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Old 06-15-2013, 10:29 PM   #1
Plaka OP
Brevis illi vita est
Joined: Jun 2010
Oddometer: 4,615
If a 6 was a 9, a dual plugging dilemma.

'83 R100RS, 1000cc motor, 9:1 compression, new pistons and full new valve job, nicasil in great shape. Every kind of riding from city traffic jamming to WFO on the highway and mountain passes---winter to 30F or so, summer to over 100F, solo, two up, big loads, everything.

14mm 3/4" reach upper plugs, 12 mm 1/2" reach lowers.

The problem is the plug heat ranges. In the past, with a worn low compression motor and 14mm x 3/4" plugs top and bottom (1/2" reach bottoms) I ran a one step hotter plug on the bottom as an anti-fouling move. Worked well. The lowers came up to self cleaning temp. quickly and started working and didn't get hot enough to make pre-ignition points.

Now I'm thinking same heat range top and bottom, and stock. Nice tight top end, minimum oil burning, risk of detonation from the compression and hotter running anyway.

BUT, such plugs are not to be found. Closest I can come is either one hotter or one colder on the bottom using a stock heat range on the top. If I go One colder or hotter on the top then I can match it on the bottom. But that stock top plug (I'm going with an NGK BR7ES for the top, same as as Bosch W5) is not available in a 12mm short reach. I can get heat ranges of 5,6,8 and 9...but no 7.

So do I go hotter or colder on the bottom? Or change the top and match it on the bottom? And if I change the top go hotter or colder? Is the heat rating absolute or relative? That is, does it account for the thread diameter and depth of engagement?

I took a look at Mottorad Electrik to see what Rick is selling, as he sells NGK. He is selling 7's for the 1000 cc motors from 81 on which would be the lower compression, and 8s for the pre-81 1000cc, which would be the higher compression. The 8 is a colder plug than the 7. He is also only selling 8s in 12mm short reach. So I infer his opinion is go one colder on the top and match it on the bottom. The NGK 8 = Bosch 4

Cutter, whose printed NFO is a bit old, goes with an NGK 6 top and bottom for a 90S and '77 R100RS, high compression engines. This would be a grade hotter.


Side issue, I have some 14mm by 1/2" reach Bosch Platinums, NIB, in Resistor heat range 8---A very hot plug...If someone is in need.
Shipping+Donation terms.


Other side issue on plug indexing. Interesting info on NGKs site. Apparently good for 1% performance increase (in HP)--if you point them the right way (towards intake or towards exhaust) and it takes a dyno to sort that. But w/ dual plugs, you could point one each way....

Methinks a .7 HP gain might register on the butt dyno. If I can pull some more with intake cooling I might get another full HP out of the thing, always nice.

Plaka screwed with this post 06-15-2013 at 10:47 PM
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