Whats the difference??

It's just some sort of heat rating, isn't it? One plug would be better suited to running at higher revs, the other better for plodding, something like that.

 

yes it is Fig - and on an NGK the bigger the number then the 'hotter*' the plug - i.e. an '8' would not oil up in traffic as much as a '9' the 'R' is for resistor which is also worth thinking about as it gives the ignition a slightly harder time of it particularly if the spark plug lead is already a resistor type ! (I use D8EA on M900 Monster and television not affected ) Best to try both Cornish but just be careful of constant high speed runs in hot weather if using the '8' and make sure plug is not running too hot.


* allthough this is the standard term used it's a bit confusing as you could see it as a contradiction of it's application - to explain in a different way :- a '9' is more capable of running in hotter conditions than an '8' (NGK only) - sometimes referred to as a 'harder' plug also.

 

I use the 9, the 8 will allow the bike to start easier, and runs smoother in traffic, but when you get to high revs they can misfire spradically on hi compression/tuned engines. My 900 was running 8's but when i got it tuned baines fitted a 9. recently when i got it back on the road, I got to try the two back to back, same route, same time over two consecutive days and found the 9 better overall for a tickled engine, but yes, the 9 is more prone to oiling as its better suited to a good spirited run.

 

saying about the (r) resistor plugs i have had seen a lot of problems with these
lovely big blue spark but engine will not start put in a non resistor engine runs fine
had a lad put a new r plug in his bike (even though it was running well on the one in the engine) wouldnt run
as soon as old plug back in ran fine
had a few bikes fail to run on new resistor plugs .....
thats my tuppence worth lol i just try to avoid them one less problem

 

taking the conversation off on a tangent briefly, has anyone gone down the road of the surface discharge plug on our engines?

 

For a standard bike is use the recommend plugs. If everything is working properly they will be fine.

 

Here's a thought...on a DS engine, how about using an 8 one side and a 9 the other? Shouldn't do it any harm, surely?