Whats the difference between plugs?

[chassis 33]

I've just replaced the spark plugs on my SD1 with a set NGK BP5EFS, which is what my local auto factor listed for the engine. However what came out was a set of NGK BP6EFS. Out of curiosity, what difference does a number make to these plugs i presume its something logical like the next size of gap one way or the other? I havent been out on the road yet, but it does seem to of gone a fair way to curing a slight misfire on idle. Regards Iain

[stevieturbo]

6's run slightly cooler, but should still be fine in your car.

The numerical part is just the heat range of the plug.

[chassis 33]

Oh right, cheers. The engine wasnt getting up to temperature on the gauge anyway, but thats more British Leyland electronics or a dicky thermostat

Not that its concerning my hugely, but should i look out for pinking now?

If i remember correctly from my thermodynamics and powertrain lectures at uni, hotter plugs equals more NOx and less CO forming??? or is it the other way round, or am i talking horlicks...i'll get my textbooks out

[stevieturbo]

hotter plugs mean less chance of fouling, although in a high power application can lead to detonation, as the plug will get too hot and pre-ignite the mixture.

So really want to run as hot a plug as you can safely.
Or as cold a plug as you can, without fouling.

[deltafox]

BP5 EFS

B=thread diameter-14mm
P=projected nose insulator
5= Heat rating- 5 is hotter than 6
E=Thread reach-(19.0 mm)
F= Tapered seat plug (is this right???)
S=Super copper core.

HTH. Ps your gap should be 0.9mm.

[chassis 33]

deltafox said:

BP5 EFS B=thread diameter-14mm P=projected nose insulator 5= Heat rating- 5 is hotter than 6 E=Thread reach-(19.0 mm) F= Tapered seat plug (is this right???) S=Super copper core. HTH. Ps your gap should be 0.9mm.

Is the hotter plug why the engine now occasionally appears to be "running on" a moment after I switch the ignition off, ie localised temperatures are high enough to detonate any fuel/air vapour in the cylinder despite there being no spark? That never happened before, although happens a fair bit on my dads 4-pot SD1.

BTW how do you know it should be 0.9mm?

Regards
Iain

[Pigeon]

Might be. What colour is the nose of the plug?

[deltafox]

chassis 33 said:

deltafox said: BP5 EFS B=thread diameter-14mm P=projected nose insulator 5= Heat rating- 5 is hotter than 6 E=Thread reach-(19.0 mm) F= Tapered seat plug (is this right???) S=Super copper core. HTH. Ps your gap should be 0.9mm. Is the hotter plug why the engine now occasionally appears to be "running on" a moment after I switch the ignition off, ie localised temperatures are high enough to detonate any fuel/air vapour in the cylinder despite there being no spark? That never happened before, although happens a fair bit on my dads 4-pot SD1. BTW how do you know it should be 0.9mm? Regards Iain

Could well be why the engine runs on. You could try a colder grade of plug to cure it.
Really you want the coldest grade of plug that wont foul, as it also gives the best power output.

As regards the plug gap; I have the NGK spark plug manual......sad git aint i?

BTW, was i right about "taper seat" plugs?