I decided to fit a set of these plugs into my folks slant six. Its a Super six engine,HEI ignition ,all very mild. I adjusted the advance curve to suit the fuel they run and got it pretty close,just barely pinging under heavy loads,so I thought it would be interesting to see if the ZFR5N plugs would help the pinging without making another timing adjustment. To my surprise they made it marginally worse,especially after a high power run,then the engine would ping under a moderate load,I'm guess due to the hot extended plug tips. So without any other adjustments I refitted the non resistor old bosch plugs (W8DC,same heat range as NGK BP5ES and ZFR5N) and the pinging reduced again. The engine isn't really a benchmark of a fine slant six,long story,built by "professionals" ,still very low miles,but the #1 plug seems to get a little wet after prolonged idle, so obviously that won't help detonation.. I will try the ZFR5N plugs again when I build up the fresh engine, hopefully it will respond better to these plugs.

You might try the same plug design(zfr) with a colder model.

Sam

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ZFR-5n is the only plug available with the extended electrode of this type...You will have to switch to a standard style plug to reduce the heat range. There was talk on a Dodge Truck board a few years back and found the extended tip plug pushes the kernel closer to the center of the chamber which acts as if lighting "the fire" about 5+ degrees advanced compared to the timing curve.

I use zfr5fix-11 - Its the fancy fine point iridum (expensive...) version. I don't recall the specs but it's reach was somewhere in between the zfr5n and standard plug. This may be enough of a compromise for you?

Thanks for that,interestingly they do zfr6fix-11 as well....e$$xpensive,but nice!
DusterIdiot,thanks again. That explanation makes sense. I was kind of disappointed with the zfr5n outcome thinking the extended nose was getting too hot and causing detonation,but it may well be a better located flame front not needing so much advance. The other weird thing with this engine is the rear 3 cylinders look pretty lean on the zfr5n plugs,the front 3 look OK except for # 1 being a little damp with the oil issue,when I run the standard plugs the colours look much more even...I don't know!
I think I will just run it as is for now,with the standard plugs,when I build up the new engine I'll do some more fine tuning with the ZFR series plugs. I have a set of 198 rods and 2.2 pistons here,Im toying with the idea of using them to see if the dished piston design will create a more efficient burn,but I am torn between using them in this engine or keeping them for a later higher performance build.Thanks again guys!

My experience was more or less the same. I had to back off the timing a good 5 degrees with the ZFR5Ns. I'm not sure if that's a good or a bad thing - I guess you would have to run both sets of plugs on a dyno to find out.

I am not sure what plugs to use for my 10.5:1 engine either - I like the extended electrode for an open chamber in theory, but with a lot of compression I'm worried about holed pistons. I'll probably just use something like BPR7ES.

That's why they work with the stocker engine since the distributor curve is a bit lazy and carburation and VE a bit on the inefficient side so putting them in will help those engines a lot.
I noticed last time I raced, that Dave Mueller was swapping a set of BPR6ES out of his 13 second '69 valiant beater racing car...so you might start with a set of those in both heat ranges and see which works best for your combinations.


-D.Idiot

Interesting tidbit! I wasn't aware of this. Makes sense though.

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Casually looking for a Clifford hyperpak intake for cheap.

This would indeed explain why DadTruck's engine went into preignition with those plugs. You know it is funny, I thought I had a full set of ZFR plugs in various heat ranges. Did they used to make more ? Maybe it was a different type plug I had.

Sam

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there were some posts on the modern dodge truck board (the 2003-current) as users were asking why this specific plug was used in everything from the last Jeep 4.0L and the V-10... They noticed that the engine acted like there was a timing change when swapping from the older standard version of the plug and the extended electrode.

-D.Idiot