Dual plug heads.
- Thread starter Tod
- Start date
Chuck Sea/Tac
Well-known member
What's the purpose. On airplanes , I assume reliability.
Brad in Germany
Well-known member
hardtimes
Well-known member
Dual plug heads can be and are used in antique aircraft. They are also used in A/B engines where a guy wants something different. If they are used in an auto application, special ignition set-ups are required for proper spark delivery, i.e.- special twin spark plug distributors or custom hardware to house two four cyl distributors, more than one coil, etc.... If used in aircraft engine application, they use magnetos (dual) to fire them off.
Redundancy will provide more chance of reliability, I guess. I've been told that even tho the two plugs exist at different areas in combustion chamber and fire almost (or) simultaneously ...that there is not much, if anything, performance wise, to be gained over single plug.
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Bruce of MN
Well-known member
If you used the stock dizzy with Brad's head and used custom bronze strips, would the doubled (?) current through the points burn them up?
That's why I am doing this for Snyder's. He has had a call for them for just that reason.
Tod
No. It would be the same amount of current, which is determined by the primary windings in the coil. If you just ran two strips from the stock distributor terminals to each plug, only one of the plugs would fire (electricity always seeks the path of least resistance, so the plug that would fire with the lest voltage would get all the spark). To fire both plugs at the same time, a second distributor would be needed, or modern coils that fire from both ends, with one coil per cylinder.
Bob Bidonde
Well-known member
I would run an easy test to see what happens with dual plugs. Remove the plug from Cylinder #1, wire a 2nd plug in series with the existing plug. Reduce the plug gaps to 0.02. Make sure both plugs are grounded and run the engine.
I suspect that there will be a spark at both plugs, but the intensity of the sparks may be different.
I suspect that there will be a spark at both plugs, but the intensity of the sparks may be different.
After 40 years in the electrical industry I would pretty much agree. Electricity will always take they path of least resistance. There is not a known exception, that I am aware of, to this rule.
If you have enough current, you could fire both plugs. One could get a better spark than the other.
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huddy
Senior Member
Aircraft magnetos are usually timed a couple degrees apart. Part of the pretakeoff checks is to disable each mag individually and watch the tach. With a small displacement, carbureted Continental 4 banger at 1800 RPM you will typically see 150 RPM drop when running only on one magneto, and usually 50 RPM from one to the other. This tells me that two plugs firing with timing a bit apart gives you a more efficient fuel burn. I don't have experience with a Model A engine in an airplane but I would think it would work the same way, except, of course, the runup RPM would be somewhat lower. (One purpose of the runup test is to ensure that both mags are firing, and also to ensure that the "kill" switch is actually killing the spark on each mag, both of which are important to know).
Y-Blockhead
Well-known member
If you look at the ignition system on 4 cylinder motorcycles with points ignition they have two coils, each with two secondary leads to two spark plugs, firing #1 and #4 cylinders together, and #2 and #3 cylinders together. They fire on every stroke, meaning they fire at ~TDC on the compression stroke and the exhaust stroke.
Both plugs seem to get equal spark every stroke, altho the cylinder with the exhaust valve open contributes no power.
I also had a 750cc Yamaha Twin Flat Tracker that had a Mag that fired both plugs at the same time.
I guess my point is that you would THINK the plug that saw no compression would fire easier that the one seeing high compression (in the case of the Yamaha, 13:1 compression ratio).
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Late model cars often use the same 'one coil per two cylinders' setup. They work differently than a typical Model A, etc., coil. With the coil type used on a Model A, the secondary windings are grounded to the coil housing or case. So when the plug fires, the current goes out the center tower of the coil through the distributor to the plug, jumps the gap to ground and returns to the coil through the engine and body to complete the circuit.
With the coil that fires two plugs at once (power stroke of one cylinder and exhaust stroke of the companion cylinder), the coil secondary windings are not grounded to the coil case nor to the engine. Rather, what would be the grounded end of the windings in a normal coil are instead connected to a second coil lead. So, to complete the circuit, high voltage current flows out one lead to a spark plug, jumps across the gap to ground, then through the engine's head to the companion plug and jumps from the ground electrode to the center electrode and back to the coil, thus completing the circuit. In this system, if the spark can't fire one plug the other one goes dead too. Thus, if one plug wire, for example, goes bad, two cylinders will not fire.
Y-Blockhead
Well-known member
Yea, that makes sense. 
BILL WILLIAMSON
Well-known member
At our CCRG meet, a Guy broke his Mallory dist cap, down to NAPA & found an 8 cyl one that fit! Used every other hole & drove it home!!!
Bill Seeneverything
Bill Seeneverything