Spark plug gap

[brett_smith]

I run Scorcher electronic ignition on a modified dizzy on my mark 2. I use NGK PB4EY high temp plugs. I am going to set the plug gap at 25 thousandths. Your recommendations would be welcome.

[jbuckl]

Thought 0.8 mm was correct.
25 thou is close to 0.635
might get a fatter spark at 0.8mm-1.0mm

[marshman]

The "correct" gap is 25 thou (0.64mm) as stated in the R.O.M. and used on pretty much all older cars. (along with 15 though points gap). You can open out the gap, most moderns seem to run at 40 thou (1mm), but then they were designed to and often have a "coil in cap" design.

Older systems, as per the Stag, have a distributor, rotor arm with distributor cap and long H.T. leads, Lord Lucas, AKA The Price of Darkness, designed these things to withstand up to around 25,000 volts. It is not just HT lead insulation but the "creepage and clearance" distances - i.e. the length of the path to earth, inside the distributor as well. The air is the only thing "insulating" things inside the distributor and 25,000V will jump approximately 25mm in the air. Opening up the plug gap to 30 or 40 thou will increase the voltage required for the spark to jump the gap, meaning the "spark" may be able to find an easier path which will cause a misfire. This will stress the H.T. parts further possibly leading to earlier failure as either the insulation around the HT leads, plug caps break down or it starts to find an easier path inside the distributor. If you go too wide, and you ignition system can cope with the increased HT voltages required, then you may also run into problems where the spark is so long that turbulance in the combustion chamber will effectively "blow the spark out", again causing misfires.

Having said all that by all means open out the gaps. I know people do run with gaps of 30 thou or more BUT I am not convinced it makes much difference to the performance. Perhaps Neil (flying farmer) can comment as I suspect he has "played" with this. Years back when I fitted electronic ignition to another car I opened out the plug gaps to 32 thou and although I was hoping for an improvement I can honestly say I noticed no difference in any thing. These days, I just set the gap to what ever is recommended by the cars designers, in this case 25 thou. Getting the ignition timing spot on (not necessarily the same as stated in the ROM - I aim for "just starting to pink on wide open throttle when "pulling" hard at low revs), along with properly set up carburettors, will make a far bigger difference.

[Wheelz]

A good fat spark depends mainly on the energy stored in the coil, and the timing of the switching of it by the ignition system (dwell angle and switching speed). Increasing the gap beyond normal is more likely to cause tracking which can lead to a misfire.

[MikeParker]

I have played with opening up the gaps over the years. I did persuade myself that with a capacitative discharge system,years ago, then increasing the gap did improve performance. With later inductive discharge electronic ignitions I really don't think I can tell the difference. Perhaps you get a longer duration spark with a smaller gap which offsets the smaller amount of mixture between the points? I run mine at 25 thou at the moment.

[singapore stag]

If you have high power lumenition system fitted you may find that the spark would be liable to blowing out at around 6000rpm with a gap well over 25 thou, unless of course you have the BHP to match.
Also if you open up the gap over the recommended figure, then you need to ensure your Leads, dizzy, plugs etc are in tip top condition.
Please don't take my view as gospel, just my gibberish. Though I have noticed this in my quest for the perfect gap.

[flying farmer]

I always found plug gaps over 25 thou were a disaster at full throttle/high revs.
OK, my engines are making far more than the standard power output but even with the Lumenition constant energy coils there is not enough coil charging time between ignition events at high rpm to produce a viable spark.
On my engines fitted with coil packs I can use a 35 thou gap. Makes absolutely no difference to maximum power but does seem to make idle a bit smoother probably because the fatter spark is better at lighting a weak mixture at low cylinder pressures

Neil