In Reply to: I don't get it... posted by Ben '74 tii on December 24, 1999 at 15:53:49:
You are quite right. There are some clean sheet designs which use two plugs per cylinder. Usually, there is some specific design objective which must be met.
Twin plugs are used to build high compression engines where gasoline is of poor quality.
Twin plugs are used when the designer wants to use cheaper, regular gas instead of premium.
Reduced long term emissions are another reason to use twin plugs.
But very few high performance race engines used twin plugs (for very long). Formula one and Indy car engines use single (very small) plugs. So do most very high performance street cars.
Locating a second plug is very tricky business. It depends on local flame speed, temperature and pressure. Often a redesign of squish bands or piston crown shapes can cure the problem. MIT (actually Sloan Automotive Labs) built an engine with 17 spark plugs. Their conclusion was that if one plug were properly located, all the additional plugs provided no benefit.
If a second plug is used, it most likley will not be fired at the same time as the first plug. More ignition advance does not provide improved combustion but may alter the location of the peak pressure angle. A good example is the twin plug rotary Mazda engine.