Garth S

So, what is the life span of a spark plug in 'typical' 928 service? How do you know when to replace them? The usual indicators are dropping fuel economy to misfire: But in a clean running 928 .... ?
Years ago when I got my '80, a talented Swiss Racer/Mechanic mentored me on numerous Porsche 'facts' - some of which I actually remembered.
On sparkplugs, he installed a set in my new car, and said "Garf, Ve haf put diesen silber in you car - you haf never to change -Ja!?"
My friend had installed a set of Bosch WR8DS, the silver electrode ones, and that was in '85. Every so many years, I've checked the plugs. This time, as the car has been down for a lengthy list of PM items such as TB/tensioner update/oil pan/etc - I pulled the plugs again , as it was 12 years since the last time!
The engine is in excellent tune, and will rip to redline with ease: fuel economy has never changed - and to no surprise, the plugs look fine. Each plug is clean of deposits other than a light brown coating, each electrode has sharp edges, free of errosion, and the gaps are still all dead even at 0.7mm (~0.028"). I'm hooked on the performance of these 'Silber" ones ( have W7DSR in the S4 too)
I have a fresh set to go in, but 'read' the plugs in the pic (set in engine sequence, lower left is #1) - they look ready to go again. What do you think?
So, how/when/what motivates you to replace plugs?

GlenL

When they look like crap (erosion or build-up) or run bad.

Those look nice. Touch them up with sandpaper, check the gaps and go. Of course, at a buck or three each once they're out you may as well put new ones in.

2V4V

Have to go with Glen.

They look ok for used plugs, but at $1/per at the local auto parts 'big box' why bother?

(this presumes the existence of a local discount auto chain in yer neck-o-the-woods....)

Greg

Garth S

Can't disagree with the comment to 'replace them anyway' - which is set to happen.
Posting this topic was in part due to the remarkable performance of these plugs ( locally, replacements run at ~$8@ vs regulars @$1 - not an issue either way). I've run a variety of brands and designs (regular, split electrode, grooved, Platinum, +4Pt, ...) in a variety of cars - and it is always obvious when replacement is required. With these 'silver' ones, there has been no sign of degradation: I couldn't be more impressed with what is a normal wear/replace item - it's like having the best performing set of tires - that never wear out!
More recent engine designs are quoting 100K plug change intervals IIRC - but these are 20 year newer designs with vast improvements in fuel and spark control: maybe the clean running 928 already had most of that 20 year jump 'in the bag' through superior design.
If the $1 plugs do that well, great! I have no experience there - but this tip from my Swiss friend has worked for me.

UKKid35

In my previous car (not a 928) I kept the same ordinary copper core plugs for about 100,000 miles, simply re-gaping every time I had to adjust the tappets (about every 6,000 miles - what a nightmare). Evenutally the car started misfiring and I started panicing, not realising that all I needed was a new set of plugs. The old ones still looked fine when I took them out but had obviously broken down internally.

WallyP

Plug failure is due to:
- Wear on the electrodes. Standard cheap plugs will have steel electrodes. Spark prefers to jump from sharp edges, and slowly erodes those sharp edges. Eventually, the electrodes get rounded, and then required voltage goes up. The side or ground electrode will eventually erode to a sharp point, which can get hot and cause preignition.
- Deteriation of the center insulator nose. Eventually, the insulator will start collecting deposits that lower the insulating value, and you will get misfiring as the voltage bleeds off across the insulator rather than jumping the gap.
- Breakdown of the internal resistor. "Resistor plugs", used to reduce RF interference, will sometimes (but rarely) fail when the internal resistor fails, causing very high resistance and resultant misfiring.

Most of these failure modes are not applicable to the premium plugs used in the 928, and few of us put very many miles on the plugs anyway.

The 928 has a VERY good ignition system, and is not very sensitive to spark plug condition. In fact, the ignition system will blast out enough voltage and amperage to destroy the plug wires (or rotor, or cap) if a plug fails...

Doug Hillary

Hi,
many years ago - in the 1960's - I commenced using Bosch platinum plugs.

This was mainly in mechanically injected Benz engines (an absolute requirement at the time) and later in the prototype electronically injected cars (VW & Benz) of the mid to late 1960's. They solved many problems then such as poor cold idle (at minus 20C) low rev fouling and rapid electrode erosion

I have used them always ever since - with leaded fuel and then unleaded fuels - and in a wide variety of engines

I always run them out to around 100k kms with inspections two or three times. I have never needed to replace any during these change cycles.
Recently I replaced a set in my Outback after 110k kms and they were still in excellent condition

I have them in my S4 of course and they have so far covered about 30kkms

They are not in my Z3 or a MY01 VW Golf 1.6 I have (Bosch do not make a suitable platinum plug for these engines)

With electronic fuel enjected engines it is just one job/cost less. When you insert them though always use a suitable thread lubricant!!!

Wally's comments are of course totally accurate

Regards

Jeff928S4

Good post Garf!!!

Jeff