MG Car Repair: Spark plug gap theory questions, spark plug gaps, spark plug gap

Question
Hi Howard,

in a couple of MGB books I own I read conflicting values on what my spark plug gap should be.  I have the 18 GF, high compression engine.  One book says 0.025", and the other says 0.032"  The engine is currently getting rebuilt.  Before I had it torn down it was running, but burning too much oil.  The spark plugs at that point were gapped 0.025".  With a fresh set of plugs on the rebuilt engine, what gap should I use?  Also, can you explain the theory behind different spark plug gaps?  It would seem to me with a bigger gap you would generate a quicker burn, however I would also think to create a good spark across an increased gap would mean you would need a higher performance coil...

Thank you in advance for any information you can provide!

Regards,
Dan

Answer
Hi Dan,
Some of what you say is true. As the gap becomes wider and wider it is, it does take more voltage to jump the gap. However, the firing of a coil works strange. If you have a standard coil and you have it on a scope so you can monitor the voltage that it takes to jump a gap you will see that as you widen the gap the voltage it takes to jump that gap goes up as you might expect except that is what happens with a standard coil. I experimented with just gap on new plugs and it usually took about 25,000 volts to jump a new Champion plug gapped at .025" with a standard MGB engine with the mixture set normal. If you leaned the mixture out it required a higher voltage to jump the gap and a richer mixture would make the voltage go down. A wider gap made the voltage go up on the same coil on the same engine with the standard mixture. What it amounted to is that the condition of the area around the plug gap made the coil fire at a certain voltage. The coil is loaded by current going around the primary windings in the coil and as the gap became more and more difficult to jump the field would build higher and higher in the secondary before it would unload by jumping the gap. If the gap were close or easy to jump (like a rich mixture) the field would unload by jumping that gap. If the gap were difficult to jump then the field will build higher before it could unload and jump the gap. High compression is also difficult to jump a gap like a lean mixture and a wide gap. I also found that a well used plug gapped at .025" may take 30,000 volts to fire and if you were to file the end of the electrodes of the plug and set the gap exactly the same but make no other changes the voltage to jump the gap under the same conditions was 5,000 less voltage to jump the gap.
When you raise the compression and hop up an engine you make it more difficult to jump the gap thus you need to install a higher energy coil. When the conditions make it too difficult to jump the gap then you get a misfire. A high energy coil has a different number of windings in the primary and secondary so that the capacity of the coil is much higher so even though you may still only gap a plug at .025" the voltage that jumps the gap may be 35,000 or more voltage. and if you gap the plug at .035" you may see 40,000 volts on a scope and some ignition systems can go much higher than that on newer cars.
Many later model street cars went to .040" gap and very high voltage coils not because the engine was a high performance engine but because of lean mixtures. The emission standards made some manufactures try to lean out the mixture in an attempt to clean up the burn but they found that there were not enough fuel molecules in the area of the spark to cause the fire to spread to the rest of the fuel to keep it lit (propagation) so they widened the gap and doing so made the coil reach the end of it's capability so thus high energy ignition system with high voltage coils.
All this caused a secondary problem, the higher energy ignition systems caused a higher temperature spark and then it started burning the electrodes off the plugs and made it necessary to use a better plug wire etc, etc.

I know this is a long answer to your question but plugs and gaps and coils are more complex then most people realize. As far as your gap on your car goes you can use .032" with little to no problems. I always used .025" as I felt that it was less likely to cause a plug wire failure or a dirty cap and rotor failure since I kept the voltage lower. However I also had to keep my mixture a little on the rich side to prevent a misfire under load with a standard coil.
Howard