What spark plugs to use on rebuilt engine?

[ertech]

Hi guys
Did a search can't find a definite answer
What spark plugs to use on rebuilt engine?
Thanks
Erick

[Speedy Squirrel]

Do you have a stock setup with capacitive ignition? The W3DPO's are still a pretty good choice. I guess for one thing, if it uses so much oil, post-rebuild, that it needs a special plug, something is messed up. Normally it will consume a little more for a few hundred miles, but not that much.

When I looked at the choices for myself, I considered these factors:

Ignition system type (capacitive or inductive)
High tension wire type (spiral wound or solid core with shield and resistor)
Combustion type (single plug or dual)
Longevity desired
Heat range

Ignition System
If you have the original or an aftermarket capacitive ignition system, the voltage rises very, very fast. In fact, it happens too fast, so most capacitive systems discharge several times for each cylinder firing to emulate a longer spark. Capacitive ignitions do not like resistance because it reduces the spark energy (voltage over time) at the gap. For stock 930's the total resistance from distributor end to spark plug is 4K.

Inductive ignitions, whether wasted spark or COP produce a long duration spark. Since they only strike once, resistance is less critical. If wires are used, they have a spiral construction to reduce EMI, and use a resistor plug for RF suppression. Typical wire resistance is 5.5K/meter. A resistor type spark plug is typically 5K. Thus, the total resistance from distributor end to spark plug is around 8K.

Wire Type
If you want to stick with the Beru metal shielded cables, then you shouldn't use a resistor plug, as the Beru metal shielded cables have the 3.5K resistor build into the connector. It can be removed, if you want to take that route. Then you could/should use resistor plugs.

Most, if not all of the aftermarket spark plug wires are spiral wound steel wire with a silicon cover, with about 5K per meter resistance. On a capacitive system it would be better not to use these wires and a resistor plug, as the combination of resistor plug and spiral wires have too much resistance.

Combustion Type
The big combustion chamber of the 930, and the giant valve sizes, make it hard to get the spark plug close to the center. For this reason, Porsche specified a longer protrusion in the stock spark plugs electrode to get it further in. This really is only a part throttle and idle thing. Not so big of a factor at WOT.

If you twin plug (not something I recommend), you can do away with this and use a standard protrusion, which protects the center electrode a little better.

Longevity
In fact, all spark plugs are copper plugs! The fancy metal is too expensive to make the whole thing out of it. The core is copper, but the tip is something else. Why? Copper erodes over time, increasing the gap. Also, newer plugs have fine wire center electrodes. Why? So that it will heat up faster and burn off the combustion products. This usually makes for a better idle. If you are not too fussy about idle and want to change plugs a lot, copper is fine.

Heat Range
There are lots of cross references on line to convert from a Bosch heat range to that for other brands.