Help With Spark Plugs....PLEASE

[65_Cobra]

Hi....I have a Superformance Cobra with an older 351W 385HP crate motor. Please let me know what spark plugs and numbers you have had success with. I hear the NGK's are good, but don't what number and if I should get cooper, Iridium, platinum, etc. Also what the gap should be.

The car currently has Autolite 3924 in them and I don't know is the correct plug.

Thanks

[eschaider]

What did your engine builder tell you to run in the engine? Is the engine misbehaving with the 3924 Autolites? If not, then why do you want to change them?

[65_Cobra]

Bought the car used. Plugs have a lot of black carbon buildup after a couple hundred miles. Trying to get an understanding of what plugs others are using with their 351W 385HP cars.

[eschaider]

Black carbon is not a plug issue, it is a tuning/fueling issue. I would take the car to a knowledgeable tuner and ask him to work his magic for you.

[Tommy]

Basically I agree with Ed. But perhaps my experience will be informative. . . My Cobra had a custom built 408 Windsor configured for track use. It used Autolite 3932 cold racing plugs. With those plugs and the absence of a choke on the carburetor I had to use extra fuel at start up to keep it running until it warmed up a bit. That would occasionally cause it to foul a plug. I live near an open road so as soon as I could I would run it up to 4000 RPM or so and keep it there until the fouled plug cleared up (a minute or less). Once it got warmed up it did not foul the plugs. . . So I suggest two things in addition to what Ed said. First, take a look at the carburetor after it has warmed up to be sure the choke is fully open. Second, when you have an opportunity to safely run the engine at higher RPM for a minute or so, take it. Then check your plugs to see how they are doing.

[65_Cobra]

Thank you for your comments and I will take it to a tuner, but what plugs are being used by members with a 351W 385HP motor and what is the gap?

[eschaider]

Here is a page from the Autolite spark plug website. You plug is in the red box. You can tell from the other brand plugs the relative heat range for your plug and where it falls in the overall hot-to-cold spectrum.



Before you start to make plug changes, I would encourage you to get your carburetor jetting, power valves, and whatnot squared away. Once you have the engine getting fueled properly, you can move your heat range around to suit your preferences. The plug you have right now should be pretty close to what you will want in the end.

Here is a pic that might help you to read your plugs. Street-driven plugs are never clean, but a proper heat range and proper engine fueling should produce a plug that looks like the plug on the left above the self-cleaning temperature line in the pic that says, "You want this look ..."

[HighPlainsDrifter]

Hi,
The gap depends on the ignition coil output, a 50,000 volt coil can use a larger gap like .045 to .050. An msd box can also run a larger gap.
However a high compression engine will need a smaller gap .035 to .040.
A high rpm engine likes .030.
so you tailor it to your combination.
Perry

[65_Cobra]

Thanks for all the help. What type of plugs do you use in your Cobra? Cooper? Iridium? Platinum?

[Tommy]

Originally Posted by 65_Cobra

Thanks for all the help. What type of plugs do you use in your Cobra? Cooper? Iridium? Platinum?

Champion's answer to your question.

I use copper because I don't expect to put 50,000+ miles on my hotrod plugs.

[twobjshelbys]

It is not advised to re-gap platinum and iridium plugs so if you need anything other than their standard gaps copper is the way to go.