Spark Plug Gap... let's think about this.

Discussion in 'Gen 3 Prius Technical Discussion' started by Nor'easter, Jan 3, 2020.

  1. Nor'easter

    Nor'easter Junior Member

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    Picked up some Denso SK16R11s for the 2010. Happily, they appear to be the genuine article (a pox upon the counterfeiters).

    The factory gap on these is pretty big at 0.044" or so, and this leads to a dumb question from a long time shade tree wrench and new-to-Prius guy...

    Given the incidence of cold start misfires on this engine, why aren't we running these plugs at something like 0.032 - 0.035"? Anyone got a good reason not to do this?
     
  2. bisco

    bisco cookie crumbler

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    how will that help with the unburned hydrocarbons causing the misfire?
     
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  3. Nor'easter

    Nor'easter Junior Member

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    Misfire is an ignition problem.

    Closing the gap increases the reliability of the spark when cold.
     
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  4. bisco

    bisco cookie crumbler

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    i'm not familiar with that cause, what does closing the gap do when the engine is running normally?
     
  5. Nor'easter

    Nor'easter Junior Member

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    As far as I know, the difference between (say) 0.035 and 0.045" on a normally running, warm engine is minuscule, probably not measurable. Now, I'm a materials & process engineer, not an automotive engineer, I might be missing something (hence the question at the top of the thread).

    The difference in spark reliability when cold is substantial. I have experience with small propane auto start engines (like you see in generators). In these engines, an 0.040 gapped spark plug (which is what the manufacturer was shipping) is endless cold start problems. An 0.030" plug (same plug, adjusted by yours truly) is 100% reliable. Smaller gap gives better ability to cope with any adverse conditions, whether that would be a little oil, condensed moisture, whatever.
     
  6. gliderman

    gliderman Active Member

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    Then by all means, change your gap. Mine works fine at factory specs.

    YMMV
     
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  7. Tande

    Tande Active Member

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    Would not the specs. of the ignition coil play a huge part in this??......just asking
     
  8. TMR-JWAP

    TMR-JWAP Senior Member

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    You may have bought Gen 2 spark plugs for your Gen 3....

    SK16R11 is for the 1.5 liter engine in the 2004-2009
    SC20HR11 is for the 1.8 liter in the Gen 3.

    or am I missing something about interchangeability?

    on another note.....every manufacturer recommendation in the universe for iridium plugs is to not adjust the gap. That being said, it's probably because most knuckle draggers will damage the small diameter center iridium electrode. But, we each pay for 'em, so we can each do whatever we choose to them.
     
    #8 TMR-JWAP, Jan 3, 2020
    Last edited: Jan 3, 2020
  9. Nor'easter

    Nor'easter Junior Member

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    We're both wrong :).

    Toyota changed heat range. The correct Toyota part number is 90919-01275 (you can look this up at e.g. McGeorge Toyota). The Denso cross reference is SC16HR11 which is what I intended to buy....

    So how the heck did I end up with SK16's....?

    Thanks for pointing that out. Fortunately returnable.
     
  10. Grit

    Grit Senior Member

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    Smaller gap size intuition? :LOL:
     
  11. Nor'easter

    Nor'easter Junior Member

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    I ran afoul of a sloppy search engine (piloted by a sleepy shopper) since I wasn't associating the application with a specific vehicle. I went back and entered SC16HR11 and sure as hell, half the hits were for SKs.

    Thanks again TMR!! I was probably not going to spot the error until I had the old ones out.
     
  12. Grit

    Grit Senior Member

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    Some people come in here with new ideas, they go practice what they preached and brought results back in here. And it led to new normal practices, I'd like to see you do that and report back. That's how the oil catch can was started in here.
     
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  13. Elektroingenieur

    Elektroingenieur Senior Member

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    As @TMR-JWAP kindly mentioned, in DENSO’s fascinating brochure, Spark Plugs: Discovering DENSO Technology (PDF), they write on page 62, “DENSO Spark Plugs are manufactured with a pre-set gap. [ . . . ] Do not change the gap on platinum, iridium or Twin Tip spark plugs, as the fine electrodes may get damaged.”

    Toyota’s Repair Manual (more info) series for model years 2010–2015 says the maximum spark plug gap for used plugs is 1.3 mm (0.0512 in.); for new ones, the standard gap is 1.0 to 1.1 mm (0.0394 to 0.0433 in.). As @Nor'easter notes, the heat range has been changed: the original spark plugs were Toyota part number 90919-01253 (DENSO SC20HR11), but the catalog shows this has been superseded by 90919-01275 (SC16HR11).
     
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  14. Nor'easter

    Nor'easter Junior Member

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    There isn't a lot of info on gapping plugs that goes beyond "adjust to manufacturer's specification". Here's one article that's consistent with my understanding: Signs & Symptoms of Improper Spark Plug Gap | It Still Runs

    As far as adjusting iridium plugs goes: they adjust like any other. The issue is that careless use of a feeler gauge could (very easily) damage the iridium tip. With just a little bit of care, damage can be easily avoided.

    My car doesn't have any history (that I know of) of cold start misfire. It's a non oil-burner and most trips get the engine all the way to normal operating temp. The OCC kit is still sitting under my dining room table, lol. If I were changing plugs today I'd gap them to 0.035" as extra insurance against cold start misfires.
     
  15. Elektroingenieur

    Elektroingenieur Senior Member

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    In general, that’s so—all the way back to the Ford Manual from 1921 (see Answer No. 31; it’s 1/32").

    Bosch gives us a few hints about how the gap is determined. In Gasoline Engine Management: Systems and Components (2015), they write (p. 186):

    Engine manufacturers use various test procedures to determine the ideal electrode gap for each engine. The first step is to conduct ignition tests at characteristic engine operating points to determine the minimum electrode gap. Salient considerations include exhaust emissions, smooth operation and fuel consumption.

    In subsequent extended test runs, the wear performance of these spark plugs is determined and then evaluated with regard to ignition-voltage demand. The specified electrode gap is then defined at a point providing an adequate safety margin to the miss limit. Gap specifications are quoted in vehicle owner’s manuals as well as in Bosch spark-plug sales documentation. Bosch spark plugs are set to the correct electrode gap at the factory.​
     
  16. ASRDogman

    ASRDogman Senior Member

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    I believe every manufacturer knows what type of spark plug and what gap it should be at.
    Any changes that "might" help could only be minor.
     
  17. Grit

    Grit Senior Member

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    It might even draw lower MPGs from those who installed off brand sparks plug and counterfeit ones also.
     
  18. elementnomore

    elementnomore Member

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    Pulled 20s out of my 2012 today, 98,000 miles. Dealer sold me 16s as current spark plug
     

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  19. bisco

    bisco cookie crumbler

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    did you regap them to .032-.035?
     
  20. Nor'easter

    Nor'easter Junior Member

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    Is anyone else impressed at how crummy used plugs out of Gen III Prii look?
     
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