction, helps to prevent corrosion and prevents short circuits due to water. And inhibits wear on the points rubbing block.

We always use as little as possible. Just enough on your finger tip to make the cam slippery. Have the same tube for many many years.

One more thing. If there is no metal transfer from the stationary point to the moving point, or vice versa, don't replace the condenser was always the rule heard when training eons ago.
Don't have the procedure handy on how to match condenser to a system. Will have to look in the Chevrolet Service manual.

Ray

[Ronald L.]

Re: Dwell & Timing

On this same subject with a 427/390 of the same year, when I checked dwell it was at 20 too. Ran well. But with a spec of 28-32 I ran it up to 28 and I noticed it doesn't seem to be a strong now.

One concern was the old Sears multi meter but a couple of times at this showed the same result. In fact the old way was to run it up until it stumbles and then back of 1/2 turn - that but the dewll at exactly 28.

I did run the RPM up to 1200 or 1500 and dwell stayed stable.

What was 20 deg of dwell doing that 28-32 isn't?

[Duke W.]

Re: Dwell & Timing

Increasing dwell retards the timing. Dwell and timing should always be checked together. Check and set dwell first, then timing.

To check dwell variation free rev hydraulic lifter engines to 5000, 6000 for mechanical lifters. More than a couple degrees variation indicates a sloppy distributor that needs help. Engines with relines above 5500 should use the 28-32 oz. break arm tension points, like NAPA/Echlin CS89 or BWD A112P - same part made by Standard Motor Products - just a different box.

Duke

[Ronald L.]

Re: Dwell & Timing

Duke, I am surprised at the dwell being at 20.

When you say set it, you mean to the spec of 28 and then check the timing - which I will go back & do...

[Duke W.]

Re: Dwell & Timing

The "spec" is 28-32. The reset value is 30.

Set the dwell to 30, then check and adjust the initial timing as required.

Duke

[Mike Z.]

Re: Dwell & Timing

I support Duke's comments 100%-dwell and timing work hand in hand, you can not change the dwell without affecting the timing. The arc rise time and arc duration (dwell time and duration/position) give the coil it's message as to when and how long to work-rise & collapse.
As contact points wear-the contacts continually try to weld themselves together and material is eaten away; rubbing against the cam lobe wears the rub block and cam, and spring tension decay, all contribute to the wear, which in turn reduces the dwell. These and other factors are the reasons for transistorized ignitions-no points to wear, means a rock solid "trigger" (at much higher RPM) for maximum performance of the coil and rest of the system, without the concern for change/wear.
A well performing distributor on my Sun Tester, will not vary dwell (contact points) less than one degree from zero to 6000RPM. But, a worn unit will vary widely and should be rebuilt. You do not need a load on the motor to do this test-free load RPM will give the readings you want. When I used to ran points; at initial stabbing/installing of the distributor, would set the dwell at starter cranking speed without firing the motor, just to make sure the dwell was close enough to properly fire the motor. Guess what-at any higher RPM, once the engine was running-the dwell remained the same.
So, the reason your car ran happy @ 20 degrees dwell; was more timing advance-not the lessor dwell, then when you increased the dwell (from 20 to 30 degrees), which reduced the advance-it felt lazy, because the timing retarded. So, get that dwell back up around 30 degrees and re-set the timing. This would also be a good time to get out the vacuum gauge and re-set the idle jets and idle speed-they also will be affected by the timing changes. Happy tuning!

Mike Zamora
#12455