Plug gaps

[Guest]

Hi all, have a pertronix ignitor with a 12v flame thrower coil. Could (should) I open the plugs up a little more than standard? If so, what do people recommend?

Cheers

Jason

[Andy Rundell]

you'll be able to run at 30 thou no problem (i'm assuming your distributer cap and leads are not falling apart)

.........Andy

[V Mad]

Opening plug gaps raises the HT voltage, which in turn puts more stress on all the HT component insulation (dissy cap, leads, coil). Higher HT voltages means more likelihood of breakdown, hence misfiring. Maybe good qualityuprated coils and leads can withstand the extra voltage but what about the dissy cap; most are probably poor quality pattern parts that do not even work well at the original voltage, let alone increased HT.

I am not aware that there is any advantage in upping the HT as the spark energy remains the same.

I would keep the gaps the same as standard.

[Guest]

Er ok thanks guys!

Jason

[Princejohny]

Opening plug gaps raises the HT voltage, which in turn puts more stress on all the HT component insulation (dissy cap, leads, coil). Higher HT voltages means more likelihood of breakdown, hence misfiring. Maybe good qualityuprated coils and leads can withstand the extra voltage but what about the dissy cap; most are probably poor quality pattern parts that do not even work well at the original voltage, let alone increased HT.

I am not aware that there is any advantage in upping the HT as the spark energy remains the same.

I would keep the gaps the same as standard.

Hi Chris
The bit where you say opening the gap, raises the voltage - I remember from 1970's physics that it's the voltage that sort of provides the pressure for the spark to jump the gap. There's some relationship between the amps as well. Doesn't it also mean that the current actually flowing is down as well, which is not good either?

Johny

[Andy Rundell]

Opening the gap up allows a more complete burn of the available mixture buring combustion, feel free to Google it to your hearts content.

For a car running electronic ignition and an uprated coil (as Stagstan is) providing the power to generate a good spark over the increased gap won't be an issue. Clearly your HT system needs to be in good order - hence my comment about distributer cap and leads.

If you had a regular points/coil set up then the regular gap is where you'd go - the original post indicates an ignition system which should be able to cope with a wider gap.

Try it and see if you notice any difference - easy to alter again and again until you get the best set up for your car.

.............Andy

[StagnJag]

Surely the HT voltage is governed by the No of turns of wire in the coil, it is basically a transformer is it not?

Rgds

Dave

[KOY 23]

Dave, the coil will generate a very high voltage, I believe around 30kV for a standard coil, sports are higher and I believe Rover V6 coils were 85kV. The plug gap of 25 thou + will breakdown somewhere around 15kv(according to my Crypton), so the HT voltage is determined by the gap. The higher coil voltage gives you more reserve to spark a wider gap to give a better burn, as Andy says. The spark has to jump a 10-15 thou gap in the distributor cap, which will not stress it. The only time excessive stress will occur is when a lead falls off and the voltage will increase to the coils max.

Chris, sorry but I cannot see your argument not to increase the gap, please elaborate.

John.

[StagnJag]

Thanks John, that makes sense to me .

Rgds

Dave

[Gresley]

I have exactly the same set up and my Stag runs a treat with a 30 thou plug gap. I change the distributor cap and rotor arm every 12,000 miles.

Brian

[Ian Durrant]

Surely the HT voltage is governed by the No of turns of wire in the coil, it is basically a transformer is it not?

Rgds

Dave

Yes it is but its fired by the collapse of the magnetic field when the points open. This collapse causes a rise in voltage on the secondary coil but its not entirely instantaneous. If your plug gap is too small the plug fires before the voltage reaches its peak and dissipates the energy. The plug gap is at its optimum when it achieves the highest voltage commensurate with the coil still being able to supply enough current at high rpm and enough voltage to fire the bigger gap at high combustion chamber pressures, like wide open throttle.

Chris VMAD is right (I believe) that the total spark energy is the same but a wider gap gives more spark area to start the flame front and a hotter (more current) if slightly shorter duration spark (I think, I did this as one of my A level physics projects but we are talking 27 years ago now)