Looking for advice on plugs. Engine is 418 small block, aluminum older W-2 heads (valve angle is 14 degrees), 12.7 compression, 950 Quick Fuel carb, 4:56 gears with Powerglide, 5800 TCI convertor with Pro Brake, 3550 lbs with me in it. Ran 9:90's @133 mph last year with NGK R5671A-8.

Autolite says the equivlant to the NGK is the AR3934. But I see they make the "X" model that they state "sports a specially machined four-corner firing surface gap style instead of the normal single electrode. Specs. are 5/8" hex, 3/4" reach, 14mm thread, nickel plated shell, gasket seat, non-resistor."

What are your suggestions and anyone use the "X" model?

Thanks everyone,

Chuck

I've never used that type plug before, but I feel a -8 or equivlant is WAY to hot for a engine with 12.7 compression ratio. I would use a -10

What happens to an engine if you use a plug that is too hot or cold? What are the effects to the engine? Does to hot of plug make the car run faster, slower, etc? I have always run 8's in my engines but this is the lowest compression ratio of all of them.

So a 10 is what I should be running? Anymore info any of you want to know? I have way to much money in this engine and I dont want to cause damage for something as simple as a plug.

Thanks

Chuck

Cold plugs can foul but helps with detonation issues. Hot plugs can cause detonation and hurt the engine quickly. For racing purposes a colder plug will benefit you.

There are some guidelines for this but it usually has some trial and error to it.

What is the autolite equivlant to a -10? AR3932?

Yes. I found a lot of previous articles and there was a cross reference chart between NGK and Autolites.

There are some references to the AR50 series plugs. Not sure what the difference is between the AR50 and AR39 series. However I do want the longer reach plugs.

I should have added that this is a strip only 71 Duster with MSD ingition and I run Sunoco 112.



Thanks

Is that the difference between the 50 series and the 39 series, projected tip or standard tip? Which would be better for my engine?

Thanks again

Chuck

I spoke to Autolite tech today. He suggested an AR3911 which is an equivelant to an NGK 8. Also said to look at the AR3910 which is an equivelant to an NGK 9. AR3932 is the NGK 10.

What to do? I did find out the 50 series is the 13/16 socket which I dont have room for.

I run the AR3910 autolite's and have good sucess with them in my 13.0 to 1 smallblock 408 w/sunoco 112 I have a set of the X plug's but have not put them in yet,,I have been told to get my jetting right first then put them in and run them.