HO spark plugs in a Renix

[DirtyComanche]

I'm doing my stock parts reorder for the fall, and noticed that the HO uses a different spark plug than the Renix.

 

The electrode protrudes farther on the HO (projected type) and it's a heat range hotter (9 for the Renix, 12 for a HO, using Champion P/Ns, but that represents about one step for most manufacturers).

 

Any comments on why they went to a hotter plug, or the projected tip (the projected tip may be why it is hotter)?  I wouldn't say I've ever had fouling issues with the Renix.  Has anyone thrown the HO plugs in there Renix and found any change (positive or negative)?

 

Champion plugs pictured for comparison.

HO:

 

Renix:

[gogmorgo]

There was a head redesign between HO and Renix. While I can't state with certainty, I imagine the shift in plugs would be related to how they sit in the combustion chamber. If the HO plugs stick out that much further due to the longer electrode, they'd see higher temperatures, hence the need for a hotter plug. I've never had a Renix head off so don't really know what they've got going on. 

As I'm sure Dirty knows (but I'll point out for others) the heat range of a spark plug doesn't produce a "hotter" spark or affect combustion temperatures, it's just the heat tolerance of the plug itself. 

[DirtyComanche]

22 minutes ago, gogmorgo said:

There was a head redesign between HO and Renix. While I can't state with certainty, I imagine the shift in plugs would be related to how they sit in the combustion chamber. If the HO plugs stick out that much further due to the longer electrode, they'd see higher temperatures, hence the need for a hotter plug. I've never had a Renix head off so don't really know what they've got going on. 

As I'm sure Dirty knows (but I'll point out for others) the heat range of a spark plug doesn't produce a "hotter" spark or affect combustion temperatures, it's just the heat tolerance of the plug itself. 

 

As I understand it, it's the other way around, a hotter plug runs hotter, but as a result of being a hotter plug rather than the design of the engine it is in.  It's not the plugs overall tolerance to heat exposure, they're all fairly well limited to the same maximum temperature, but rather it's the expression of how fast it can shed heat (or cool itself, I guess would make more sense).  So if the plug is exposed to more heat than typical, you would normally want a colder plug, as that would result in the plug operating at the correct temperature.  Plugs foul if they are not kept hot, so if there is issues with fouling typically a hotter plug is tried (assuming there isn't other reasons for the fouling), as it will stay at a higher temperature than a colder plug, given all other factors are the same.  Likewise if a plug is too hot it may experience premature failure or cause detonation, in which case a colder plug is needed for reliable operation.  I had always believed that keeping the plugs hotter, as in as hot as was safe for all usage, was considered better from a standpoint of plug life and ideal ignition (for both fuel economy and power), however some reading I've done recently indicates I might be wrong on that.

 

Modern high intensity ignition systems are pretty dummy proof as far as being able to make an effective spark, so I doubt if there would be much real world advantage, maybe there would only be real world disadvantage, and any advantage is assuming that you also don't cause the engine to detonate by doing this.  In fact, from some of the reading I'm doing, it sounds like a few of ZJ guys (which ran the same HO plug I'm talking about) swapped to the Renix style plugs to avoid detonation.

 

I'm also not sure how different the Renix vs HO (and when I say HO, I mean 91-92, I don't compare newer stuff normally) combustion chamber is.  I thought they were the same, and it was really just the ports that were changed.  But I'm ears if anyone actually has a good comparison.

 

This is just my curiosity talking on all of this too.

[eaglescout526]

I’ve been running this MoPar number since I’ve been working at the dealer:68304020AA. It’s the H.O. Style plug you have posted. 

[cruiser54]

NGK ZFR5Ns in all 4.0s. 

 

NGK has always had the same heat range. 5.

[DirtyComanche]

1 hour ago, cruiser54 said:

NGK ZFR5Ns in all 4.0s. 

 

NGK has always had the same heat range. 5.

 

That's listed as being for the later HO (99+ for XJ, 97+ for TJ).  Did Chrysler consider it a superseding part number but the aftermarket parts systems didn't catch it?

[cruiser54]

I have no idea.

 

They went to NGKs when they needed good spark plugs for the waste spark firing of the coil rails. Ya know, when the plug fires on the exhaust stroke also?