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MG MGB Technical - Spark Plugs - please increase my knowledge

1977 MGB, Ivor Searle 1950 rebuilt engine, Weber DGV carb, sports coil, plugs gaped to .035. Documentation that came with the rebuilt engine specs a "N7YC" plug. I'm hoping to get a spark plug education.

I had historically run BOSCH 4016 WR7DP spark plugs, since discontinued. Late in the 2016 driving season I changed to NGK BPR6ES per several recommendations on this site. I was never particularly happy thereafter due to a hesitation and occasional miss under load.

On the first tank of gas this season the hesitation was much more pronounced. I was originally thinking stale gas left over from the winter or that a carb adjustment could be the issue, but on a whim I reinstalled the BOSCH plugs and the problem immediately went away.

I'm asking for opinions on what's going on. Is there something different about the plugs that resolves the problem.... I only recently noticed the "7" in the plug spec'd for the engine... or are the BOSCH's covering an underlying problem that should be addressed?

Anything you can contribute to my education will be appreciated.

Brian Denis

Brian Denis

Even some standard engines seem to run better with one make of plug than another (although I've never found that on either a roadster or a V8), so with a modified engine I'd imagine that is even more likely.

The original plug was N9Y and its equivalents given as Bosch BP6ES and NGK W7D and the various options such as Resistor and Copper core. However on some heat charts the NGK equivalent would have been a 6.

N7 is slightly 'cooler' than N9, so might be preferable if the combustion chamber runs hotter than 'normal'. The Bosch equivalent would be a 5 but the NGK 7 is shown as a direct match. We are talking very small variations here, just one heat range different.

If you are happy that everything else is correct i.e. timing and mixture, then don't go looking for trouble, just use the plug that works best.

Paul is absolutely right, but bear in mind the BP"R" plugs are resistor plugs and if you are using any of the modern silicone, etc., plug leads they actually also act as resistors. The resistance is there mainly to suppress RF interference. Having a double resistance, i.e., plugs AND leads is not good.
The heat range signifies the plugs ability to conduct heat from the plug tip to the cylinder head and hence to the water jacket. Hot plugs are good for"anti-fouling" if your engine is putting too much oil in the combustion chambers. Cold plugs can help if plug tips are getting too hot which can lead to pre-detonation. It's also worth pointing out that running too lean a mixture can lead to overly hot plug tips, mainly because the incoming fuel also acts to cool the plugs. Needless to say different manufacturers denote their heat range differently, but usually with a number. NGK; the smaller numbers go hotter; Champion; the other way round.
Allan Reeling

The double resistance is actually for two different things. Lead resistance (either carbon string or silicone) is historic to prevent interference with AM signals externally and internally, plug resistance is to prevent interference to modern on-board ECU systems. NGK says:

"As well as reducing electrical noise for radio, television and mobile telephones etc, many modern ignition systems require resistor plugs to stop electrical noise from interfering with the vehicle’s on-board electronic control units (ECUs). Resistor spark plugs should always be fitted where specified.

NGK automotive resistor spark plugs contain a ceramic monolithic resistor. The resistor filters out excessive electrical noise generated by the ignition system. The most effective place to situate a resistor in the high tension circuit is as close to the spark plug as possible making the spark plug an ideal place to house the resistor.

In nearly all cases – apart from some very old low output ignition systems – resistor spark plugs can be used in place of the non resistor versions with no detrimental effect on engine performance, power output or vehicle emissions."

So 'R' plugs are perfectly usable with suppressed leads of either type on our cars, I've been using them for years.

Paul, I wonder what NGK mean by "very old low output ignition systems?" I can't remember exactly where I read the piece on resister leads and plugs, but it seemed to make sense. Anyway I don't fit them, not an ECU in site???
Allan Reeling

I think they are talking about vintage and veteran systems. For our cars it doesn't matter whether they are 'R' types or not, and as ECU-equipped cars are by far the norm these days those plugs are more readily available.

I'd expect ECU-equipped cars that don't use coil-on-plug systems to need suppressed leads, and there must be many of those around.

However it is certainly the case that resistor CAPS, used with copper leads, should not be used with suppressed leads.

i only use NGK plugs on all my engines having had the sort of problems and sometimes worse than the ones you describe. But note that BPR---S types are resister and BP--S are not. MY 1840 and 1950 S/C both run happily on BP7ES
jim soutar

My 10 cents worth
Having a Morris 1000 racer on the rollers several times changing things around as far as the rules allowed it had to run a standard ign. system which included a normal 3ohm 12v ign coil
We tried resistor plugs and they would missfire under full load at higher revs and tried all sorts of plug gaps--and ended up with .025" to get good performance right through the range to 7500rpm
With the std 12v ign coil(Lucas sports coil) there was no way in the world that .035" plug gaps could be used, from 4000 rpm onwards on full throttle you could hear the ignition breaking down
I admit this is a full on high compression racer but it must relate somewhere down the line that the ordinary old basic ignition systems simply aren't strong enough to overcome any form of extra resistance
We had one of those Powerspark electronic pointless modules in it for a try and it was magic and big plug gaps weren't a problem but the rules are the rules---bugger

That'll be 10cents thankyou
William Revit

Gentlemen, thank you for taking the time to respond to my issue, I certainly got my moneys worth and Willy, your check is in the mail. Interestingly the symptoms are exactly as Willy describes with his racer. Maybe the next time that I'm ambitious I'll try a set of NGK non-resistor plugs and putter with the gaps if necessary. But for now I'll head Paul's advice and leave well enough alone.
Brian Denis

This thread was discussed between 26/04/2018 and 28/04/2018

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