Protruding electrode spark plugs

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BOETJE

Junior member
Location
NEW ZEALAND
Anybody knows if protruding electrode spark plugs can be used in a 750 SF-2 , with the big ( Ø 41.5 mm inlet ) valves ?
I forgot to check that when I had the head on the bench..
 
Any reason you wouldn't just run std plugs? I've run NGK B9ES in my SF since it was 2 years old - raced, toured, commuted, fanged, raised compression, fitted SFC cam and valve train, 2/1. What's the advantage of protruding electrodes?
 
Andy Wagner's catalogue was where I first even heard about them. I tried a set years ago in my 3C. Didnt notice any difference other than me shitting myself with worry on the first start. The P bit stands for Projected
 
Almost as nervous as when I had some rear shock mounts welded to an Alloy swingarm I added to my XT500. I was really nervous the first few rides with that mod.
 
I always have used N2 , or NGK B9 in the past ( late 70's early 80's )
Sometimes an engine runs /idles better with protruded electrodes I have noticed .
Flame starts more " in the center" of the combustion chamber , All depends on the swirl/ squish pattern though.
Since I had a couple kicking around , I thought I might as well try.

I have always been dumbfounded that such a low horsepower / low compression engine needs such cold plugs.
I suspect it has something to do with the cast in combustion chamber, that doesn't allow the pug to get rid of it's heat
 
I presume the only thing you're concerned about with the protruding electrode is piston or valve clearance. Put them in the engine and turn it over by hand. If it doesn't go "clunk" and stop turning, then it should be OK to run.

As you're moving the ignition point a couple of mm closer to the centre of the combustion chamber, the flame front will presumably arrive at the piston crown a wee bit earlier, so you may need to think about ignition timing. I'd guess that the "P" type plugs would not need quite as much advance as the normal type.
 
Its probably like indexing a plug, aiming the plug open end at the inlet valve, if it make you happy why not.
 
Yeah...never quite understood the indexing thing...
They are in there , ( after some measuring ) and nothing went "clunk ".
Might be different if the tip falls off....
 
I've never quite got the point of indexing plugs so the open end of the side electrode faces in a particular direction. I've seen recommendations to face the plug gap towards the inlet valve, and others that say to face it to the exhaust valve. I dunno why facing towards one valve or another can make much difference because both valves are closed when the plug fires. So I did some further reading, only to find that some engine tuners don't refer to the valves at all. Some say the open side should face the squish area, others say it should face the piston crown. The only conclusion I can draw from all the different recommendations is that nobody really knows, and that the whole indexing plugs thing is nothing more than a myth.

The only indexing that makes sense is in a 2-valve head where the plug is off to one side of the combustion chamber. If it overlaps the edge of the squish area, there's a possibility that piston clearance could be an issue in a high compression engine. In that case, it may be necessary to to keep the side electrode away from the piston crown.
 
BOETJE said:
They are in there , ( after some measuring ) and nothing went "clunk ".
Might be different if the tip falls off....
Click to expand...
I use NGK B8ES in my Jota, but one day I had a brain fart and put a couple of them in my Ducati MHR.
The Ducati's normal plugs are NGK B7HS. Those of you familiar with NGK codes will already have spotted the potential disaster. B8ES have long threads while B7HS have short threads. That meant the plugs I'd put in the Ducati were protruding 1/4" further into the combustion chamber than they were supposed to. I actually started and ran the engine before I discovered my mistake. Fortunately, there was sufficient clearance between the plug and piston so nothing went bang, but I almost shit myself when I realised what I'd done.
 
On one of the Snowy Rides, my bike suddenly started running on 2. The bad plug for pulled and the gap was closed up. It was late in the day and we had approx 50k to go to get to the accommodation so I gritted my teeth and rode there on 2. That night I was asking around as to what might cause this and one helpful comment was maybe something had hit it. With a new plug, I made it home but after using a Borescope we found a large chunk of piston ring land missing adjacent to the inlet side eventually put down to Detination. Happy to avoid stuff hitting pistons
 
BOETJE said:
I have always been dumbfounded that such a low horsepower / low compression engine needs such cold plugs.
I suspect it has something to do with the cast in combustion chamber, that doesn't allow the pug to get rid of it's heat
Click to expand...
Might be more to do with 40 deg full advance? I don't think the SF head runs hot - have a look next time you have the head off at how much air circulates over the top of the combustion chamber, all these amazing casting inlets and outlets. The twins don't need oil coolers - definitely not on the road, and even arguably for racing. And for n air cooled twin they aren't really that 'low' on horsepower. Once you ride them over 6000rpm they don't feel like a detuned plodder, that's for sure.
 
I use BP in my Jota. For no other reason that [when I had the head in my hands] I felt the spark-gap position using standard plugs was a bit low - slightly sunken, if that makes sense. I found the P type ran very slightly hotter, probably half a grade. The arse-o-meter didn't really notice a performance benefit, but it just looked better in the metal, so I stuck with it. No problems to report.
 
I ran B9ES in my jota for a while when I couldn't get 8s. I was expecting them to get all black and foul up but the bike ran perfectly well on them. I think Clem's right that these old tractors are pretty tolerant.
 
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