Spark plug advice

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HemiDenny

HemiDenny

HDK Suspension
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What is a good plug to start with for a 408?....10:1 compression. Lumpy (don't know specs) hydraulic cam.

Might be over carburated....thinking 750 cm Eldelbrock.

Thanks in advance for any input.
Denny
 
I use Autolite 3924 on my heads,
What are the heads you are using?
 
Cast / steel heads. Look like a factory head. I bartered for the engine, owner bought the car for the body and a 528 Hemi install ...so he knows less about it than I do.
 
HemiDenny said:
Cast / steel heads
Click to expand...
I run Champions in my Mopar's . Just sayin. I know there are probably better quality plugs out there ( like maybe NGK or Bosch) but I don't like the heat ranges they build. If you do try a non-Champ plug make a run and pull a couple plugs to read how they are burning. I reduced compression on my freshly built 273 and tried a Autolite plug. I had a rough/rich idle issue and come to find out the plug was a heat range colder than the original N9Y Champions. I bumped up a couple heat ranges and all is well.
 
HemiDenny said:
Cast / steel heads. Look like a factory head. I bartered for the engine, owner bought the car for the body and a 528 Hemi install ...so he knows less about it than I do.
Click to expand...

Obviously plug preferences are like asking what oil is best to use, but just for kicks I’ll tell you I run NGK in everything.
My 9.5 compression Magnum has had NGK GR5’s set at .050 in it for 5 years or more and they have been flawless.
(Thinking to put new ones in anyway):D

I am running a Holley Street Avenger 770 that performs great in all ranges, and I know for fact a 750 isn’t overkill on a 408.
In fact my own opinion is that a 408 could use all of an 850.

BTW, one of our local Mopars4us group picked up one of your K members, A arms and power rack and pinion setup.
Impressive stuff.

F1AEB18C-EABE-4097-9933-416B0A272162.png
 
HemiDenny said:
What is a good plug to start with for a 408?....10:1 compression. Lumpy (don't know specs) hydraulic cam.

Might be over carburated....thinking 750 cm Eldelbrock.

Thanks in advance for any input.
Denny
Click to expand...
i'm using champions on my 426 stroker no problems 7,000 miles and the plugs look fine , cheap and they work
 
MOPAROFFICIAL said:
Stock heat range or one step colder.
How much timing you can run can impact choice.
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THIS!!!! ^^^^^^^^^^^^^^^^

100% all day long everyday!
100% dead on bulls eye. No doubt about it.
Choice of plug is your call. I run Champs without issue.
 
RN12YC, is, IIRC, the Edelbrock-recommended plug for my alloy heads.
If I told you that I installed them at .045, in my 367, in 1999, would you believe me?
How about if I said they currently have over 100,000 miles on them?
I use a big Yellow Square-top Accell SuperCoil. In a pinch, I think you could weld with it. lol. I have not looked at the plugs since winter of 2004/05.

EDIT; see post #12
 
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thanks for your input. just looking for a decent place to start and thought I would ask the experts.
 
Yup. I agree on the Champions and what hooha said in post #6. That'll probably get you right on the money.
 
AJ/FormS said:
RN12YC, is, IIRC, the Edelbrock-recommended plug for my alloy heads.
If I told you that I installed them at .045, in my 367, in 1999, would you believe me?
How about if I said they currently have over 100,000 miles on them?
I use a big Yellow Square-top Accell SuperCoil. In a pinch, I think you could weld with it. lol. I have not looked at the plugs since winter of 2004/05.
Click to expand...


If it’s an Edelbrock or other aluminum head it will be an RC12YC and not the N series. The 13/16 hex won’t fit in there.

EDIT: the OP says he has iron heads so he can use the N series plug.
 
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HemiDenny said:
What is a good plug to start with for a 408?....10:1 compression. Lumpy (don't know specs) hydraulic cam.

Might be over carburated....thinking 750 cm Eldelbrock.

Thanks in advance for any input.
Denny
Click to expand...
Denny I have never been a fan of Champion plugs it always seem like I could never get the heat range correct. For the past 5 years I have found that Autolite 64's work well in my warmed up 10.5:1 iron head 345 SBM. I also ran Autolite 63's in in my 512 stroker B body - no issues.
Cheap and all the parts houses have them available.
Good luck
 
MOPAROFFICIAL said:
Stock heat range or one step colder.
How much timing you can run can impact choice.
Click to expand...
This might be a stupid question but how do you know the heat range I'm looking at the champion 405 copper plus plugs I dont see a heat range in the description
 
dartfreak75 said:
This might be a stupid question but how do you know the heat range I'm looking at the champion 405 copper plus plugs I dont see a heat range in the description
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What is the number designation of the plug? 405 is the stock number.

You should be looking at an RC12YC or equivalent IIRC your engine.

R = resistor
C = plug type (3/4 reach, 5/8 hex is a C)
12 = heat range
Y = tip length
C = copper core

The C and the N series Champions are the exact same plug EXCEPT the N series is a 13/16 hex and the C is a 5/8 hex.

I’m not a fan of the 13/16 hex. I was hoping that Champion will kill the N series plugs and keep some others they have evidently killed.
 
yellow rose said:
What is the number designation of the plug? 405 is the stock number.

You should be looking at an RC12YC or equivalent IIRC your engine.

R = resistor
C = plug type (3/4 reach, 5/8 hex is a C)
12 = heat range
Y = tip length
C = copper core

The C and the N series Champions are the exact same plug EXCEPT the N series is a 13/16 hex and the C is a 5/8 hex.

I’m not a fan of the 13/16 hex. I was hoping that Champion will kill the N series plugs and keep some others they have evidently killed.
Click to expand...
Rn14yc is what the 405 are so does the heat range go up or down based on the number? Would 14 be a higher (hotter) heat range than the 12?
 
yellow rose said:
R = resistor
C = plug type (3/4 reach, 5/8 hex is a C)
12 = heat range
Y = tip length
C = copper core

The C and the N series Champions are the exact same plug EXCEPT the N series is a 13/16 hex and the C is a 5/8 hex.
Click to expand...

I learn something new literally every time I visit this forum. Love it.
 
dartfreak75 said:
Rn14yc is what the 405 are so does the heat range go up or down based on the number? Would 14 be a higher (hotter) heat range than the 12?
Click to expand...


Higher number, hotter plug with Champion plugs. NGK is the other way. The bigger number is colder.

I would do everything I could to use a a C series so you don’t have the dreaded 13/16 hex. And I’ll tell you why.

Because as much as I say use the C series, when I went into town Wednesday to grab some plugs and other junk to fire the 340 thats on my run in stand, and they only had N series plugs in the heat range I need.

The headers I’m using to fire this pig I’ve used since 1980, so I figured WTH they will be fine.

They go in there, but they are a PITA. I guess when I was younger it didn’t matter, but fighting that bigger socket just pisses me off.

That will be the LAST time I ever buy that dinosaur plug. If they don’t have the C series I’ll either order them, or if I need them in a hurry I’ll use Autolite’s or AC’s.

I now officially HATE the N series plugs to death.

Just an FYI.
 
I forget how the autolite plug works with there heat range stuff
 
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