Marine Blue

Some great info, I have some reading to do tonight.

j.kenzie@sbcglobal.net

Very interesting. I'm due for new plugs on my 84. What plugs should I use? Also, it looks like getting at the center plugs on each bank of cylinders will be pretty tight because of the spider. What it the trick for getting in there with a spark plug wrench?
Thanks

hacker-pschorr

The US spider is curved in just the right way to sneak the wrench through. IIRC one or two of them you have to put the socket over the plug, then the extension will fit between the intake legs.

dr bob

I found the original-type copper plugs made a noticeable difference in NOx emissions over the 4-prongs and the single-electrode platinums.

The mult-prongs were introduced to allow longer in-warranty plug life on new cars, a response to EPA requirements for emissions systems warranties. The longer life means that the dealers won't have to cover plug replacement, or risk the damage to cats caused by mispires with worn plugs.

Meanwhile, a single-electrode plug offers a greater exposure between the spark and the fuel charge. It wasn't that long ago that the hi-perf crowd spent a lot of time selecting plugs for individual cylinders, based on what direction the ground electrode is facing. More power in certain engines when te electrode is 'pointing towards' the intake valve. This is especially true in 2-stroke race applications.


Generally, I like to stick with the plugs that came in the cars. My vintage S4 seems to like the copper single electrodes. I can remember working on Chrysler cars that just wouldn't run right except with Champions in them, ditto Fords with Autolite/Motorcraft and GM's with AC plugs. I did hot-box testing on Champion plugs to find the threshold of conduction on a series of "identical" plugs and they were all over the map compared with NGK (for that particular study), yet the subject Chrysler 383 engine would idle perfectly with the Champion plugs and almost not at all with the better-matched NGK's. The research was done for Standard Oil of California (Chevron) with a look towards what the gas needed to look like to meet the tightening tailpipe standards of the time (mid 1970's). The plug choices played a huge part in how the cars would run, spew, emit, etc. Remember that we had carburetors, points and condensers, vacuum and centrifugal advance fighting with vacuum retard, exhaust gas recirculation/contamination in the intake charge, etc. Way different from EFI, decent electronic ignition, etc. that we enjoy these days. Plugs choices don't make as big a difference these days, but they still make some.

IndyMatt

This is what I seem to remember:

Copper is a better conductor so that is why some still prefer copper over platinum or even iridium.

Platinum and Iridum are harder and last longer repectively.

So some still prefer to use copper and just change them out more often.

Lizard928

I wont run anything but copper in any of my vehicles.

The copper plugs will fire hotter and run better every time.

RED SHARK 1990

Hello dr.BOB and ROB when we worked my car in january we pulled all the plugs and found that the spark plugs were in great shaped and burned correctly-it was plat,iridium 4 from bosch.They work great and last pretty long.Updated new retrofit tensioner and belt system with 4,000 plus miles so far...excellent KEN...AGAIN i wanted to thank to many sharksters that came to DR.ROB to help me out.Thanks again..LUAN

Chuck Schreiber

Randy,
Not sure of part# or $ as I trashed the box and Roger still has not invoiced me. I'm sure Roger will chime in here and provide information as he always does!

Funny, he mentioned to me that most times when you change the plugs on the GTS, you'll find single prong plugs there and the GTS specifically calls for the Bosch 3 prong. The plugs are in a Genuine Porsche Box so I'm sure whatever the $ comes to, it will be several times more than those in a Bosch box!

Dean, Afshin, I think they are specific to the GTS and not sure what the S4/GTS call for. Roger??

ROG100

I agree with Dr. Bob stick with what Porsche specified for the car.
I used Plat 4's on my GTS when I first bought it and had idle issues.
Changed to the correct plugs and been perfect since.
The Porsche 3 prong are 999 170 189 90 and called WR7 DTC.
Not cheap at $10.75 each.
There is a more cost effective alternative from NGK BP6DT at $3.50 each.
Jim M is using these in his GTS.
Plenty in stock of both.

Jim M.

So far I'm happy with the NGK's. About 5K miles and it still runs great.

Chuck Schreiber

$10.75 a plug?? You gotta be S**ttin me!!

I'm returning them for the NGKs!!

Marine Blue

Roger, do the earlier cars also use the three prong or is it the coppers? I did some reading last night and I'm thinking copper is the way to go but just want to be certain. FYI you should seen an order coming your way in the next two/three weeks for some parts.

hacker-pschorr

They are copper

W - Thread type
R - With Suppression Resistor
7 - Heat Range
D - Thread Length / Spark Position
T - Triple Electrode
C - Copper

I use WR7DC - same plug with a single electrode. You can use either one in every 928, I just prefer the single version. One step cooler would be WR6DC (or WR6DTC for a triple).

NGK heat ranges go the opposite direction, the closest match in an NGK to the WR7DC is BPR6ES. Next range cooler is BPR7ES. Nice and confusing

Most of the big power boosted cars I know of use an NGK 8 or 9 with a buried center electrode. It's a plug designed for high RPM snomobiles, my local farm supply store sells them for $1.00 each
I have a set on the bench I've been meaning to try out. #1 issue with this type of plug on a street car is fouling. Todd has been running them for many years on his 928 with no issues:
https://rennlist.com/forums/4957108-post42.html

ROG100

WR7DC+ - Copper 999 170 156 90
These are the plugs for the later cars - 83 to 91
I have plenty of them for $1.95 each
WR8DC - Copper - 77 to 82 999 170 170 90
For the earlier cars - $1.95 each.

chrly924s

I know that most of the 951 crowd uses NGK copper plugs so that's what I've got in my 951. The copper conducts heat away from the plug better and reduces the risk of detonation(pinging) that a platinum plug can cause.