Spark plug conundrum

[Ashful]

Several of you may remember that I hot-rodded my log splitter a few years back, dropping the cycle time from 11-12 seconds down to 8 seconds. This was done by increasing the size of the pump, required line sizes, and replacing the engine with one of nearly double the original displacement.

It's time to replace the spark plug on said engine, and I've run into a surprise. The engine is a Briggs Intek 21R707, and the manual that came with it calls for two spark plug options (for engine type 210000):

Briggs & Stratton 491055 = Champion RC12YC, for regions requiring a resistor type plug
or
Briggs & Stratton 5066 = Champion RC12PYC for those wanting a long-life platinum plug

All are heat range 12, flat seat, all the same dimensions.

However, when I removed the original plug from the engine, I found it marked Champion XC92YC. This plug also has all the same dimensions, but is heat range 92(!). Engine was bought new from Jack's Small Engines, and I don't believe they messed with it, so I'm scratching my head as to what they might've been thinking at Briggs.

At first inspection, the plug seems to be doing well in the engine, not coated over with black carbon as you might expect for a plug that far above the recommended heat range.

I'll continue to dig for an explanation, but given the wide experience and knowledge base of this forum, I'm wondering what the rest of ya'll think about this.

 

[Ashful]

Bueller?

 

[Jags]

Just a thought, but could that plug have been spec'd for a specific region (i.e. Alaska use vs southern florida?)

 

[Ashful]

Just a thought, but could that plug have been spec'd for a specific region (i.e. Alaska use vs southern florida?)

That would make sense. Just wish I could find some literature to confirm it.

I did contact Jack's Small Engines, where I bought the engine, to see if they had any idea why they shipped me an engine with a plug different than the literature. They had no idea, even implied it may have just been what Brigg's had in stock. I think they're letting current COVID parts shortages influence their assumptions, but this engine was bought in 2017.

I'm leaning toward just replacing it with long-life platinum version the plug listed in the literature that came with the engine. The only thing that gave me pause is that the old plug actually looks pretty good after 5 years at 10 cords per year, despite the heat rating being WAY off the recommended plug.

 

[Jags]

go with the long life dude and carry on smartly.

 

[SpaceBus]

I would just keep using the plug that was in there. Sounds like it's in good shape and no need to replace. Get a platinum plug for peace of mind, or replace the old and and keep it for a spare. Obviously it works fine. Heat ratings are probably not that important in a small naturally aspirated stationary engine. Plug temp ratings are much more important when you are talking about a forced induction gas engine and you are fighting detonation due to high cylinder pressures.

 

[brenndatomu]

despite the heat rating being WAY off the recommended plug.

When looking those 3 plugs up on a conversion chart, they all convert to heat range 5 in NGK brand (just what I'm familiar with)
so I say grab whichever one the local parts house has n run it...personally I'd lean toward going back to the one it came with since it has performed well and still looks good.
And just FYI, even if you did select a much "hotter" plug, it really doesn't affect the engine, just the plug itself...it would not make the engine run hotter, or the combustion chamber burn hotter, it just has to do with how the combustion chamber heat is dissipated through the base of the plug to the cylinder head...so how "hot" the plug itself runs...if that makes sense...

 

[Ashful]

Yep, makes total sense, I understand heat ratings. That’s why I’m surprised a 92-rated plug looks good in an application spec’d at 12. I’d have expected the plug to be somewhat gunked up, being that far off the mark.

The fact that NGK converts all three to the same temperature is a surprise, but may hold the key to the answer. Is 12 and 92 not as far apart as I may have assumed? I’m not sure what the scale is for Champion’s heat ratings system.

 

[brenndatomu]

Is 12 and 92 not as far apart as I may have assumed? I’m not sure what the scale is for Champion’s heat ratings system.

Its the same...1-25 is "automotive and high performance" series plugs...76-99 is "special and racing" series plugs.
The second number is the actual heat range...so the 2 in 12 and 92 means same heat range.
26-50 is "aircraft", and 51-75 is "racing"...

 

[Nate R]

X is a different resistor for better RFI, IIRC. This may have swapped for that in some build specs to pass stricter RFI in some areas (Canada, I think?) As someone who works in R&D on small engines for a living, I can tell you any of the 3 choices would be fine for your application. I wouldn't bother with the platinum if you're going to actually change them out on a schedule like it sounds like you're doing.
Kudos to you for actually maintaining the engine and trying to do the right thing. Service/owner's manuals often cover a wide array of engine specs, and it's likely when they wrote the literature, everything in that line DID use an RC12YC. But likely when some specs were changed out, they didn't change the literature.

 

[hedge wood]

I started using NKG plugs probably thirty years ago when we starting sand railing with VW air cooled engines. They just held up the best of all the different brand of plugs we tried. They are in all my air cooled engines. Just last summer was getting ready to put a Grasshopper mower on the market with a Kohler 25 HP engine. Thousand hours on the motor and I looked in the record book and I had put NKG plugs in it at five hundred hours. So and I bought a set to replace them I couldn't believe how good the old ones still looked after five hundred hours of warm weather running.

 

[brenndatomu]

NKG plugs

Big fan here too...now Champion, total opposite opinion.

 

[SpaceBus]

Big fan here too...now Champion, total opposite opinion.

I bought a set of Champion plugs when I was a teenager based on stuff the old timers said about them. Big mistake, my truck wouldn't even run right. I couldn't figure it out for the life of me so I called a mobile mechanic. He put NGK's in it and suddenly the truck worked, and I felt pretty dumb.

 

[Ashful]

Thanks, guys! I knew the collective knowledge this forum would shine thru. I had no idea on 12 and 92 being the same heat range, or the fact that X was just a different resistor. Thanks!

 

[Ashful]