Spark plugs

[nzmike]

Hi all, my S1 4.2 has just had a full engine rebuild. As part of the rebuild, we fitted fast road cams and upped the compression ratio to 9.6:1. Would be interested to hear from other owners with modified engines as to which spark plugs you have fitted. We have set the carbs carefully but we are finding the engine starts misfiring after a few hundred miles on a set of plugs, and at present we are unsure if it needs colder plugs or if is fouling its plugs.

[Heuer]

If you have any sort of electronic ignition then you must fit Resistor plugs - BPR 5 or BPR 6.

This is the difference between standard and resistor plugs:



"At the moment the spark jumps the gap it causes a high frequency burst of energy, this is known as RFI (radio frequency interference). This is why resistor spark plugs were introduced in the mid 1960's. Placing a resistor within the spark plug suppresses the RFI. Without resistor plugs in your car you can experience static on your radio as well as interference with other sensitive electronic equipment. Some later model vehicles as well as newer Powersport engines must use resistor plugs for a proper "talkback" to the electronic ignition. As a rule, performance is in no way impaired by resistor spark plugs."

[mtnjag]

What plugs are you running?
Points or Electronic ignition?

My 3.8 is slightly over 9:1, fast road cams. ported/flowed head with larger intake valves, points, UO needles, solid Cu wires, NGK BP6ES plugs.

I do have NGK 1kohm resistor caps on the plugs similar to the Champion caps but lower resistance to eliminate audio amp interference.

[Heuer]

I have a 4.2 with 9:5.1 CR, fast road cams and EDIS/Megajolt. When I first installed the EDIS I had bad misfires cured by Resistor plugs even though the HT leads were suppressed silicone.