by Richard Atwell
You've probably heard that you can adjust the dwell on your Pertronix Ignitor I (simply known as the Pertronix) by varying the gap between the pickup module and the rotating magnetic sleeve. You've also probably heard from equally as many people that the dwell cannot be adjusted. True or false? Read on...
The Pertronix is an electronic replacement for the points in your distributor. I've installed several in different vehicles over the years and I feel the pros outweigh the cons considering the price and the fact that I've never had a failure:
There are fancy CDI systems out there that cost a lot more and there are copies of the Pertronix like the HotSpark and Compufire/Accufire that cost less but as they say, YMMV.
I've not been able to replicate comments that leaving the ignition on with the engine off burns out the unit. If this was true at one time, it was the fault of an older design or perhaps an electronic unit mistaken for a Pertronix branded unit. As always, disconnect the battery before doing any work on the ignition system.
Although you can vary the gap, it has limited range and effect. However adjustment is far less picky than points so it's worthwhile to try for yourself.
The gap set using the 0.85mm plastic feeler gauge provided produces a dwell of about 54 degrees. When I reduce the gap to 0.40mm I can lower the dwell by 1 degree to 53 degrees. Because of the radial play in the shaft of a worn distributor reducing the gap any further runs the risk of the pickup module contacting the magnetic sleeve that is rotating with the shaft especially if you haven't rotated the shaft to check that the clearance is even (the sleeve may be out of round). The easy way to check this is to reposition the sleeve four times rather than bump the engine.
When I increased the gap to 2mm I can increase the dwell to about 58 degrees. Enlarging the gap further runs the risk that the mounting plate will contact the distributor body which will limit the movement of the advance plate or produce a scratching noise when you rotate the distributor.
These adjustments explain the variations in dwell reported by VW owners (dwell meter accuracy not withstanding). Naturally, most people are following the installation instructions, using the feeler gauge provided and ending up with a similar dwell value. How does this explain dwell readings as low as 45 degrees? Given my own experience I can only attribute it to a manufacturing fault because I could not lower my dwell below 53 degrees safely.
NOTE: Remember that adjustments in dwell produce adjustments in timing so you must retime the engine after your final adjustment. This is a normal procedure after the initial installation because the dwell/timing will vary from the value your points were set to as the Pertronix trigger location has been moved to a position different from where the points opened.
If you look on the inside of the sleeve you will be able to see part of each cobalt magnet that is embedded in the plastic. I've wondered if it's possible to shave the sleeve to generate a smaller dwell without lowering the structural integrity of the magnet but I'm loath to try it without a spare sleeve on hand. Pertronix sells spares...
I recently came across an article suggesting that you can vary the spark duration by changing the gap.
You can read it yourself at Ken's Buggy Barn.
Wanting to see this for myself, I put the secondary ignition under test measurement equipment to see how varying gaps would perform.
The results? Varying the gap had no effect on the spark waveform at idle. Because the dwell changed (smaller) the timing changed (retarded) and vice versa but that was all.
What Joe witnessed on the test bench at his company may have been true at one time but with my 2002 made unit it at least no longer applies. It's not clear from the article if he simply tested the unit on the bench or if he tested these changes with a working ignition system. He did increase the dwell with his adjustment and that obviously cured his problem.
Given the nature of the coil, the suggestion to increase the Pertronix gap makes sense but not for the reasons he is quoted as saying. The increase in dwell produced the improved results and I would imagine the increase would be a benefit for any rebuilt engine designed with a higher redline. Of course, if those engines are running CDI, then the dwell period is pointless because the coil is no longer used as an energy store. It is simply used as a step-up transformer powered by the large capacitor inside the CDI box.
What is dwell? This is the quantity that is adjusted when you gap your points. Although you are using a feeler gauge to set the maximum size of the gap when the points are open, it is the time that the points are closed that is really being adjusted.
BTW, I've never met anyone that was able to set the dwell perfectly the first time using a feeler gauge. As soon as you check it with a dwell meter, you'll find that you need to develop a system of alternate measurements with your feeler gauges to get it close to the ideal spec on the first few tries.
When the points are closed this is called the dwell period because it's the length of time that the points "dwell" in the closed position. In German, it's called "schliess winkel" which literally means closing angle. During this time, the primary side of the ignition coil is grounded and because the coil is an inductor, current flows increases through it until a maximum current is reached.
When the circuit of the primary side of the coil is disconnected its magnetic field collapses which induces a magnetic field in the secondary circuit. When the field grows big enough to jump the spark plug gap (voltage) a spark is induced at the plug and current flows from the secondary side through the plug to ground (the engine case). The condenser in a points-based system exists to prevent the secondary spark from jumping the points to ground as there is some mutual inductance involved and the points provide an easier path to ground than the spark plug.
Like any circuit, the voltage, current and resistance present are all related and have an impact on spark produced. The resistance of the system exists to control the voltage/current in the system and shape the spark waveform. For the purpose of this article, these quantities are fixed in a stock system. So what are we trying to maximize?
The voltage coming from the coil, has to be large enough to ionize the mixture between the gap. Set a larger gap and it ignite more mixture but it will require a larger voltage. However, the size of the gap is a trade-off: the gap should be as large as possible to ignite the slow burning mixture (smooth idle) but not so large that the turbulence of the mixture will blow it out at higher rpms (misfire). The dialectic constant of the gap and therefore the voltage required to jump it, varies with temperature, pressure and the air fuel ratio of the mixture. The engine designers select a spark plug gap that is tailored to the design of the combustion chamber and the engine's compression ratio (enough for naturally aspirated engines).
The higher the current level in the coil, the longer the duration of the spark that can be maintained in the secondary. Since coil energy increases as the square of the current flowing through it, every milliamp (mA) matters especially at higher rpms. Since the current level is determined by the dwell angle, we need to set the dwell correctly.
Let's assume the points are closed exactly half the time. At idle speed, say 900 rpm, a 4-cyl's dwell period is about half of 90 degrees (360/4). 45 degrees is swept by the shaft in the distributor within 16ms:
At 4500 rpm it only takes 3.3ms and 5400 rpm it takes 2.7ms to rotate through 45 degrees. Electricity moves nearly instantaneously right so why does this matter? The key is that the coil (inductor) takes time to charge and the dwell determines the delay before charging begins as well as the charging duration. At the speeds the engine can spin, this becomes significant.
Look at this graph to help understand why:
The blue line represents the secondary ignition voltage waveform. You can see the spark fire at 0ms and it burns for about 1ms from left to right. The red line depicts the coil primary current waveform that begins to build its charge starting at x (-10ms). See how it takes about 4ms for the coil to saturate (also notice how the secondary voltage drops below zero during the the time is coil is recharging but that's another article in itself).
At 900rpm, there is plenty of time (16ms) to energize the coil for a full spark but at 4500rpm there isn't enough time (3ms vs. 4ms) to build 100% coil energy. Because the coil current is about 5A instead of 7.5A a weaker spark is produced. Since the spark energy is the square of the coil current we've lost about 56% of our spark energy. Luckily as the rpms increase, so does the turbulence in the combustion chamber which helps to propagate the flame and burn the mixture more easily but at some point our weak spark factors in and the result is a big drop in power.
If you build a bigger motor especially one with a higher rev limit, you can see that a better ignition is a must. Why is the classic ignition setup used? It is cost effective, compact and simple. The points/condenser are the only part of the system that have gone the way of the dodo but only because of government mandated emissions regulations that require ignition systems to be maintenance free for 50k miles. If not for that, I'm sure new cars would still have them factory installed because of their low manufacturing cost.
The engineers specify a dwell period slightly longer than half the mathematical rotation to satisfy all engine operating conditions. Like many systems, the design is a trade-off. Let's calculate the time it takes to charge for various dwell periods as specified by VW:
The difference may not seem much but as the graph in the earlier section shows you can see how the spark energy will decrease as the coil gets further from its saturation level because of timing.
My theory why the dwell spec varies as the points wear (used vs. new) comes from the construction: points have a radiused tungsten surface. As they wear they pit on one side and a tit forms on the other. As this happens the preferred position from which the spark will jump is no longer the center of each contact. Because of this it becomes harder for the spark to bridge the gap and so a larger dwell period can be tolerated which is preferred in order to better saturate the coil at higher rpms.
The distributor design is about 100 years old along with the rest of the ignition system that Charles Kettering invented. The cam on the shaft of the distributor has an even rise and fall from the heel of the cam to the toe (raised edge). To increase the dwell further simply make the gap smaller but why is there is specified limit to the minimum gap and maximum dwell?
The engine and ignition engineers have to select a dwell period that produces a suitable spark at both idle and redline speeds. The limitation exists because the points (the switch) are mechanical in nature. We've seen that if the gap is too large (small dwell) then the coil won't have enough time to charge but if the gap is too small (large dwell) then the spark may not occur (misfire) because current flowing through the primary side will jump the points for too long and fail to break the circuit as they are supposed to do. This is why a maximum dwell is specified by the manufacturer.
The strength of the spring also determines the maximum rpm that the points will stay closed (as they bounce the dwell then becomes erratic). A common trick for higher revving engines is to replace the VW points with pair from the early Porsches that have a stronger spring (both parts are Bosch). You can identify them by the striped wiring and 01 030 part number:
Be sure to keep your cam well lubed with this set of points. They are also a little harder on the shaft bushings.
According to VW/Porsche, dwell of 49 degrees is perfect for new points (generates the most power) and 47 degrees for used points.
You'll notice that the number never exactly equals 1/2 of 360 degrees divided by the number of cylinders. For example, in a 4-cyl the distributor rotates through 90 degrees for each spark plug but the ideal dwell is never exactly half: 50 degrees vs. 45 degrees. The same goes for 6-cylinders: 36 degrees instead of 30 degrees. The goal is to get the most out of the coil (close the points for as long as possible).
8-cylinder engine have 8 lobes on their distributor which mean the period the points open and close lasts only 22.5 degrees which isn't enough time to build up the necessary spark energy. The ingenious workaround for this problem was to use a second set of points wired to the first and place them offset from another cam lobe. As the first set of points open, the second set remain closed to continue the current build-up in the coil. When the second set opens, the circuit is finally broken.
Moving back to the topic of 4-cyl VW engines, why do some people insist that the dwell be set to 45 degrees instead of 50? Their reasoning is that as the rider on the points wears on the cam, the gap becomes smaller and this increases the dwell. The logic is to start with a less than ideal dwell and gradually reach the ideal value rather than start out at ideal and drift from there.
In some countries VW included the dwell on the engine lid sticker and said to set it to 47 +/- 3 degrees. Who should you believe? Set it yourself and find out.
The basic problem with this logic is that the timing drifts as the dwell changes so the effort to limit the number of times you adjust the dwell is now taken up by time spent readjusting the timing. For every 0.10mm change in the gap, the timing drafts 3 degrees. I'm sure there are people out there who will argue which is better but my suggestion is simpler: install a Pertronix and never worry about drifting dwell or timing again.
Since the Pertronix can switch must faster than the points due to the rising and closing action of the cam, Pertronix Inc has decided to set the dwell a little higher to allow the coil to saturate better at higher rpms. Since no spark jumps, the system is better in control of the side-effects. You can feel the improvement right after installation during your first drive. The dwell for Pertronix usually reads 53-54 degrees.
So I've explained how the coil charges, how the dwell period for points is specified and how to adjust the dwell slightly with your Pertronix. So why not run as much dwell as possible with Pertronix instead of abiding by the plastic feeler gauge?
Do it! First realize that the gap works the opposite with the Pertronix: larger gaps increase the dwell because the design of the electronic switching mechanism. Second, we can get away with this because the Pertronix is an electronic switch and too large a dwell won't cause it to misfire like points would. That's 3 advantages: fixed dwell and timing and a better than spec dwell.
Increase the gap as much as possible without contacting the distributor body. Without knowing the how the electronics inside the module work exactly I can't say what limit can be reached but this seems to be moot anyway because the module will hit the distributor body first.
You may find the module support plate hits some part other than the body during your adjustment. Since it's only aluminum, file it if you find yourself wanting to experiment with larger values.
04/10/05 - Created
06/03/09 - Added introduction
09/07/11 - Fixed broken photos, added translate button, updated footer