Here are some notes about our oldest ignition system, the Dual Points distributor. I made the first of these models nearly 20 years ago. We went on to produce several hundred of them. In the last seven years there have been superseded by our E/P and E/P-X distributors, which themselves are evolutions on this basic design.
Dual Points distributors served the Corvair flight community very well. Many thousands of hours have been flown on these ignitions, and there has never been a single forced landing nor accident attributable to the design. (we have had people fail one side because of pinching a wire putting the cap on, but they flew on the other side) The design uses two sets of points from a Corvair mounted 180 degrees apart. either one can run the whole engine smoothly.
We mount the condensers remotely on the coils. When I introduced this, there was a giant debate on the internet claiming the condensers being on the end of 20 inches of wire would case some sort of ‘delay’ in the ignition, even though I pointed out that electrons travel down wires pretty much at the speed of light, and 20 inches vs 186,000 miles per second is a very short interval, the debate lasted years. Meanwhile, many happy people went out and flew countless enjoyable hours without noticing.
In the 1960s, companies like Mallory made dual point distributors for racing Corvairs, but these had three lobes not six, the goal being much shallower ramps on the point cam that would allow 7,500 rpm operation. such a distributor can not provide redundant ignition.
Today, we sell only a handful of D/P distributors a year. They are a special order item, but the remain popular with some very old school builders and some builders Down Under. They work well, but I highly encourage all builders to use E/P series distributors instead, they run smoother and have comparatively little maintenance. (D/P points need cleaning or replacing every now and then, but on the E/P the points are a back up and pass no current normally and may go 1,000 hrs. of operation without adjustment.) All the engines we build and sell are equipped with E/P series ignitions. Read all the articles and decide which system you like, they are all well flight proven.-ww.
In the above 2006 photo, a Dual Points distributor P/N 3301(D/P). The screwdriver points to one of the two 8/32″ screws that hold down the Points Plate. Two things I tell builders relentlessly, but are sometimes not heeded: 1) Never adjust the points to make the gap .019″, the gap on all distributors come from us pre set to a specific dwell, not a gap, and if you let anyone talk you into jamming an old feeler gage in the points, you will upset the pre adjusted timing. I have had 40 or 50 people do this and then rationalize it by saying “the gap looked small.” If people want to do this, I will fix it, but it does tell me who reads directions and who wants to argue rather than learn and understand. 2) Never take the two plate screws loose for a look inside, it will have the same effect as doing #1. Builders can replace points on these in the field, but it is done by matching the existing preset gap on the original points, not by using some book value.
The above photo shows a Dual Point Distributor in the machine. If you look closely at the 11:00 o’clock position you can see the illuminated arrow pointing at the degree wheel. the distributor machine was made in 1950. The items piggybacked on the top row allow the simultaneous operation of the electronic side of the Distributor while superimposing the EI picture on the scope. Every single distributor we ever send out the door is test run in this machine.
When running a Distributor on the machine, I can vary the rpm it’s turning and observe its advance directly. When your Corvair engine is idling, the advance weights in the Distributor are held shut by springs. The advance at this point is referred to as the static timing. I set the Distributors so they have little advance below 900 rpm. As the engine comes off idle, the mechanical advance inside the Distributor’s body makes the spark occur earlier. This is the mechanical advance at work. All the mechanical advance needs to be in by 2,400 rpm or so. This way, you can tie the tail of your airplane down, run it to full power and check what the total advance is at the propeller’s full static rpm. Total advance for engines running on 93 octane fuel should not exceed 30 degrees. For engines on 100 low lead, 32 is the limit. Beyond these numbers, the engine could be aggravated to detonate.
Each of our Distributors is marked on the underside with its mechanical advance and the beginning and ending rpm of its curve. Thus, if you have an engine you’re going to run on 93 octane fuel, and your Distributor says “18-1,000-2,400,” use a timing light to set the static timing to 14 degrees below 1,000 rpm. With the plane tied down, raise the rpm above 2,400 and verify that the total advance does not exceed 30 degrees. A dire warning: Never touch the ignition wires while the plane is running and turning a propeller. There is a remote possibility you’d get a high voltage shock and inadvertently flinch into the propeller. It’s a very remote possibility, but a builder in Australia did it and was lucky to keep his fingers.
For a better understanding of ignition timing please click on this link:
The above photo shows four point cams. Occasionally people ask if they can recurve distributors at home. It would be a very difficult process, and while you might achieve some results, a lot of the fine tuning we do to distributors is very difficult to see. Off the end of the screwdriver is the part of the point cam that the counterweight touches. Notice the four different profiles shown here. There are six different common Corvair profiles. The upper two are ground to match templates we’ve developed to produce ignition curves that suit aircraft use. The upper two point cams appear shinier because they’re nickel plated. We later switched to chromed ones we use now on all models.
Above is a wiring diagram that shows the basic layout of my ignition system. This page is taken from our 601 Installation Manual, so it includes some of the wiring associated with fuel pumps. The key elements of the design are redundancy, low power consumption and low voltage tolerance. It’s also immune to voltage spikes and high temperatures.
With our system, notice that you can fail one of the coils or one set of points and still have 100% power available through the backup system. Once every few months, a builder will propose a system that has three pickups and a wasted spark system using three coils and two plug leads from each coil. I know these systems well, and they’re not safe to use in Corvair engines because with three coils and three pickups, you’re statistically more likely to have a failure and when you do, you’ll be immediately down to 66% power. However, the dyno shows that dragging two dead cylinders means you’re really down to 50% power and some Corvair powered airplanes will not climb on half power.
The above 2001 photo shows the firewall mounted electrical box from our test mule at the time, The Skycoupe. We put this together so all the electrical components and flow cool air over them. The Wagabond has something similar. This function is done on aircraft like Zeniths by having the coils and the MSD 8210 behind the firewall. The only difference in today’s method is the use of Bosch Blue Coils, readily avaiable from Great Plains Aircraft. The Accel coils shown above must have external ballast resistors, which are internal on the Bosch Blue Coils. The top shelf houses the MSD 8210 coil switch. There’s some discussion about the use of the Mallory equivalent of this part. The system will work with either; it does not care. If you look at the wiring diagram above, you and identify most of the parts in the system.