Whaddya think about these plugs?

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cruiser

cruiser

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Hi All: I just replaced the plugs in my very stock 1974 Duster. The new ones are the same as the ones that just came out: NGK ZFR5N plugs gapped to .035" with the gaskets removed and torqued to 30 ftlbs. as per the FSM. Attached is a photo of the old plugs from 1-6 top to bottom (front of the car to the back). These plugs had 18 months and 9456 miles on them - mostly highway driving. The engine is very stock and had the head rebuilt about 15K ago. Compression seems good, timing set to 10 degrees before, AFM and idle speed set correctly. Stock and recently professionally rebuilt Holley 1945 carb and auto trans. Runs very well at highway speeds, idle somewhat lumpy at 750 RPM, especially when in gear stopped at a red light. New plugs have improved the lumpiness a bit. What is your opinion of the wear on these plugs. Thanks!

IMG_4560.jpeg
 
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Seems typical on the color spread because of the intake manifold design starves 2, feeds two right, runs rich on the last two.

Is that the order they came out in?
The top and bottom seem a bit dark and rich to me. I’m actually not very familiar with how they run in a /6 despite having owned two.
Are your valve seals are in good condition?
 
Yeah, the valve seals and seats were replaced in addition to the typical head and valve work. As far as the order that they came out in, I started with the #6 plug and worked my way forward. The photo shows the plugs in order from front to rear (top to bottom in the picture).
 
My NGK catalogue states BP5FS as the original plugs.
Not surprised that the above plugs with greater protrusion into the chamber helped idle quality.
Might cause contact with the pistons with some engines....
Would expect the end plugs to have lighter porcelain colour because the longer runner length usually leans the mixture.
 
@slantsixdan do these plugs look correct to you? I don't think they're supposed to have that extended electrode, are they?
 
Those are the ones he preached to everyone as the "absolutely ultimate" plug for a /6....
Never tried them myself, good old champions have always worked for me....
 
@RustyRatRod those are NGK ZFR5Ns, the best plugs for a stock/stock-ish pre-'75 Slant-6 head in many cases. The 'preaching' (ok lol it's more of a technical explanation but wtvr) @volaredon mentions is here.

@cruiser y'member those times it's been explained to you that the early Holley 1945s were crummy carburetors and there were a bunch of revisions and tweaks to them before they started being less crummy around '77? Well, you're looking at evidence. There was a TSB to install a "1 and 6 plate" in the early 1945s as a kind of crude hack to alleviate the № 1 and № 6 cylinders running rich.

'74-'76 intake manifolds were also, ah, less than optimal for some dumb reason—the whole point of the buncha-bananas design was to promote even cylinder-to-cylinder mixture distribution, but then someone came along and "fixed" it for 1974, which aggravated the overrich front and rear cylinder condition. This was fixed for '77 and further improved later on.
 
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I have never once understood why the 2bbl carbs on those motors weren't installed 90 degrees from the way it was. That's never seemed right to me.
 
Plugs look pretty normal. If my memory serves me correct, an old school trick was to run a plug a point or two hotter on the outer holes.
Back when I was playing with the 1bbls on the slant, the early 1920 Holley carb worked really well.
 
Johnny Dart said:
Plugs look pretty normal. If my memory serves me correct, an old school trick was to run a plug a point or two hotter on the outer holes.
Back when I was playing with the 1bbls on the slant, the early 1920 Holley carb worked really well.
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You beat me to it. I was gonna say why not run a step hotter on the outboard plugs.
 
If you want to apply the one-and-six plate, here's the TSB:

1945 one-six-plate_1.jpg


1945 one-six-plate_2.jpg


1945 one-six-plate_3.jpg


And it looks like it won't be hard to get hold of the parts package; see hereand here.

You can run hotter plugs in the front and rear cylinder, and they might stay cleaner longer, but that won't make those cylinders stop running rich.
 
slantsixdan said:
@RustyRatRod those are NGK ZFR5Ns, the best plugs for a stock/stock-ish pre-'75 Slant-6 head in many cases. The 'preaching' (ok lol it's more of a technical explanation but wtvr) @volaredon mentions is here.

@cruiser y'member those times it's been explained to you that the early Holley 1945s were crummy carburetors and there were a bunch of revisions and tweaks to them before they started being less crummy around '77? Well, you're looking at evidence. There was a TSB to install a "1 and 6 plate" in the early 1945s as a kind of crude hack to alleviate the № 1 and № 6 cylinders running rich.

'74-'76 intake manifolds were also, ah, less than optimal for some dumb reason—the whole point of the buncha-bananas design was to promote even cylinder-to-cylinder mixture distribution, but then someone came along and "fixed" it for 1974, which aggravated the overrich front and rear cylinder condition. This was fixed for '77 and further improved later on.
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Thank you kindly. I remember you addressing the issue in the past, but I could not remember if it was a good or bad thAng. Moochas grassy ***.
 
You would run colder, not hotter plugs in the 1 & 6 holes. Because those cyls are going to be running leaner, caused by the long runner length. Leaner will be hotter, so use a colder plug. Personally, I would use the same plug for all 6 holes.
 
Bewy said:
You would run colder, not hotter plugs in the 1 & 6 holes.
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LOLnope.

Bewy said:
Because those cyls are going to be running leaner
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…except they're running richer, for known reasons that can be fixed.

Bewy said:
caused by the long runner length
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And yet somehow, millions of Slant-6 engines without this year-specific problem don't run lean in 1 and 6. Huh. Habbout that. Lard works in mysterious ways, I guess.

Oh, hey, wait a minute, you're in Australia! So this must be one of those things like how water spirals the 'wrong' way out the sink and clocks spin the other direction, the Slant-6 leans to port instead of starboard, V8 engines have the intake manifolds on the outside and the exhaust on the inside of the vee, etc. But the OP's up here, so…

Bewy said:
ersonally, I would use the same plug for all 6 holes.
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Amen, hallelujah.
 
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The intake runner length isn't the huge issue everyone thinks it is. In fact, there are several articles from Chrysler explaining just how good it is. It just ain't "that" bad.
 
RustyRatRod said:
The intake runner length isn't the huge issue everyone thinks it is.
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Whoever finds it entertaining to pretend they know it's a big issue can go spend some time with the pathetic-aѕѕ log-type intake manifolds on most other carbureted inline-6s and then come back and tell us what they've learned about how much of an issue it's not on the Slant-6.
 
Which years were the intake manifold for the single barrel carbs the best? Anything Post 1977? or do I want to go find one from the early 80's?
 
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