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Antique Gas Engine Discussion

Can two spark plugs run from one wire


This is a little different take on the sparking question--I have two spark plug holes--they are...

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  #1  
Old 08-23-2005, 10:33 PM
Ihorse Ihorse is offline
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Default Can two spark plugs run from one wire

This is a little different take on the sparking question--I have two spark plug holes--they are just opposite from ea other--can I splice a lead, and run two sparkplugs from it--or will damage to the mag happen?? will the spark be half as much--or twice as much???
thanks d,j,
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Old 08-23-2005, 10:57 PM
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Default Re: Can two spark plugs run from one wire

you can do that, splice a wire to hook up 2 spark plugs, if you wanted to, but my question is, is that, if this happens to be a single cylender engine, Why 2 spark plugs? you only need one to fire the engine.
hooking up 2 spark plugs will not hurt a mag but it might make it work just a tad bit more, thats all.
on the other hand, if this is a 2 cylender engine, then you cannot run the spliced wire to both the plugs, due to on 2 cylenders, the fireing order is offset. one cylender fires before the other. if you had the 2 cylenders fireing at the same time, one piston on the power stroke and one on the intake, the one on the intake will fire while at the same time the other piston is on the power stroke, knocking the engine out of time and possibly causing serious damage to the engine, for instance, busting a hole in the top of the piston, that is the reason 2 cylender engines have a what i call dual wires on the mag or coil OR the 2 cylender engine has a coil on each cylender side, that is to keep the engine in time and to correctly fire the engine.
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Old 08-23-2005, 11:04 PM
John Davidson John Davidson is offline
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Default Re: Can two spark plugs run from one wire

Years ago this was done using a spark plug with 2 binding post. It did not go to ground. So the hot wire went to one post on that plug. A wire from the other post went to a normal spark plug that has a ground. So the circuit went from through the non grounded plug to the grounded plug and the both fired at the same time. Now to find a plug like that is the hard part. They are collector plugs today and are expensive. We call them "series' plugs. A hot mag should jump both gaps.
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Old 08-23-2005, 11:13 PM
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Fred_M Fred_M is offline
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Default Re: Can two spark plugs run from one wire

If it is two plugs in one cylinder I have to go back to my basic theroy on fuel burning. We like to say the fuel explodes in the cylinder, but it don't in a properly running engine spark ignited engine, it burns. In fact when it does explode its called pinging or detonation. Some times pinging is caused by hot carbon glows setting off two flame fronts (plug supplies the other) and when the flame fronts collide they cause an explosion at the last instant. Two plugs would set up the same combustion fronts. Our 2 stroke bikes use to have two plugs per cylinder, but only one was used at a time, the second plug was a spare for fouling. The primary plug fouled you put the wire on the second plug and kept going. Fred
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Old 08-23-2005, 11:47 PM
Dennis K. Nash Dennis K. Nash is offline
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Default Re: Can two spark plugs run from one wire

If this is a single cylinder engine, it might be a set up like some motorcycles were. With a spark plug in each hole, the lead went to one plug and the engine ran off that one. If that plug became fouled you simply switched the lead to the other plug and kept going.
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Old 08-23-2005, 11:58 PM
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Default Re: Can two spark plugs run from one wire

Well.... If you use 2 Ordinary spark plugs one will spark first then the other won't fire due to the fact that the air between the points will ionize and kill the voltage so it will not fire the other plug. The one with the closest gap will fire and the other won't.

Unless one of the plugs is of the type that John has mentioned and it is the series type.... But offhand I don't know where you will find one of those...

There were some mags made that have a 2 spark coil and these will fire 2 plugs at the same time BUT you will need a wire for each plug... I believe the 2 cylinder Wisconsin had a 2 spark coil. Maybe if you use resistor plugs you may get a weak spark from Both Plugs... BUT Magnetos and resistor plugs are Not Very Friendly as the resistor will cause the voltage to go too High and can damage the coil...
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Last edited by Ken Majeski; 08-24-2005 at 12:12 AM. Reason: Add More
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Old 08-24-2005, 12:02 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Can two spark plugs run from one wire

High performance and aircract engines use 2 plugs per cylender. May tag twins have 2 plugs/cylenders, fired by one magneto. first things first - 2 plugs - 1 cylender - you can fire both plugs at 1 time if you use a 'Pass Thru Coil or magneto. This type of coil has 2 ignition wires. power comes out of 1 lead, jumps the gap on # 1 plug, passes thru the block, or head, passes thru # 2 plug, and back to the coil thru the second lead. The coil, or in the case of maytag, the magneto, hi tension circut is completely independant of engine ground! If you pull off a lead and the engine keeps running, the coil is burnt out. Kohler makes a 2 lead Pass Thru Coil - 12 Volt, for their twin cylender engines. Generac generaters also use a similar Pass Thru Coil, and I believe Techempseh also has one, as well. Back in the 1970s B&S made a Pass Thru coil for use on MOPECO heater blowers for Public Service Electric and Gas. One wire went to the engine plug, and the other went to an igniter plug that fired a propane heater in the unit. The MOPECO heater - blowers were used to provide fresh warm air to manholes for work crews. The Pass thru coil is also used on 2 cylender snowmobiles when both pistons reach TDC at the same time. The Kohler 12 V Pass thru coil is not cheap - Last one I bought was $125. I used it on a water cooled maytag twin I own.
Andrew
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Old 08-24-2005, 12:04 AM
JoeFisher JoeFisher is offline
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Wink Re: Can two spark plugs run from one wire

Gentlemen, if you use two normal plugs, the spark will take the shortest gap to ground. the other plug will never spark. you only get both plugs to fire if one is specially made to be run in 'series'. two standard plugs will be in 'parallel' and spark will take easiest way to ground. the motorcycle two-stroke heads with two plug holes were actually made to use the second threaded hole for a compression release braking device for racing flat track races. i am an old racer-17 years- and we used them often. Joe Fisher -HOT in central Texas
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Old 08-24-2005, 12:22 AM
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Default Re: Can two spark plugs run from one wire

My Little 4 cylinder Ford Ranger has 8 spark plugs. 2 in each cylinder. I would have to look at it to see just how it works. BUT I can tell you that each plug has it's own wire and power source. Seems there are plug wires and dist caps all over under the hood....

On the other hand my 6 cylinder Dodge has NO spark plugs and runs Fine also.
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Old 08-24-2005, 12:23 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Can two spark plugs run from one wire

To answer your original question, If you are using a standard coil, with one side of the Hi Tension circut internally grounded, the answer is no. Under ideal conditions, BOTH plugs would have to have the EXACT same gap, the leads to each plug would have to be EXACTLY the same length, Wire resistance must match EXACTLY, internal local compression must be EXACTLY the same, fuel to air ratios at each plug must be EXACTLY the same, and the ciol must deliver enough energy to fire both plugs at the EXACT SAME TIME Even under ideal sientific conditions, there is no guarentee that both plugs will fire off a common hi tension lead. There are too many variables! The only way to fire both plugs simultaneously, is to use a Pass Thru Coil or Mag. As I stated in the earlier post, Hi Performance engines use 2 plugs per cylender. They usually have 2 forms of ignition. Real early engines had a battery and coil , with points set on one plug, and a magneto on the other plug. Air craft and Dragsters use 2 sets of magnetoes. Older high performance engines used 2 sets of distributers, batteries and coils. Anmerican La France made a V12 engine. It had dual point - dual distributers for the engine. Each distributer had to be set up independantly, Left bank, and Right bank This engine also had dual carbs. It usually took 3 8 hour days to tune the engine, including at least 5 hours of road testing for the timing alone!
Andrew
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Old 08-24-2005, 02:52 AM
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Default Re: Can two spark plugs run from one wire

So many issues - I hardly know where to start!
Two cylinder engines issue:
I have a couple of Onan W2C engines. 2 cylinder - inline. Both crank throws / pistons are at TDC at the same time. One is at the top of its compression stroke and the other is at the top of its exhaust stroke. There is only one set of points, no distributor and 2 coils. Both fire at the same time (Actually somewhere around 20* before TDC, but you get the point) One is simply wasting its spark - no damage to anything. The same will hold true for a 2 cylinder opposed type engine. Harley Davidson engines work the same way. They use a 2 lobed cam for the points that allows the 'working' cylinder to fire at the proper time while the other one wastes its spark either a little early or a little late - depending on which one needs the spark on that particular revolution. They only have one dual (secondary) lead coil. If you pull one plug wire while it is running the other cylinder still fires and the engine will still run, although not nearly as well.
Now for 2 plugs off 1 mag:
Once upon a time I built an abomination of a motor. I cut down crankshafts and made adapter hubs to couple them together and with some channel iron framework joined two 2hp B&S horizontal shaft motors together to make an inline "Twin". I fabricated an intake manifold to feed both of them with one carb that was a real work of art (NOT!). OK, it was butt ugly, but it bolted up. Long story short: both plugs fired off of the one magneto. Both cylinders were actually running / firing. It did not have a Magnetron coil - just a standard coil with points.
Here's an interesting closing shot:
While an engine is running, if you carefully pull the plug wire off and hold it 1/8" or so from the end of the plug and let it jump a gap from the connector to the end of the plug, the engine will keep running, and in some cases actually sound better. You are jumping 2 gaps with one mag.Yes, this would be series and my Frankenmotor was parallel. Way back, they used to sell "Spark Intensifier" devices for cars that did this exact function. They were a glass chamber with appropriate connectors on both ends that provided a controlled gap for the spark to jump before the plug.
Thank you and good night,
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Old 08-24-2005, 04:23 AM
David Greenwalt David Greenwalt is offline
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Default Re: Can two spark plugs run from one wire

Hi Ken, the '86 Nissan pick-up I used to have also had two plugs per cylinder. This was to burn fuel better, less pollution. If your Ranger is set up like my Nissan was, I believe when you look under the hood you won't find any of those carbon canisters used to control pollution. I put over 200,000 miles on the original 8 plugs. My dad was always after me to change them, but I figured as long as it wasn't missing I wasn't gonna touch them. The fuel injectors went before the plugs did. David.
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Old 08-25-2005, 10:29 PM
Doug Barbee Doug Barbee is offline
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Default Re: Can two spark plugs run from one wire

John is right about the Harley. I have been riding one longer than I care to admit. It has one coil with two plug wires. Both plugs fire at the same time, one cylinder on compression,the other on exaust. Thats why Harleys pop and spit fire if the timing gets a little off. Get a coil from a shovel head, 1980 - back. Doug
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Old 08-28-2005, 12:14 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Can two spark plugs run from one wire

The Harley Coil is a pass thru type. The Hi tension is independant of engine ground. If you pull off 1 plug wire, and the engine continues to run, you are burning out the secondary windings internally. Eventually the internal wiring will short permamantly, if the condition persists. This coil is probably cheaper than the Kohler one!
Andrew
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Old 08-28-2005, 03:44 AM
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Default Re: Can two spark plugs run from one wire

I used to drive a 1949 Autocar firetruck that had dual ignition. It did not run right if only one set was used. It has a 501 Blue Streak inline flathead 6 cylinder in it. Single Holley carb that weights a ton, and stright 4" pipe with no muffler. What a running truck it is.
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