If a 6 was a 9, a dual plugging dilemma.

Discussion in 'Airheads' started by Plaka, Jun 15, 2013.

  1. Plaka

    Plaka Brevis illi vita est

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    '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.

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    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.

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    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.
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  2. ME 109

    ME 109 Long timer

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    Why was 6 unhappy?
    Because 7 8 9.
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  3. Plaka

    Plaka Brevis illi vita est

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    Don't you need to change the air in your tires?









    ( :lol3)
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  4. disston

    disston ShadeTreeExpert

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    I don't pretend to know the answer to this one (like I pretend some other times :lol3). But since you mention Cutter. I believe he has stated that he realizes the paper on dual plugging is old but maintains the info in it is still correct. I guess you could try it the Cutter way and change later if that doesn't work?
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  5. pommie john

    pommie john Long timer

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    Any reason you can't run a spacer ( thick washer ) on the plug so you can use a long reach plug in the short reach hole?

    We used to use copper spacers when I was racing Yamaha 2 strokes.
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  6. Plaka

    Plaka Brevis illi vita est

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    Cutter mentions this. He also states it should not be done long term but as a stopgap only. Screws up the heat range. Also two gaskets to leak instead of one (my own thought). He gives the dimensions and says Honda dealers carry them.
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  7. Plaka

    Plaka Brevis illi vita est

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    His spark plug tables haven't kept up with the newer bikes...Only go to '79. Also some of the plug designations are obsolete--but you can figure out where he is going. I'm thinking High compression R100...so pretty similar to the '77.

    I could do trials of this and that, but it eats cash (which I have little of), I may have to order some of them off the net which eats time, and I end up with leftover pugs I can't use. That's where those 14mm short reach platinums came from. One heat range hotter was enough, I prepared to go two up front---wasted time and money.
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  8. supershaft

    supershaft because I can

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    Not all heat range conversion charts are the same and the ones that are usually are for repeating the same mistakes as others. I learned that lesson a long, long time ago using different brand plugs in my dirt bikes.
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  9. Plaka

    Plaka Brevis illi vita est

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    It's looking like NGK 8's on the top and matching 8s on the bottom. I figure colder is better than hotter. The motor will run plenty hot anyway so the worst that can happen is I have to warm up longer to get the plugs firing well. However if I go one hotter anywhere I risk creating a pre-ignition inducing hot spot in an already detonation prone motor. That could mean engine damage.

    Also thinking of going with the irridiums. I always liked my Bosch Platinums. Better flow around the plug nose. Haven't found indexing washers yet.
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  10. chasbmw

    chasbmw Long timer

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    On my 90/6, with 14mm dual plugs, I use hotter running plugs on the bottom as an anti fouling measure, using iridiums on the 90/6 and NGKs on the 1070. The 90/6 was using a bit of oil as the rings did not bed in properly first time round, so last winter had the cylinders lightly glaze busted. Seems better now.

    Jim Cray who did the work and is the main man for dual plug setups in the UK, told me straight that he has never had any head cracking problems by using 14mm lower plugs.
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  11. Plaka

    Plaka Brevis illi vita est

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    I had upper and lower 14mm on my 75-->90/5. On this one I let Ted Porter make the call and he called 12mm lowers.

    I can get NGK Irridiums in an 8 for the top, but only the plain janes for the bottom. I'll start with plane janes everywhere. I'm using resistor plugs and could be nice to have fresh resistors every so often.

    I always liked the bosch platinums, but you need 14mm to run those on the bottom. I like to burn a little oil, the motor was designed for it. But I got a feeling this is going to be a hot runner. It has the stock oil cooler but no thermostat, the Briel cooler on the pan and I'm considering some dead loss water cooling as well. I Have it fitted with a lot of temperature instrumentation. Will have to see.
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  12. supershaft

    supershaft because I can

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    I have seen and machined a lot of heads for 14mm bottom plugs. I have only seen around four crack. I have never seen one crack around the top 14mm plug. All four cracks were between the bottom 14mm crack and an exhaust seat. A couple of the bottom plugs were machined noticeably off center and closer to the exhaust than the intake but I know from experience that getting all that lined up takes some time that apparently not all are willing to make. I have seen a lot of bottom plugs favor one side over the other. I had 12mm plugs put in my dual plug setup.
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  13. RGregor

    RGregor Been here awhile

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    Hellp Plaka,

    I've been running botton 14mm plugs in my modified 90/6 engine a long time ago. No problems with cracked heads. Heard of but never saw one.

    With the 1000ccm and 1070ccm engines I always ran 12mm bottom plugs (heard of cracked heads and didn't want to see mine crack).
    Long plugs with an aluminium spacer.
    Always NGK BP7ES for the top 14mm plug and NGK D7EA for the bottom plug.

    No pinging/fouling or whatsovever problem at CRs of 11.7 for the 1000ccm engines (tested up to 12:1) and 11.2 for the 1070.

    Best regards,

    RG
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  14. Plaka

    Plaka Brevis illi vita est

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    Thanks for the NFO!.

    I also ran 14mm lower plugs in my /6 heads with no problem. These are drilled 12mm so I'll work with that.

    I got NGK BR8ES for the top. Doesn't have the extended nose of of the BP~. See how it works. Then DR8HS for the bottom. Local auto parts places had them and I figure if I can get them in this hickhole I can get them anywhere. Cheap enough to try out and not cry if I want something else.

    if they work well I'll go to Platinums or Irridiums. I'm reconsidering spacers for the lowers to make that happen. I'd feel comfortable with threaded steel spacers installed on the plugs.

    I have some GR5 plugs I just took out of the old heads and I think I'll take a hacksaw to them. The lower body may be just right to fabricate a set of spacers. I have neither a lathe nor a surface grinder so I would have to have the work done by a machine shop.
    #14