Resistor spark plugs
 

In my rebuild (78 3.0L SC), I used NGK BPR5EY spark plugs, and my cylinder#6 does not fire well. I initially accused the spark plug as the culprit, because it had a 5 kohm internal resistance, about twice the resistance of the wire. Then, I discovered that this resistance is there by design, to prevent EM interference with computer engine management. Now, my SC does not need that, as it does not have a computer. So, I was wondering if I should change my spark plugs to low impedance ones, and can these resistor spark plugs be causing problems on a SC?

Aurel

If you don't need the resistor to prevent radio noise that would cause static on a sound system or EMF/radio frequency interference with a computer, then put in non-resistor spark plugs. If you can get some copper plugs in the proper heat range that have no resistor, run those and you will have slightly hotter spark and less heat stress to cause wear on electrical components. Your plug electrodes may wear out slightly faster.

All of this is in the small marginal gain area. You probably won't feel a difference between new plugs whether they have resistors or no.:)

Aurel,

"Flieger" is spot-on,....:)

You don't need (or want) resistor plugs on an SC since you already have RFI suppression in the plug connectors since these have an internal resistor.

Use plain old copper Bosch or NGK plugs and make sure your plug wires/connectors are in perfect condition.

I have a Porsche Spec book and it says to use Bosch WR5DC or WR5DP plugs. I was led to believe these plugs are resistor types. Can someone explain why Porsche recommends them but others say don't?

I am using a set of Magnecors (8.5mm) and the WR5DC's, is this combination not good and if so am I loosing some power or engine response somewhere because of it?

Come spark plug wire replacement time, you can also use stranded copper or stainless conductors, not the more fragile radio resistance conductors. The service life of metal core wires depends on the integrity of the insulator not the conductor.

Sherwood

A resistance is placed in each sprk plug circuit to shift the RFI generated to a frequency that doesn't interfere w/ electrical equipement k=like radios.

It's interesting to note that thrugh '81 no resistor plugs were speced and '82-83 resistor plugs were specd. I know that on my '76 the resistance was in the connector so you could use steel core wire w/o concern. I also don't see a change is wire or connectors for the '82-83. It's quite feasible to use 2x the resistance w/o issue, that is until a combustion issue stresses the system at which point it won't fire as well.

Your original question was regarding a poor-firing cylinder, what do you mean by "doesnt fire well?" Is it rich or lean? Are you using CIS or carbs? Did switching plugs fix the issue?

Well, I swapped in a new plug, and it did not solve my problem. From the look of the old plug (beige), I would say it was running lean. Now, It sounds like I have leak in there, exhaust or head leak. The car is due for valve adjustment and retorquing head bolts, so hopefully I will find out the problem then. I have CIS. It is not backfire or misfire , just pop pop sound of a leak.

Aurel

Doing my valve adjustment, I found one intake valve very loose. I still have the other side to do, but I hope that was my problem.

Aurel