they are both the same style and reach the only difference is the heat range with the 10x being a bit hotter plug. This time of year when the air is cooler and air/fuel tends to lean out a bit either would be ok. I’d probably just stick with the 10x on gasoline.
From the Autolite Site "Autolite Heat Ranges - The larger the last digit in the part number, the hotter the plug is. Example a AR3935 (good street/strip plug) is hotter than a AR3932 plug which is very cold used for lots of nitrous."
AR3910X is about the coldest plug on the planet. I bolded and underlined 'very cold.'
The 3910X is 2 steps colder than the above listed 3932.
I think the only reason the 3910X became popular is for the extremely minor amount of compression increase due to the
OvalTech is correct.
It seems AutoLite changes #s for heat range between families, in this case the X series is coded differently than a non X series.
The 3910X is still a cold plug and sort of unnecessary in a low compression Clone.
We run the NGK 5671- series. I run them in the big car so I use them in the karts too. They run from -10 cold to -7 hot. We use the 8 in unrestricted motors and the 7 in our plates. There is another NGK that’s a laser iridium it’s a BPR5EIX I believe real hot plug for purple back to red.
Indexing isn't that kind of a hold over from the flathead .
it may provide a minuscule benefit .
Certainly not hard or big enough to offset a dns or dnf .
Once the fires lit the jobs done .
No fire no work .
I switched to extended nose plugs after several blubbering restarts where
the 3910X was fouled almost to the point of not working during the multiple
slow laps. WOT it does OK and as FT1 says 'after the fire is lit the job is done.'
There are some interesting developments in extended nose plugs.
I have a plug that will change idle RPM by +~300RPM just by switching.