Which do you like and how long between installing a new set?

 

This may help:- http://www.jackals-forge.com/lotus/993/993_plugs.html
I use Bosch plugs and dizzy cap, Beru plug leads. If diying get the Snap-on wobble extension plug socket or even better and cheaper the Gearwrench 80546, allow all day and much cursing.
944 takes 10 mins and I use NGK plugs.

 

FYI, the current Bosch FR5DTC plugs recommended by most come pre-gapped at 0.8mm which is out of spec for 993 which requires 0.7mm.
regapping all 36 electrodes is a PITA
ask me how I know.....


P.S. change every 25kmiles

 

Hi Dry Fly,
I replaced mine at 80K miles because I needed new lifter cartridges, the valve covers were leaking and I had committed to new wires, along with a clutch.
At 80K the original plugs & wires looked good were not throwing any OBD Misfire Codes so apparently the plugs are very long-lasting.
For the above reason, I would suggest not replacing them until you have 80K or more miles on the old set.
Now others will rush in and say I'm crazy, I can live with that!
Andy

 

Plugs are arguably the best deal in replacement parts for their cost and their importance in the scheme of things..

while the plug will spark for 100k miles, when your gap is out of spec, you risk flash over inside the coil due to higher breakdown voltage. Much more expensive.
rather than regap every 25k I chose to replace. Unfortunately, this time I still had to regap. Grrrrr.

 

0.7 to 0.8mm ?
The 0.1mm gap even make any difference?

 

The owner's manual says the spec is 0.7mm +.1mm. I thought I read here previously someone (Steve Weiner??) who said the FR5DTC's that come pregapped at 0.8mm are still in spec and can be used out of the box and no regapping is necessary.

 

I use FR5 DTC

You're crazy!!!!

All I would say is that if you leave plugs in situ-for too long, there is a good chance they will seize. Something to do with the two different metals (plug and casing) reacting over time with heat/electricity/ether/antimony/wizardry - y'know chemistry. Bimetallic corrosion? Galling?

How long is too long?

Anyway, all you have to do is loosen the plugs and retighten if you don't wish to change.

Which is an awkward task on a 993

ETA

I did it the first time (for me) in 2012 as part of the 60k service. One of them was half-seized in place - probably the difficult one behind the pump? which may never have been changed in the past - it is alleged that some Porsche garage/shop specialists/main dealers don't bother changing that 12th plug as it is too difficult for them to make money or the mechanic is too rushed. Which is why I do it myself.

I've just done it again a few months back - well I did the bottom six when I changed the hydraulic tappets and cam-cover seals, and someone else changed three when they swopped out the distributor belt and serviced the twin dizzys, replaced the caps and rotor arms with new Berus.

I know they did this, as they are a small Classic car specialist/garage, and I supplied the parts as they do not hold any parts at all..

So I still have three to do

ETA2 here is my 'flexi-joint' from when I changed in 2012. The tank-tape is to ensure that the connections stay connected !!!

 

I’ve always used & recommended Bosch FR5DTC plugs. For the next round of maintenance on my beater, I will install NGK BCPR7ET plugs. Times change.



Andreas

 

Hi Iain,
Never heard of flashover before thanks for the education:If a coil or spark plug had this issue I would think the OBD system would catch it. My car has its original coils and I have never experienced any issues other than a misfire code from old distributor caps that once replaced and the code cleared it, it doesn't come back. Not sure I have experienced a flash-over but if it was a problem wouldn't it show up as an emissions issue with an oxygen sensor or if bad enough a misfire both throwing a code if not a CEL? If not maybe it simply is of no consequence. I get the concept of regaping in low mile intervals if our systems ran at 6000 to 9000 volts like in the old days but our cars run at somewhere between 20,0000 and 50,0000 volts making gapping a much less critical requirement and so short intervals not so much of a requirement.
All opinion here as my experience is just practical having owned and serviced the many cars I have owned over the years including three Porsche. I suspect a technician using a dyno or some other advanced diagnostic equipment would support your assertion.
Andy

Andy

 

Good to know, I'm crazy that is. All I can say is in all the dozen or so cars I have owned since 1976 none have ever experienced a seized spark plug including those with alloy engines and three Porsche, what can I say. As Dirty Harry the mechanic would probably say to me:I guess I do!
Andy ;-)

 

An extensive discussion of the evolution of sparkplugs and how their seizing happens and is counteracted:
Anti-seize on spark plug threads

Andy

 

You are the research hound! Top marks.

But unless you did you own spark-plugs last-time, you might not know if there is any 'anti-seize' paste or plating on your currently fitted plugs.

So 'from now on' we can find out. I put a merest dab of paste on the Bosch when I fitted them in 2012, and 9/12 have come out already.

I have a new but 15 years old set of NGK for the Golf sitting in my garage, and they have a shiny plating on them.

I'll take some photos. Well why not?

 

I replaced my plugs with the recommended Bosch ones at a bit over 60Kmiles simply because the manual stated that was the interval. They looked great, gap was nearly spot on, and show nary any signs of wear beyond the normal "dusting" you see inside and on the ground straps. Likewise, they came out without drama whatsoever, with not even a hint of impending seizing. Given the PITA the job is, I will not change the plugs again at 60k miles it seems totally superfluous and a wasted of effort. And given Andy's observation may even wait till 90k miles

Edward

 

The voltage required at the electrode to produce a spark in a highly compressed mixture of gas & air is a pretty complex thing to calculate but at a simplistic level the voltage is proportional to the gap. A bigger gap requires a bigger voltage. This voltage originates at the secondary winding of the coil(s) where it is in close proximity to the grounded chassis and the low voltage primary. The coil manufacturer provides insulation between the high voltage windings and everything else but this insulation has a limit. If the limit is exceeded, the spark, looking for the path of least resistance, can punch through the insulation compromising it's ability to isolate and providing an alternate path for discharge. Worst case it could discharge through the primary to the coil driver pack and take that out.
When this happens it will be detected by the ECU and cause a misfire code so you'll know when it happens - but now the diagnosis is harder because multiple parts have failed.

I have seen this in a couple of other cars where diagnosing a misfire, I discovered the coilpacks were bad. On deeper investigation, I also discovered that the spark plugs which I hadn't replaced at recommended intervals had larger than spec gaps.
When I did the first smog on my 993, the tech pointed out that while I was passing, he could see an occasional spark on the disty cover. Observing in the dark confirmed that some of my spark was escaping. Now of course, this is a different problem, most likely a bad/cracked plug wire but my point is that the improvement in how the car ran after replacing plugs, wires, disty covers & rotors was startling. I prefer to keep my car running in this condition.

As for starting a components use at the limit of spec (0.7 + 0.1mm) I find this counterproductive. The plugs will only be in spec for the first 1000 miles or so and will them spend most of their useful life out of spec. Sure you can do it and I'm sure Bosch exceeds the requirements in their coil design but why would you? This is an easily maintained (relatively of course) part of the engine and so I choose to keep this system in spec, on target. Am I OCD? Yes, absolutely.