I need some help with a spark plug choice , brand and heat range . The motor is a 406 with 11:4 to 1 and 227cc cnc'd track 1 heads , I'm running a MPFI from holley , anybody here have an idea where to start with plugs for this thing .
Mark .

AC 43N would have been factory for a 396 and 427.

Sounds like you will have better fuel control with the mpfi. But you did not state what type of plug sealing or seating you have for your sbc? I will guess that you have the same as all the new cnc alloy heads with the small plug and the sealing washer. Try Autolite 3924 for starters or Bosch 7557. These plugs have the stepped sealing surface with washer... If not , go back to the stock design.

Go to Mfg.s website and see if they recommend any special reach plug.

Sorry guys for the lack of info , they are 3/4 reach with the washer . Do they make a plug with the 3/4 reach but using a 5/8 socket .
Mark .

Try an Autolite 3924. My combo is very similar with the same compression and slightly smaller heads. The 3924 stays clean and the mark on the strap is is exactly the right spot with 35* timing.

Call NGK (http://www.ngk.com/default.asp)
sparkplugs.com (http://sparkplugs.com/)
P.O. Box 772
Sun City, CA 92586
Toll Free Phone Number: 888-800-9629

Different ignition systems make a difference. NGK has some very specific plugs designed for certain applications. They should be very helpful.

Steve

Your 1st set will be a starting point...and start a little hot.
After a week or so of use...or a couple runs.
Read your plugs..u need them as cold as possible without signs of fouling.
Do NOT head out to the highyway, and plant your foot...this creates great heat, and what is there, will glaze onto the plugs and will take a while to burn off to give a true reading...
I digress..noticed when u go on a trip, after about 80 miles the engine picks up and starts to sing nice..u can get this to happen earlier by hitting the highway, then slowly increase speed over several miles, preventing glazing and cleaning plugs.
Back to subject:
Eventually u will get to a colder plug that works well on the 4 inside cylinders and fouls on the 4 cnrs...the cnrs have longer runners causing this...run a hotter plug 1 heat up on the cnrs.
If u want to get real fussy, mark the plugs as to what side the open gap is. Then sort thru a heap of them, so when they are torqued up, the gap faces the valves.
If u drop a plug on the floor, throw it away.

I installed a set of Autolite 3924 , these are also going to be the ones I first breakin the motor with . Thanks for the help .
Mark .