Spark Plugs - should they have a resistor or not?
 

Hi all,

I just pulled the spark plugs of my 1966 911 because I suspected fauled spark plugs again (it appeared two plugs were fauled up pretty bad).

It appeared that my mechanic installed Bosch Platinum WR4DPO plugs. The online catalog of Pelican states that my model car should have W4DPO plugs (i.e. no resistor).

What's the difference?
Would non-resistor plugs have a lesser tendency to faul?

Thanks for any advice,
Ed

Resistor should not make a difference. I have no experience with them but I will be installing a set to clear up static in a CB radio

After reading some other posts about spark plugs and wiring, I decided to measure the resistance of my wiring. Here are the results:
1 - 1990
2 - 1960
3 - 2240
4 - 3800
5 - 3810
6 - 3730

Notice that the wires for cyl. 4-6 are significantly higher then 1-3, probably because the wires are longer.
Based on what I've read so far in other posts, I would at least expect the resistances to be closer to each other.

Is this normal?
I don't know if the wiring is OEM or not. The wiring looks new, but has no markings (brand, partno.) at all, so it's hard to tell. Since the PO restored the car I suspect the wiring could be 'substandard'.

Ed Hut
1966 911 2.0

If the wiring is copper, there should be no resistance in wire (Well, 0.something). The only resistance comes from the resistors that are attached to the wire ends.

forget those cold heat range plugs unless you have real high compression or a turbo. just use the W7DC or W8DC normal plugs. they stay clean better. cheaper, too.

[This message has been edited by john walker's workshop (edited 09-29-2001).]

Thanks for the reply John. Can you give me a pointer on where to find more information about the various types of Bosch spark plugs? I don't know anything about it basically, didn't know these were 'cold' plugs. In another post I found a link to a site of NGK spark plugs, which had a detailed sheet about plugs. Is there a similar site for Bosch (couldn't find anything about it on www.bosch.com).

I assume the following:
a) the 'C' of W7DC means copper as opposed to the 'P' of my current plugs that which means platinum.
b) the digit '7' is a heat range indicator, higher means hotter.
c) not having a 'R' in the type means no resistor.

Am I correct so far?

How to decide which type of plugs to use? Is this (sort of) an exact science?
I definitely don't have a high compression (anymore). Engine has about 130K miles on it; acompression is about 125psi (all cylinders are near this value). I'll be up for an engine rebuild soon I guess.



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Ed Hut
1966 911 2.0

you are correct about the bosch #s. basically, you need a plug that just runs hot enough to stay clean. so it's a bit of trial and error. bosch #s go up for hotter, NGK go down. BP6ES is popular for most applications. some engines run so dirty they need WR9DP to keep them running right.(one with low comp, hot cams, and carbs for instance)

Just read the FAQ on the NGK site. Very helpfull indeed.

Also found www.boschusa.com (the link was somewhere in Pelican's online catalog). Both Bosch and NGK (www.ngksparkplugs.com) have an online part finder.

Now that I have this information I cannot wait till monday to get me some spark plugs.

Any opinion about the resistance values of my wires John?


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Ed Hut
1966 911 2.0

[This message has been edited by EdHut (edited 09-29-2001).]

as stated above, wire wires have no resistance. you must have carbon core wires. trash em. are you just checking the wire alone without the dist cap or plug connectors? they have resistors in them. plug connectors have 3K-5K ohm resistance. some cap connectors have 1K. they burn out also and cause a running problem occasionally.

I tested the wires including the plug connector at the spark plug end but without the dist.cap at the other end (i.e. I pulled the plug from the dist.cap). Didn't want to remove the connectors from wires for just testing them. After what you've just said I may do that after all.

Ed,

I would recommend replacing the wire portion of your cables with bulk 7 mm copper-core wire, available at most larger auto supply stores. The following thread discusses the issue. Bear in mind that pre-74 ends, the Beru connector for the spark plug, and the distributor connector just screwed directly into the wire, whereas the '74 and later wires with the braided shield -- used crimp-on ends for the wire, and the Beru connector had a different style with machine-screw threads rather than the tapered wood-screw style threads of the early connector.

http://www.pelicanparts.com/ultimate/Forum3/HTML/006551.html

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[This message has been edited by Early_S_Man (edited 09-30-2001).]

OK, just tested the individual components of the wiring and guess what? The wires are fine, resistence of next to nothing and below http://www.pelicanparts.com/ultimate/wink.gif Also the wires look new, so no need to change them.

Same goes for the distributor cap ends; also looking fine and each measuring 1K ohms resistence.

The Beru connectors is quite another story. Judging from the way "Beru Germany GWS 9" is printed on the rubber end caps I've got 4 different ages of them. The oldest looking ones have a resistence of about 1K ohm, the newer looking ones about 2.7K ohm.

So instead of getting new wiring (which was 7mm copper core by the way) I'll be getting a 'sixpack' of new Beru connectors. Plus a new set of spark plug of course http://www.pelicanparts.com/ultimate/cool.gif

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Ed Hut
1966 911 2.0

Don't be cheap with the wires. Buy new ones. 4 meter will be enough and it will cost about 3$. Change them, they might have tiny cracks or such that leak and spark goes to engine chassis.