spark plug and carb question

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Nov 14, 2012 | 12:47 PM

66crawler

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spark plug and carb question

What are some good plugs for a semi mild built 390? Motor has 10:1 comp, mid range cam, electronic ignition and headers, manual trans, and around 400hp. Also thinking of ditching the ebrock carb and going with a Holley, not sure on what cfm though. I've heard anywhere from 600-750.

Nov 14, 2012 | 03:15 PM

MIKES 68 F100

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From: Antelope Valley ,CA

I am just running the autolite plugs just gapped at .44 for the electronic ignition .
i've had a number of carbs on my built 390
holley single feed 750 is what she likes #3310s

Nov 14, 2012 | 05:36 PM

Beechkid

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From: Southern California

If you have old style heads, I have always had real good performance out of splitfire (conventional) plugs...but they are not going to last as long as the "modern plats".

as far as the carb goes.....general rule...take the CID and double it...that will put you in the ballpark for the CFM range.....but the formula...is...
CID x max RPM.....divide by 3456 and multiply by the volumetric efficiency (I use .90- it is a little high - street engines are typically .80)....

Now for me personally, I add 20% to the cfm with a street-type built engine....I prefer the jet down than jet up...

so let's say max RPM is 5500.....
620 cfm plus 20%= 744- ie 750 cfm

Nov 18, 2012 | 09:46 AM

NW 150

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From: Ohio

My 390 likes the holley 750 on top. Much better low end than with the previous 600. Also I use the autolite 45s, not my pick I just went with whatever the store had.

Nov 18, 2012 | 05:48 PM