By Ron LaDow
I wish I could credit this properly: I'm sure it was in a Henry Manny F1 review in Road and Track: "If you need two plugs, there's something wrong with your chamber". It related to a Maserati, and is suitably snotty, but there's a point there.
356 engines improve with the addition of a second spark plug in each cylinder. They improve in overall efficiency, both power and economy. And since the application does not effect engine breathing (with the resultant compromises), it improves matters at just about every RPM which has been tested. Improved overall efficiency means a 356 runs stronger, cooler and with less fuel.
Firing Later, and More Evenly, Maximizes the "Rod Angle"
The reason has to do with the fact that the mixture in the chamber must burn at a given rate, not explode. The burning causes the pressure to rise in the chamber, and that's what pushes the piston down that hole, connected by the rod, thereby rotating the crank.
Ideally, the pressure will maintain at a high level for some time after ignition, and should peak when the piston has pushed the crankshaft ~35° to 40° after TDC in a 356. This is because the rod is arranged to push the crankshaft to the best effect in that area (in shorthand, this is the best "rod angle", comparing the angle of the rod to the angle of the crank).
This should tie it together: Since the mixture burns, it must be ignited such as to reach peak pressure at that good rod angle. In a 356, the bore is relatively large, the plug is way over in that corner, and the actual internal geometry of the chamber is just plain ugly as far as burning is concerned. That means a single plug has to light the fire early: 32° to 36° before TDC being common.
(Aside: the exact 356 chamber geometry was designed into the first RR Merlin WWII aero engines, called the "Ramp" chamber. It was rejected as being too sensitive to octane and 'knocking.')
Well, lighting the fire that early gives the best power numbers for a single plug engine, but it does so at the cost of an early pressure rise; some of the pressure is actually employed to push backwards on the piston/crank, before TDC. Really bad rod angle...
It needs emphasis that the test procedure determines the ignition timing, not the other way around. When the numbers are best, that's the best timing for that configuration. In a single plug, it's just that the peak pressure at a good rod angle over-rides the earlier loss.
From all this it is obvious that any modification which allowed a later ignition without pushing the peak power past the good rod angle will help. It should also be obvious that lighting the mixture at points diametrically opposite in the chamber will yield the quickest total burn time across the chamber and therefore the quickest pressure rise and therefore require the least timing. Like 23° to 25° before TDC.
At the risk of addressing a straw man, any "hot-spark" parts (i.e. electronic ignition systems such as MSD, Petronics, etc.) fitted to a single plug engine may or may not help that engine, but they physically cannot reproduce the effects of two plugs.
The Next Best Thing to a 4-Cam
So for a 356, fitting a second plug means we can reduce the timing lead and gain pressure on the piston at good rod angles with the resultant gain in overall efficiency. But also, for reasons that need more detail than anyone should deal with, less timing allows a lower octane fuel in a given chamber, or allows a higher compression in a chamber with a given octane rating. As detailed elsewhere, compression ratio also improves overall efficiency; until the limit is hit, higher is better. It follows that twin-plug ignition allows complimentary gains by raising the compression ratio on top of improving the peak pressure/rod angle. [Larger view of added lower plugs in separate window]Did I hear: "How much improvement?" Well, it depends, but suffice say that on the dyno, this modification provided the greatest improvement over the entire RPM range compared to any other single modification, bar none.