Spark Plug confusion

Discussion in 'Bubba's Outboards' started by Pogo, Apr 2, 2006.

  1. Pogo

    Pogo New Member

    Messages:
    96
    State:
    North Carolina
    I recently went in to buy a couple new plugs for my 2001 Tohatsu M15D2 and ran into the NGK rep who often visits Marinas in the area.

    As I told the parts guy that I wanted two NGK BR7HS-10 plugs, the rep commented that I'd get better performance over a wider rpm range using BPR7HS-10 plugs.

    When I asked him why, he replied that the "projected tip" puts the spark further out into the mixture where it more easily burns the entire fuel charge and does it more consistently.

    He said a lot more ... most of which made sense (sounded good), but now I'm confused.

    I admit that his theory is correct, but the "P" tip extends about 1/8 or a bit more than the standard plug. I'm concerned that there isn't enough clearance to handle that much extension into the combustion chamber.

    Out of curiosity, I went to the NGK site and, sure enough, they recommend the projected tip plug for my engine. I can't find anythng about this on the Tohatsu site, but the "sticker" inside the cover says "B7HS-10" so I've been running either B7s or BR7s.

    I know that technology changes and I don't know what NGK recommended back in 2001 when this engine was produced. I only know what they are saying now. If, however, a change would, as the rep says, give me cleaner, fuller burning (like after a period of low speed trolling) and smoother running, then I'm all for it.

    I could, I suppose, put some clay on a plug to mimic the projected tip size and pull the engine through a couple times then check to see if there was any contact, but ... I figure one of you guys should already know the answer.

    In short, guys .. if I switch, am I asking for trouble?
     
  2. explayer

    explayer New Member

    Messages:
    372
    State:
    Tucson AZ
    I would say yes your messing with trouble
     

  3. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    Send the question to Tuhatsu or NGK and ask about the clearance.

    It's not always good to have a more complete burn. The 2 stroke relies on the unburned fuel to maintain cyl head temp, and combustion chamber temp.
     
  4. cat daddy 791

    cat daddy 791 New Member

    Messages:
    141
    State:
    Kansas- Wichita
    I Would Install The Plug, Crank It Over Without The Plug Wire On It And See If It Hits. It Won't Hurt It, If It Does Close The Gap On The Plug. Just Take It Back To The Parts Store And Replace It With The Same One That It Came With. Ngk Makes Great Plugs And I'm Sure They Did Their Homework Before They Listed It As The Recommended Plug For Your Application.
     
  5. Pogo

    Pogo New Member

    Messages:
    96
    State:
    North Carolina
    O.K. .. I finally bit the bullet and figured this out for myself ... sorta.

    I took a standard reach plug, built up the nose with modeling clay to mimic the shape and reach of the projected tip plug, installed it and pulled the engine through by hand.

    There IS clearance, but it's so very small you wouldn't believe it. So small, in fact, that I'm going to do this again after I've run the engine up to temperature which will allow for thermal expansion. I mean, we're talking a couple thousandths here.

    So, for those of you interested, yes, there is clearenace, but so little that I'm still not satisfied. At RPM and warm, everything swells and the rods get just that much longer ... enough so that I'll do this again with an operating temp engine.

    Other engines might be just fine, but if what I'm seeing is typical, with this engine, I'll stay with the short reach standard plug and put up with a loaded up plug after a period of trolling.

    So much for the plug manufacturer's tables ...

    Micrometers and dial indicators can be wonderful things :)