...so I have a few questions with regard to reading the spark plugs. First, how many miles should I have on the car? I've put probably 200 miles on since the first crank of the new motor. Second, does it matter which plug/cylinder I check, or should I check multiple plugs?

Thanks

 

Check several.

In a street car, it's not quite as straightforward as a race car. Whatever the engine did last before reading, is what you're going to see; not necessarily representative of its total tune. Most likely you'll see the evidence of what it ran like puttering down your street and oozing into your driveway.

That said, really gross mis-tuning leaves evidence. Too rich will build up black fluffy-looking deposits sometimes, or wet greasy-looking others; too lean will leave the porcelain bright-white and maybe even shiny, etc. Oil is obvious: the threads will have oil on them instead of gasoline and combustion residue.

For a typical street-driven motor, about the best you can hope for, is that they come out sort of tan or grey, with no major buildup of any kind, and mostly dry threads. You won't get a really "precise" indication no matter what.

Plug reading for racing is done by looking down at the very bottom of the porcelain where it meets the outer shell of the plug. You usually need a magnifier with a light to see it well. Most racers trying to optimize their tune that way will turn acoupla hot laps and cut the ignition clean and pop it out of gear at the moment they want to know what it's doing (end of straight, coming up off a corner, etc.) and coast into the pits, so as not to destroy the evidence of what they want to see by low-load low-speed operation and idling.

Note that a dual-plane intake splits up the cyls in a particular way. 4 are fed by the left side (2 on each side), the other 4 are fed by the right. Center cyls usually run WAY richer than front & rear in a wet-flow setup (carb or TBI) so you want to get a mix of all those variants if that's what you've got. 2 each L & R plenum, 1 each center & end of each, for a relatively complete picture.

 

Mine is a race engine and I only read them for the heat range. A plug running in the proper heat range will have a blue dividing line half way up the ground strap.

Something as simple as changing the ignition timing will change the heat range requirement.

 

Lots of cool info. thanks guys

 

I always made a wide open pass in high gear, shut it off Rolled to return road and pulled and replaced a couple of plugs and drove back to the pits. Read them from there with lite magnfine glass. Goggle reading plugs from there to get pictures to compare to.