Spark plug voltage requirement

[Truckedup]

I've heard many times that the voltage needed to fire a plug in normal conditions is around 12K volts....I assume that's a engine in a mild state of tune...How about firing a typical modified N/A V-8 with 11 to one compression at 7000 rpm? A bit more?...

[#84Dave]

Good question! We out here would love that answer for 10:1/12:1/14:1 static compression ratios. The ratios used by a large % of the racing community. -Dave-

[Truckedup]

I've seen 10K on a guy's scope hooked to a stock SBC...Can't find much on the Internet about actual operating voltages at full power

[BOOT]

I didn't read it, just skimmed but after a quick goolge found this http://www.dragonfly75.com/motorbike/JNM/CDI.htm

[user-23911]

The required voltage changes with engine load/cylinder pressure and AFR.
If you're playing around with the engine idling, it might be 12Kv.
If it's under full load with a turbo, it's NOT 12kv. It's a LOT more.
But as the required voltage increases, the duration decreases and it's the lack of duration where you end up with a misfire.

[user-23911]

http://en.wikipedia.org/wiki/Paschen%27s_law

[maxc]

What about plugs that don't need a gap? https://www.youtube.com/watch?v=mhVyvMYCA2I go too 1:47

[user-23911]

It's photoshopped.
It's using AC high frequency high voltage rather than DC high voltage. It still needs 2 electrodes.

You can do a similar thing with a normal ignition coil by driving it at high frequency to give a corona discharge but looks nothing like in the video.

[Truckedup]

The required voltage changes with engine load/cylinder pressure and AFR.
If you're playing around with the engine idling, it might be 12Kv.
If it's under full load with a turbo, it's NOT 12kv. It's a LOT more.
But as the required voltage increases, the duration decreases and it's the lack of duration where you end up with a misfire.

What's a lot more? You've seen the voltage on a scope?

[maxc]

viewtopic.php?f=15&t=39413&start=15

[user-23911]

What's a lot more? You've seen the voltage on a scope?

Of course. Mine make lots more volts....joules too.

A typical 8 to 1 engine with 1mm plug gap will start misfiring at about 1 bar of boost with 30kV + coils.
It's the lack of duration that causes the misfire.
Modern driver circuits all have the primary side clamped at 300 to 400v to protect the driver chip, that's what limits the secondary voltage to 35 or so kV depending on turns ratio.
Now....if the 30+ kV wasn't required, then the primary side would be clamped at 100 or 200v to give extra protection to the coils and electronics.

It's just another example of the ongoing BS and confusion that exists in the motor industry.

[Truckedup]

Interesting, I built a 454 with 8-1 pistons, mild cam and GMC type blower for a mud truck. It makes about 5.5 psi boost at 4500 rpm...The engine is never reved higher than 5000 rpm. The ignition is an older used Mercury Marine electronic distributor with an cannister coil, not a CD multiple discharge... Never misfires....

My original question was N/A engines...

[les327]

Youtube,

secondary pattern superchared 455

Firedome 8
Full throttle i think it had 9:1 14 lbs. Boost. firing v is uneven at part throttle i beleve due to un even distubtion it was an olds 455 put down about 450 on my mustang 600 chassis dyno the mixture was around 11.5:1 taken at end of pipe at full throttle.

https://youtu.be/vOmPW-ze3Tc

Plug gap .035"
HEI EFI motor

[user-23911]

It's just numbers on a screen ...it means nothing.
Most people don't know that you can't directly measure the HT voltage.
Not only that but a proper oscilloscope can't take any more than about 200v peak input, or with a 10 times probe about 2Kv
As soon as you connect something to the HT, the original voltage gets shunted by the extra load, mainly capacitance.
The only way to read it accurately is at the primary side then to get the true figure you multiply the voltage by the turns ratio.

It's the equivalent of trying to measure a crankshaft journal with a ruler.
The end result is gigo.

[les327]

It's just numbers on a screen ...it means nothing.
Most people don't know that you can't directly measure the HT voltage.
Not only that but a proper oscilloscope can't take any more than about 200v peak input, or with a 10 times probe about 2Kv
As soon as you connect something to the HT, the original voltage gets shunted by the extra load, mainly capacitance.
The only way to read it accurately is at the primary side then to get the true figure you multiply the voltage by the turns ratio.

It's the equivalent of trying to measure a crankshaft journal with a ruler.
The end result is gigo.

i
My scope uses a cap coupled secondary probe and is trigered off primary . The scope is calibrated with a rig for dist machine that allows you to adj the pri pk. with a 100:1 coil. it is plus or minus within 200 v secondary . I normally look at it as an indicator not per say a volt meter. Was it calibrated in 5 years ,no. I just thought it might be intesting for some.

But yea numbers on a screen for what their worth.