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John@Outfront Mtrsprts

lets learn about something today......issue on the dyno

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every now and then I have an issue that kicks my butt on the dyno.  Current customer, just freshened up his motor (550hp stroker) made the first few pulls and sounds great until you get to 4-6psi of boost or higher and all hell breaks loose. popping, miss firing  really sounded like one plug.  *new plugs were just installed with motor.  some times its easier to just change parts.  First plug wires, then coil, then fuel pressure check under load.  still have the same issue.  now we are looking at ecu or injectors but im out of 120# injectors until Monday :(  so just because its easier then wait) and car on dyno we substituted some smaller injectors and did a quick retune for smaller  injectors.  same thing :( now its ecu or wire harness time.  on a whim we pull out the new plugs and found a hair line crack in the porcelain.  that was the issue.

plug gap is super critical to turboed motors, we never use iridium or platinum plugs, once fouled they may not come back as easy.  many don't know that the coil only puts out what is needed to jump the gap,  spark can jump easier in a vacuum then with pressure.  we are living in 15psi of pressure (one atmosphere) and in the motor while running the pressure goes up way more at time of combustion then outside the motor where you might find the same coil can jump a much larger gap. so the gap  in the motor (at the sparkplug) must be small with the added pressure.  now in a turboed car the pressure can be doubled even up to four times the pressure than normally found.  as you can see as the pressure goes up, so is the requirement for the coil to put out even more power.  this continues until the spark is too weak or the pressure is too high.

but the spark is just like water down the river.  it takes the path of least resistance.  the crack you see opened up a path from the electrode through the porcelain and out to the hex of the sparkplug base, but it would only occur when the pressure in the cylinder is so great that jumping through the porcelain "outside the motor" where pressure is less but distance may be way more.

I like coils as they can be seen like people (they rise to the occasion) when work needs to be done.

Cheers

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Edited by John@Outfront Mtrsprts

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sounds like a cracked plug.

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I always said you were a smart man.................................

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I have had the ceramic crack inside the cylinder causing it to spark up in the plug body  against the inside of the threads. My buddy didn't believe me till we changed the plug

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Nice find, easy fix. NGK coppers are all I’ve ever run in big boost engines as well, usually 0.016-0.020”, depending on ignition setup. 

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Was this crack due to heat? Possibly due to something mechanical hitting it (a wrench?) Possibly just a defect from the factory?

 

Any ideas?

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2 hours ago, journeyman said:

Was this crack due to heat? Possibly due to something mechanical hitting it (a wrench?) Possibly just a defect from the factory?

 

Any ideas?

i feel it came out of the box that way.  the plug was installed on a subi sohc. the sparkplug tube is super long so no way to hit it, go sideways to break it

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I'm guessing no cylinder misfire data from the ecm?

Would have seen the short spike on a scope too. LOL

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I have had this happen at my dyno shop here at home, but was lucky catch.   It was my HRT and after a few pulls my dyno guy wanted to look at a plug to see if all was good.  I pulled one out and all was good so I put it back in.  The very next pull the same thing happens that John just described.  First thing  my dyno guy said is you cracked the plug putting it back in.  Sure enough when I got it out there was a hair line crack just where this one is shown.  I believe, In using the small plug socket if you get it misaligned any at all when torquing, it will put side pressure on the plug.  Subi's have deep plug cavities and this is easy to do.  

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I love it when I learn something new, Thanks! 

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I would have been throwing money at it. I never think of small things like this. Good info!

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I've broke a plug or two by the slightest angle in my socket/ratchet. They are sensitive sumbitches. Could also be the cause of a couple brewskiis. Thanks for the ed though. This is awesome to keep in mind when diagn'!

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On 11/9/2019 at 11:42 AM, Legit Duner said:

I would have been throwing money at it. I never think of small things like this. Good info!

When running into issues like this, I always start moving components around.  Whether or not the problem moves with the components help lead to you the next steps in diag.

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