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Old 09-20-2012, 04:19 PM
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Spark plug help for SBC 406

I have been doing a lot of searching and have not been able to find much consistency in the results so I thought it best to just ask the experts for my particular application.

400 bored 30 over
lunati lightweight rotating assembly
235 265 lift cam
202 camel hump heads
victor jr intake
1:5 rockers on intake and 1:6 on exhaust
roller rockers
800 barry grant carb
all msd ignition

Engine has under 100 miles on it. Just got it in a nova I bought. Had autolite 353 plugs in it which i hear are more for marine and small engine applications. What plugs and gap do you guys recommend? Thanks in advance!

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Old 09-20-2012, 08:38 PM
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what heads,CR,and clarify camshaft? are you having issues?what ignition?
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Old 09-20-2012, 10:25 PM
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Spark plug help for SBC 406

The heads you have on the 400 cid are early model (before 1970) 202 camel hump heads right? Well if your compression is 11.0 or higher you need a colder plug. The pre-70 heads use R44 with 11.0 or less compression. Use R42 if high compression of 11.0 or higher. On 1971 and later heads use R44T on 11.0 or less compression. Use R42T if 11.0 compression and higher. You can get these plugs cross referenced at your Auto Parts Store to Autolite plugs. Try to avoid using R44TS plugs. The S on the end stands for extended tip.
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Old 09-21-2012, 02:22 AM
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For the SBC heads that use a 13/16" plug with the 3/8" reach
use a champion J10Y to J12Y or Autolite AR73 or NGK R5670-6

For dragstrip use a Champion J63Y

gap at .035"
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Old 09-21-2012, 05:41 AM
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Thanks for the responses guys. Sorry I should have noted they are the older heads with 13/16"gasket seat plugs, also running 10:1 compression. If it changes anything I am running a MSD 6AL Ignition. I was told its an MSD distributor, but I have not had it out or even the cap off yet to tell the model. If there is any other information that would help narrow down my starting point please just let me know!
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Old 09-21-2012, 09:33 AM
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any last recommendations before I head to Autozone? or should I just pick on from the list recommended?
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Old 09-21-2012, 11:14 AM
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Well that was a wasted lunch break. Autozone didn't have ANY of the plugs recommended! They attempted to cross reference and the only plug they came up with was the autolite 353 ( which is what is in it now that i was told weren't good). Guess i am stuck ordering them
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Old 09-21-2012, 12:03 PM
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If you read your MSD instructions you'll notice they say to open the gap up wider than the stock .035" gap. It says to adjust them between .044" to .060" with the MSD. I run mine at .045", and that seems to really work best for my setup. I'm running Champion RJ12YC in my 327 with the old 2.02 heads, and a Isky 270 cam.
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Old 09-21-2012, 12:57 PM
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Spark plug help

Check out this website it list all the plugs cross referenced to an AC R44. Alternative spark plugs for AC DELCO R44 If your plugs are on the list use them. I have the MSD 6AL Box with a MSD Pro Billet Distributor. I gap my plugs at .040. The bigger the gap the longer the spark has to travel.
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Old 09-21-2012, 01:06 PM
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Quote:
Originally Posted by cdminter59 View Post
Check out this website it list all the plugs cross referenced to an AC R44. Alternative spark plugs for AC DELCO R44 If your plugs are on the list use them. I have the MSD 6AL Box with a MSD Pro Billet Distributor. I gap my plugs at .040. The bigger the gap the longer the spark has to travel.
awesome thanks!!!
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Old 09-21-2012, 02:43 PM
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These are the old SBC heads. The r44ts plug is not for this head. It has the wrong reach (.460") and a tapered seat.
Do not get all nuts over increased spark plug gap.
The larger the plug gpa the more likely the voltage will jump to ground instead of jumping the spark plug gap.
The spark always takes the easiest path to ground.
Like from the rotor down to the advance mechanism or to a other cylinder that is not under compression pressure.
Especially at WOT when cylinder pressure is high.
This increased spark plug gap just increases the likely hood of of cross fire/ ionizing condition in the cap or across the spark plug wires. (especially if there is any moisture around).
And shortens the service life of the cap and rotor as it promotes carbon tracking in the cap from crossfire.
Once carbon tracking from in cap crossfireing is established the likely hood of engine damage from the plugs fireing at the wrong time is greatly increased. They only fix is a new cap rotor, wires etc and resetting the gap smaller again.
The larger the spark plug gap the more critical the cap rotor phasing alignment (which is never perfect and changes with vacuum advance)
Anything much bigger than a .035" gap rarely gives a real performance increase.
Reguardless of how much mega voltage you got. MSD BOX. Big spark plug gap engine performance gains are grossly overinflated.
Any real gains from testing with larger spark plug gaps (Usually show up at idle) indicate a need for air fuel mixture-distribution correction... (The engine really just needs to be tuned better) The large spark plug gap is a crutch and causes more severe issues at WOT especially as the ignition system ages or moisture is present.

The downside is not worth it. (shorter distributor service life potential for increased crossfire etc @ WOT)
It just costs you more money from the need for more frequent distributor component replacement.

it is very critical when setting adjusting the spark plug gap that the ground strap length be correct.
It should not extend past the edge of the center electrode.

Last edited by F-BIRD'88; 09-21-2012 at 03:08 PM.
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Old 09-21-2012, 03:02 PM
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Thanks for the great detailed response and explanation on gap. Helped me understand alot better. Just picked up some Autolite 303's. Will, put in tomorrow, rejet my carb as nd hope to nail the timing down
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Old 09-21-2012, 05:37 PM
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Spark plug help

Quote:
Originally Posted by Daubers View Post
I have been doing a lot of searching and have not been able to find much consistency in the results so I thought it best to just ask the experts for my particular application.

400 bored 30 over
lunati lightweight rotating assembly
235 265 lift cam
202 camel hump heads
victor jr intake
1:5 rockers on intake and 1:6 on exhaust
roller rockers
800 barry grant carb
all msd ignition

Engine has under 100 miles on it. Just got it in a nova I bought. Had autolite 353 plugs in it which i hear are more for marine and small engine applications. What plugs and gap do you guys recommend? Thanks in advance!
Can you restate your lift and duration of your camshaft. Those figures you have don't seem right.
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Old 09-21-2012, 06:17 PM
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Sorry typo. Duration is 285/295 lift is .507/.534
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Old 09-21-2012, 06:32 PM
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nice moderate combo,sounds like fun,,,

street car?
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