"Hot" Coil - Worth it?

[BigBill]

All you need is a automotive std ign coil and run that extra wire to it.

A 20,000/30,000 volt coil will eat spark plugs, kill carbon ign wires, plus rotors and caps. Don’t ask me how I know, a street driven race car does wear this stuff out.

[Greg Armstrong]

Bill, it sounds like you are talking about a ballast resistor bypass, which doesn't work with an internally resisted (3.0 ohm) coil. Also, isn't the stock coil about 20,000 volts? With a wider gap at 40,000 volts, I can see the possibility for decreased plug life, but I think it would take years, and plugs are 2 bucks each. If the engine starts and runs better I'd call it the cost of doing business.

[Larry B]

You're right Greg that just adding a wire on a 12V system using a 3 ohm coil will not increase voltage to the coil at cranking. With the engine cranking whatever the battery voltage sag is that is what the voltage will be available to the coil regardless of where you source the positive voltage. I have done a few conversions on small tractors that utilize a ballast resistor and a 1.5 ohm coil and an extra relay. On a 184 I replace the existing coil with a 1.5 ohm coil and a ballast resistor. I then add a riding lawnmower relay wired this way.

one side of the relay coil to ground
other side of relay coil to terminal on starter
one large relay terminal to one side of ballast resistor
other large terminal of relay to other side of ballast resistor

In normal operation current from the ignition switch flows through the resistor and coil resulting in a 3 ohm load and acts the same as a 3 ohm coil. When the starter is engaged and 12v applied to the terminal on the starter it also applies 12v to the coil of the relay engaging it and bypassing the ballast resistor. Say the voltage drops to 9 volts at cranking. 9V / 1.5 ohm = 6 amps 9V x 6A = 54 watts. So now the coil is using 54 watts vs. about 27 watts with a 3ohm coil. This conversion can be done without doing any wiring under the dash panel.

The original post was about "HOT" ignition coils. What is a "HOT" coil? Too bad I don't have any of my old J C Whitney catalogs from the 70's with all the snake oil ignition devices. An ignition coil is plain and simple a step up transformer and depending on its resistance and the voltage applied to it will pass a certain amount of current and that will give you the power in watts it can dissipate. You can change the winding ratio to increase the voltage which in turn will reduce the current. At some point you reach diminishing returns. The other variable is efficiency. Transformers are not 100% efficient , there is always some loss. This is called insertion loss. The lower the insertion loss the better the transformer and the more it will cost. You get what you pay for. Back in the 60’s and 70’s there were “hot” coils that were around 2.5 ohm. These coils did provide more energy to the plugs and were hotter, literally hotter. 13.5V /2.5 ohm = 5.4 A 13.5v x 5.4A = 72.9 watts. These ran hot enough you didn’t want to touch them and if you turned the key on and the points were closed and you didn’t start the engine for a while you usually cooked the temper out of the point spring and overheated the coil and shortened its life.

We talk about 20,000 volt coils and 40,000 volt coils. Do we need 40,000 volts to fire the spark plugs in a C60 engine? Absolutely not! So why would you need a coil that says it is 40,000 volts? That coil will help in a low voltage engine starting condition. The 40,000 volt number is referenced to 12 volts and you have the coil using about 48 watts of power. Now consider it is Jan. 2 and 0 degrees and you want to move some snow and the battery is not in perfect shape and the cranking battery voltage drops to 8.5 volts. Now you get 8.5V / 3ohm = 2.8A 8.5V x 2.8A= 24 watts. Now the coil is only using 24 watts of power. About half of what it uses at 12 volts. So now that 40,000 volt coil might put out something like 25,000 volts. If the coil can’t generate enough voltage to fire the plugs then the engine won’t start. If you always have a good battery and you don’t get much voltage sag when you crank the engine then you won’t see and difference in a 20,000 volt coil or a 40,000 volt coil. Some folks run higher voltage coils and widen the plug gaps. That works fine but it also raises the voltage needed to fire the plugs so the wider plug gaps will make it harder to start in a low cranking voltage situation.

[BigBill]

By bypassing the ballast resistor with another wire sends a full 12volts for starting only to the coil. You need the solinoid with the two smaller studs. One runs the solinoid the other is for the coil on starting.

My 154 & fcub Are outside and no problem starting. Both are 12 volts.

[Greg Armstrong]

Had a very, very, very... Wait for it... Very hard time starting this past weekend. Granted, it was really cold, but I knew something wasn't right. Tonight I pulled the NGK AB-6 plugs and put in a fresh set of Autolite 3116 plugs. WOW!! It fired right up and ran like a champ. The NGKs were seriously carbon fouled. Plowed the whole lane tonight so the wife could get to work... It's amazing how $9.80 worth of plugs in the correct heat range can change your world.