The Stovebolt Site Forums The Shop Area Engine Shop ignition dwell setting ?

guys.
i hate to sound dumb, but it has been probably 25 years since i have worked on a points type distributor, so i need some help.

a friend has a 54 chevy truck with a 70's 350 engine in it, with a points and condenser distributor.
i am gonna give him a hand this week tuning it up, and need to knwo what the dwell setting should be for this engine ?
what would you set the points at, to get it started first ? .020" ?

and i cant rememebr what to hook the 2 wires from my dwell meter up to. both wires from meter to the coil ?

i never thought i would forget these things, but i have been messing with electronic distributors too long.

thanks guys.

leonard

dwell would be 30 and points .019

The red lead goes to the points side of the coil (negative terminal) and the black clamps onto any good ground point on the engine. Start the engine and measure the dwell angle.

I would think just adjusting the points thru the little window on the dist would do the trick..

thanks joker...

do i hook the 2 wires from the dwell meter to the coil, negative and positive ?

thanks

leonard

one to the neg of the coil and one to a good ground..

thanks much joker..

leonard

you are welcome..

Don't overlook calibrating your dwell meter, if it requires it. Most dwell meters, especially those that are battery-operated, require the meter circuit to be adjusted/calibrated to a "Set" line before using it. Some meter instructions specify connecting the leads together to calibrate the instrument. That won't hurt, even if yours doesn't require that step. If your meter has a black bar at the end of the scale, the pointer should rest in that bar when you turn on the ignition switch and close the points before starting the engine. The pointer resting in the black bar indicates a satisfactory level of distributor resistance, which if too high (pointer below the bar- in the numbers) will cause the dwell reading to be incorrect. And don't forget to select the correct dwell scale for the measurement- if your instrument has multiple scales.

Harvester

The actual dwell setting for this engine is 28-32 degrees. Because dwell angle increases as the points wear, it is recommended to adjust the dwell angle toward the lower end of the range(28). Also remember!! After you adjust the dwell you MUST re-time the engine......dwell angle directly affects engine timing and should ALWAYS be checked and adjusted BEFORE engine timing is dynamically set.

30 is on the money, and if in doubt
time it by ear..........you can't go wrong then

don't ever doubt a Navy guy

wow guys, thanks so much for the detailed info.

my friend thought he had a points style distributor, but actually has an electronic ignition. LOL.
so i didnt get to use my dwell meter yet, and who knows when i will get the chance to again, but when i do, i now have great info to go on.
the meter is probably 30 years old, and doesnt use batteries, and not sure of the quality of the meter, but next time i ever get to use it, i will check calibration as you specified.

also, what actually is "dwell" ? if the setting is 30, is that the degree or something, that the points are closed or open ?

also, probably a stupid question, but is there an adjustment setting for dwell on electronic distributors ? i never have know there to be, since the ignition module doesnt seem to have any type of adjustment.
just curious..

thanks again....
leonard

"Dwell" is the number of degrees of distribtor cam rotation between lobes during which the points are closed and coil primary current flows. The definition also applies to electronic ignition systems, but rarely is the dwell in those adjustable- although with some, the dwell increases and decreases as engine speed increases and decreases. The reality of dwell is that it is a convenient way to measure, and adjust, the 'On' time for coil current. Dwell angle is also referred to as 'dwell angle', 'cam angle', and 'cam dwell.'

The importance of proper dwell adjustment is that the coil requires a certain length of time for the primary winding to reach its full, or at least useable, magnetic field strength (magnetic saturation) to provide reliable spark. Since the dwell angle adjustment affects the dwell time period (coil current interval), it is a fairly important setting. The actual dwell angle setting is a compromise that gives adequate coil magnetic field strength at high engine speeds, a short enough period to provide acceptable point life at low engine speeds, and enough point gap to prevent flash-over as they open. The condensor supresses point arcing upon opening, but the gap must prevent flash-over when fully open.

Maybe this is more than you wanted know, but there is more that can be said about dwell, coil saturation, point gap, dual points, condensor function, the effect on ignition timing, etc.

Hope this helps.

Harvester

The earlier references

heck, I was wanting you to see how the meter worked?

no adjustments on electronic ignition, at least none that I know of..........no points..

Harvester,

Thanks for the explanation.

I'd like to hear more about dwell setting on a 235/261 dual-point distributor set-up.

Tim

wow harvester,
thanks for the very detailed info. you know your stuff.

and thanks to you joker also.. i learned something new.
yea, i was looking forward to using it again myself, after all these years. LOL.

leonard

Joker,

I am second-guessing, but, does time it by ear.......... mean time-it to the fastest idle or time-it by rotating the distributor in both directions (until you get roughness/hesitation) and then backing off a little from the maximum advance direction? What about timing to maximum vacuum (at idle)?

Tim

that would work... when it sounds right it usually is right..

I have never used a dual point deal but here is one thing I found about setting points.........interesting..

To set dual point ignitions you first block the "secondary" point set. You can block it by either not having it in the distributor, or using a thin piece of cardboard such as a matchbook cover between the contacts of the secondary point set. With the same (Safety Precautions) as above: ignition coil wire removed, the cap and rotor off the distributor, and a dwell meter properly connected to the coil, you can spin the engine. The best way to do this is with a remote starter button. The only other way to do this is with someone assisting you. Crank the engine and watch the reading on the dwell meter. You will be setting ONLY the primary point to the required setting at this time.

After you have set the primary point set to the required dwell, install or unblock the secondary point set. You will now spin the engine again and by adjusting ONLY the secondary point set, adjust the the secondary point so that the dwell reading matches the "combined" dwell specification. You do not have to touch the primary set again.

Example: You have a V8 engine application and your dwell settings are 26° (each point), and a combined setting of 33° ± 2°. You will set the primary point to 26° with the secondary set blocked, then by adjusting ONLY the secondary point set, adjust it so the dwell reads 33° (or between 31° and 35°).

OKAY......Dwell is the period of time, indicated in degrees of distributor rotation that the points remain closed (for each cylinder) 8 cylinders X 30 degrees dwell = 240 degrees points are closed for each rotation (360 degrees). This dwell period allows the ignition coil to magnetize the primary windings to full saturation. When the points open this magnetic field collapses across the secondary windings inducing high voltage available to fire the plugs. Too little dwell would result in low coil output at the high voltage tower (secondary).

Electronic ignition systems have a set dwell period which is determined by the electronic circuitry (transitors ie electronic switches which act as the points). Therefore, unless the electronic module is changed out the dwell cannot be adjusted.

Newer computer controlled systems can have replacement "CHIPS" to alter dwell, timing curves, fuel mixtures etc.

buy a pertronix conversion to eliminate the points. Dual points distributors were a PITA. Then again points were as well.

It is important to distinguish between "dwell angle" and "dwell time." Dwell angle remains fixed after adjustment, within the bounds of normal variation that some distributors have (non-electronic ones), but dwell time changes as engine speed changes. Dwell time is the time period that elapses while the distributor cam rotates through the dwell angle. Dwell time is actually the important factor in ignition system performance, but dwell angle is easier to measure, so that is the published figure. The connection between dwell time and the ignition system is the coil saturation time, which is the time period required for the ignition coil primary winding to develop a magnetic field that is suitably strong to fire the spark plugs. That time period is fixed by the coil primary winding (primary inductance), but varies with coil design. Racing coils have fewer primary windings (lower primary inductance) and therefore saturate faster than those with higher primary inductance (a "standard" coil). So a larger dwell angle actually provides a longer dwell time, which permits a stronger magnetic field to develop in the coil primary winding at high engine speeds, the result being satisfactory ignition system performance at high speed.

Enter the dual point idea: since point arcing/gap and coil saturation tend to be opposing factors (a large gap means short dwell angle/dwell time), a second set of points in parallel with the first accomplishes the increased dwell angle without sacrificing the point gap necesary to prevent flash-over with the primary magnetic field collapse. Dual points are usually offset from one another by some number of degrees, which results in the increased dwell angle (dwell time) necessary for high speed operation- that is to say above about 6000 RPM engine speed. A typical dual point, 8-cylinder distributor provides about 40° of useable dwell angle, up from a typical maximum of about 35°. That doesn't sound like much gain, but combine that with a low-inductance (low resistance) primary coil and the current-carrying capacity of parallel points, and you have a good high-speed system. I have always used a distributor machine when setting dual points, but the method described by Joker will work also.

On the topic of timing with a vacuum gauge, I have found it best to use a tachmeter with the vacuum gauge. The process I use is to advance the timing until one or the other reaches a peak, then retard the timing for a loss of 1"Hg vacuum. I have found that leaving the timing at the peak vacuum position leaves the timing substantially advanced. I have found the same result by using only RPM as the indicator. I have also found that RPM and vacuum rarely reach a peak at the same point of timing advance, so when one of them peaks, it doesn't matter which, I retard the timing for the 1" vacuum drop.

I guess this is all for academic interest, for whatever its worth.

Harvester

Harvester,

It might be of academic interest to some but it is of practical interest to others. Thanks for the information and for clearing answering questions.

So, if maximum rpm that I will push my 261 is less than 4,000 rpm, do you see any advantage using a dual point distributor?

Your last paragraph confirms what I have empirically confirmed to be a useful and easy technique for setting timing. I have found that the vacuum technique (with a little "back off" from the advance-side of maximum vacuum) is better than the maximum rpm method (a method commonly done "by ear").

Thanks,
Tim

Tim, with your engine, I don't see any advantage to dual points. They are nice to tinker with, but your engine won't go fast enough in stock form to gain any advantage in performance. Also, the original Delco ignition coil (or a proper OEM replacment) has a low enough primary inductance to keep spark plugs firing beyond about 5500 RPM with 6-cylinder dwell. Around that RPM, you will probably start getting misfires mostly because of point bounce, and maybe valve float. Also, I think the carburetor may limit the engine speed to something around 5000 RPM under load, but I'm guessing at that number.

Its a little off the subject of Stovebolt engines, but the MoPar/Chrysler 8-cylinder, dual-point distributor/ignition system was one of the best OEM systems around before before the Delco magnetic pulse electronic system that evolved into the present HEI system. But the Delco OEM dual-point system was no slouch, and would keep up with any of the after-market units, except maybe the Mallory system. And it was/is considerably better than a good many of them, and at a far lower cost.

Hope this helps.

Harvester