Thread: Spark plug gap

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What is the right gap for a first gen. small block 3970010 ?

Interesting, you have a block with spark plugs in it? Compression, ignition and knowing what the engine was designed for helps more.

That block was used from '69-'80 with either 2 or 4 bolt mains. Stock HP from 185 to 370. A wide range of components went into it. Correct gap? with just a block number would only be a guess. If it's all stock start at .035 and see how it runs.......

Sorry, HEI ignition little more than stock but has stock heads, hhas headers air gap intake roller rockers 600 carb. 373s lunati cam 462/489

everybody gone?? I had it at .40 before and now since I have changed to the lunati cam was wondering would .40 still work?

If you have 0.40 gap you probably mean 0.040 gap since 400 thou is a bit much.
Yes, it will work but wont be as efficient as the correct gap. Like Denny said, with an HEI, use 0.045 or so. The extra gap allows a higher voltage to build in the system before the spark jumps the gap. The amount of energy is a function of the dilectric strength of the medium (air and fuel) and of the gap. This is why HEI is better, it has a higher energy capacity so the gaps can get bigger and therefore more energy it takes (spark energy). For a given medium, the energy that it takes to jump the gap has nothing to do with the ignition system....the volts are the same. However, how that energy is delivered does depend upon the system (rise time, duration of spark, etc)

thanks robot and i just checked my old ones and they were at 0.045. The guy that just installed the engine in this other car had some of the gaps at0.050-0.055 may why it wasn't running right. I went and got some of those accel header plugs because they are shorter and the delcos were making the boot touch the headers and burn up even with the plug boots on them. Do u know if these are good plugs? hope so they are 30 bucks im putting them on now

you can run a bigger gap with HEI but it gives more wear and tear to the coil... I run mine at .036 with an MSD... also the larger gap is more prone to have the spark blown out under more extreme applications.... as long as you aren't missing at high revs I would stick with the smaller gap.

two cents,

Chris

How about a set of these E-3 Diamond Fire plugs

When HEI became standard in 1975, GM recommended .065.....but, the next year or so lessened it to .045.