STi spark plugs?

From what I've read/seen/heard, a lot of those alternative spark plug styles are as helpful as the fake blue "xenon" headlights. (In that, despite packaging claims, they don't actually do anything better for your car, and may in some cases be worse.)

I replaced the spark plugs on my '95 Outback a few years ago, and although I originally bought Champion plugs, I heard some horror stories about Champion plugs in Japanese engines. So the Champions went in the back of the wagon (and, as far as I can remember, never got returned to the store, doh!) and I ended up getting NGK (oem) plugs from my dealer - and they actually cost a bit less. I've put on about 50,000 trouble free miles since then.

-Matt

The OEM stuff seems best, Bosch=horrors.

I've always followed the common wisdom of US plugs in US cars, Euro plugs in Euro cars and Japanese plugs in Japanese cars. It's always worked for me.

A good deal of the newer cars on the market are spec'ed with high quality Japanese plugs (more often NGK but sometimes Denso). My father's '96 Buick Regal actually comes with (AC rebranded) NGK platinums for the 100K mile change period. Some German carmakers are known to use NGK plugs, and a lot of BMW/VW specialist shops use them.

It's worth noting that while the beneficial claims of a "hotter spark" from certain aftermarket plugs are almost certainly so much snake oil, there really is such a thing as plugs that are classed as relatively "hotter" or "colder" than others. "Hotter" plugs tend to be better for cold starts, but are more apt to lead to pre-ignition in high-compression or forced-induction engines. In such high-performance applications, you almost *always* want to use *cold* plugs, so as to reduce the likelihood of dangerous pre-ignition. "Hot" plugs are of a benefit only in old, tired engines that aren't running very high combustion chamber pressures and don't like to start when it's cold.

Unless your engine has been modified, use OEM plugs. The specious benefits offered by other plug manufacturers don't outweigh the potential damage from pre-ignition, IMHO. If your engine *has* been performance-modified, always use the plugs recommended by your performance upgrade expert -- ask them, if necessary. In most such modified engines, the OEM plugs will probably be a safe bet, but colder plugs might permit higher boost at the theoretical (though in practice unlikely) expense of difficult cold starts.

Whether the spark itself is "hot" or "cold" isn't nearly as important as whether the fuel-air mixture actually begins its burn when the spark is fired. If the burn tends to begin before the spark, you need "colder" plugs. If the burn doesn't begin at all, you need "hotter" plugs. If your engine is experiencing neither of these conditions, then changing to either "hotter" or "colder" plugs will give you no benefit at all, and might actually *cause* some problems.

- Greg Reed

[Not an automotive engineer, but did stay at a Holiday Inn Express last night...]