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Re: Spark plug type
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 16 Sep 2003 10:12:52 Share Post by Email
In Reply to: Spark plug type, Todd [Profile/Gallery] , Mon, 15 Sep 2003 18:36:10
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As folks have mentioned, the -11 just means it's pre-gapped. As Michael says, they leave the factory with an accurate gap. But there is no guarantee how they've been treated since then.

No matter what, I ALWAYS check the gap before I put a plug in, so I wouldn't bother with -11's. Pre-gapped plugs might be useful for a shop or an engine assembly line where saving one step (measuring) may save a few bucks over the course of a year. But for the individual, it isn't worth it.

Ah, spark plug gap. Yes, there is a spec. But it is also individual to each car. That gap setting is for new cars with an expected engine-to-engine variation. That changes with time, and your engine isn't the same as when it left the factory some 12 years ago.

In general, a bigger gap provides a longer spark, which can provide more complete burning. However, the bigger the gap, the harder it is for the spark to jump. High pressure (boost)makes it harder for a spark to jump. So you want the longest spark you can get, BUT, if you make the gap too big, the spark may not be able to jump at high pressure, and you'll get misfire under boost, which sucks.

The max pressure in the cylinder varies for a lot of things - outside air pressure, temperature, load, etc. Yes, the turbo system controls around a boost limit, but it's not a perfect control. So some days you might get more pressure than others. Notice it's boost - not pressure. Boost is how much air you're shoving into the intake manifold. Pressure is when that piston drives up in the compression cycle. More boost means more pressure, yes. But intake charge temperature, water content, engine temperature, piston speed, leakage around the rings, valves, and (hopefully not!) head gasket can all affect that pressure (compression ratio). And that changes.

OK, enough for showing off - so what? In general, with a stock engine I gap to what the manual reads, erring to make it a bit tigher. Yes, a longer spark is better, but the minimal advantage of a 0.05 bigger spark is much, much less than the disadvantage of engine miss under boost. If the engine has been modified for more boost, run a narrower gap. If you get even a hint of engine miss at boost, drop the gap down.

Or, do as Michael does, and gap to 0.89. But keep an eye out for engine missing, as that indicates too wide a gap.

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