Spark Plug confusion

[Pogo123]

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.<br /><br />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.<br /><br />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.<br /><br />He said a lot more ... most of which made sense (sounded good), but now I'm confused.<br /><br />I concede 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.<br /><br />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.<br /><br />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 and smoother running (like after a period of low speed trolling), then I'm all for it.<br /><br />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.<br /><br />In short, ... if I switch, am I asking for trouble?

 

[WillyBWright]

Re: Spark Plug confusion

If they don't fit, the sparkplugs will push the gap to nothing, they won't fire, and the motor will die. All in the first few revolutions, so it probably wouldn't even start in the first place.

 

[John_S]

Re: Spark Plug confusion

I just checked the NGK website for both of my Nissans (18E & 40C). Prior recommendations for both was B7HS-10 or BR7HS-10, and gapped to 0.040. The NGK lists for the 18E the projected tip BPR7HS-10, with a gap of 0.032. The 40C still has the the old recommendation. Not sure why one changed, and not the other, and why the gap is reduced with the projected nose.<br /><br />As far as projected tip, in general, what the rep said is equivelent to what I found in Dennis Moores book on small block chevy performance. I will be installing the projected nose plugs in my mercruiser, this spring. <br /><br />I would like to try them in my Nissans, if there are not any clearance problems. I don't know your engine, but on both of mine, the plug is angled into the combustion chamber. It is probably easy enough to get the piston at TDC and measure the clearnce with plug removed. Worse case is when you torque the plug down and the tang would be oriented toward the piston.

 

[Pogo123]

Re: Spark Plug confusion

Thanks for the replies, guys.<br /><br />John ... your Nissans were made by Tohatsu, but it's good to note that they recommend the same plugs. NGK is consistent, anyway. <br /><br />I do wonder, however, why they call for a "-10" plug (pregapped to 1mm or .0396") and then call for a .032 gap. A plain HS series is a touch cheaper and doesn't guarantee that 1mm gap.<br /><br />And I agree with you about the old tuning ideas. I raced Gran Prix motocycles for decades and found a lot of what the rep said to agree with ideas I had known from those experiences in engine performance tuning for years.<br /><br />I've queried both Tohatsu and NGK, but haven't heard anything back yet. Failing any response, I'll do the modeling clay trick, pull it through and see for myself just what the clearance is.<br /><br />I don't know, John, if you have a small problem with your Nissan 18 running smoothly after a period of low RPM, such as in trolling. My 15 has always stuttered a small amount until I "blew it out" after low RPM operation. Supposedly, the projectd tip plug will help if not cure that.<br /><br />One other consideration I have is the "total burning" idea. 2 strokes rely on unburned fuel for both lube and cooling. I'm not sure what sort of chamber temp change messes I might get into by burning all (or nearly all) the mixture. That might, at the least, call for a different heat range plug.<br /><br />And to think ... all I did was ask for two plugs ... <br /><br />We'll see.

 

[John_S]

Re: Spark Plug confusion

If I troll for an extended time, with either of my Nissans, or any of the Evinrude/Johnsons 2 strokes I've owned, have had some missfire when going to put it on plane. Once its cleaned off, runs great. Of note, these are all premix engines, no auto oil mixing or variable oil injection. If I troll with the engine in the shallow water running mode, it seems to be worse. Probably because these are float bowl carb motors. I wouldn't expect it to fix that issue 100%, but any improvement is welcome.<br /><br />

One other consideration I have is the "total burning" idea. 2 strokes rely on unburned fuel for both lube and cooling. I'm not sure what sort of chamber temp change messes I might get into by burning all (or nearly all) the mixture. That might, at the least, call for a different heat range plug.<br />

I am not an engine expert, but I believe most of the 2 stroke lubrication happens before the mixture enters the combustion chamber. ie when the fuel/oil mixture is in the crankcase. The heat range is the same on the regular and projected tip plug. Shouldn't be an issue. I have run one heat range hotter plugs in my old evinrude/johnsons, when doing extended trolling, and short full throttle runs. I did get less fouling, but was always worried about forgetting I had them in there, when doing long WOT runs.

 

[Pogo123]

Re: Spark Plug confusion

O.K. .. I finally bit the bullet and figured this out for myself ... sorta.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />So much for the plug manufacturer's tables ...<br /><br />Micrometers and dial indicators can be wonderful things -