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Hi folks, I am replacing my spark plugs, it currently has Autolite AR3933 racing spark plugs, I am replacing them with the same ones just to be able to check on them while tuning the car and fixing timing, I have a dui distributor, what should I gap the spark plugs to? Online it say 0.055” what do you guys recommend? Also what spark plugs do you guys recommend? I plan on changing them again if needed. I Appreciate any input
 

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1965 Mustang GT. 11.898 @ 113.646, all motor, three pedals
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33’s are on the cold side. Which is fine, if that’s what your motor needs. .055 is pretty big, but that too depends on the rest of the package. Post some motor details and let’s see what we’ve got
 

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As I don't trust advertising or claims, and my engine is the one that needs it to work right, I test mine for gap to get max performance. There is a relationship between plug gap and voltage. The wider the gap, the higher the voltage climbs to jump it, until it just can't anymore. The reverse of this is using a small gap and the voltage never goes very high (even if it could), as it's not being forced to use it. I find the limits of my stuff and squeeze the coil for everything it has by testing.

Where plugs are hard to fire is at two places in the range; just off idle, and at max rpm. Accel from idle has maximum cylinder filling (lots of time to fill them) with maximum cylinder compression pressure, and plugs that are getting borderline voltage will have a feel much like a pump-shot fuel hesitation or power sag when you punch it. This is the one that typically shows-up first, and once I've set that, I test for high-rpm.

The other end is at redline, where the cylinder filling (VE) is lower, but the time between firings is so short the coil can't saturate fully for full energy. Voltage drops and the spark gets weaker. You see this as a sag in power as you enter high rpm, and most easily seen with time-slips as top-end mph levels-out or falls-off. If doing back-to-back runs without timing, you can feel it like a light headwind (or worse). This one shows-up with very high rpm, the wrong or old coil, or an ignition with poor dwell control.

I find these max limits, then reduce the gap .005" for racing, or .010" for street and longer plug life between changes or maintenance. That's me. Do your thing.
 
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