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Discussion Starter #1
I'm looking for spark plug suggestions. I have a 350 with '68 double humps that have been slightly ported and I believe it has 10.1 compression. I also have a Comp Cams 280/494 in it with 1 5/8 headers along and a Holley Sniper FI system. I'm guessing in the 350 - 370 hp range?

Doing some research I see that the ACDelco R45S or Autolite 3924. I'm leaning towards the ACDelco but wanted to get suggestions. Also, what gap would you recommend with either?
 

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Seeing how R44's which aren't made anymore I would run Champion J8C instead of the AC's.

Just an FYI: AC spark plugs aren't made by AC anymore
 

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Discussion Starter #6
Seeing how R44's which aren't made anymore I would run Champion J8C instead of the AC's.

Just an FYI: AC spark plugs aren't made by AC anymore
Looks like the J8C is also Champion 841. Do those definitely fit? A couple sites I came across said they don't.
 

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Small block AC plugs in the 60's were typically 43 & 44's which is then turned into R43 & R44 with a resistor

R45S: RJ12YC ~ Hot
R44: J8C/RJ8C ~ Med ~ came with 327/350hp
R43: J6C/RJ6C ~ Cold ~ came with 302

With my 327 and how it is built and the carb is adjusted, I run a cold plug (RJ6C) with zero issues. Personally, I would run a middle-of-the road heat range and go from there.
 

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Any suggestions?

NGK plugs are colder the higher the number, hotter the lower the number. Starting with part # BKR6E-11 (Heat Range 6), a colder plug would # BKR7E-11 (Heat Range 7), a hotter plug would be # BKR5E-11 (Heat Range 5). For NGK Racing Plugs, the Heat Range is located AFTER the hyphen.
I run R5671A-7 stock #4091 in my stroker. my compression is 10-9-1 and I tried a #8 and it was to cold
 

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The NGK 3332 (XR5) equivalent to the AC Delco R43/R44 plug for the 70 Z28.
I've used this and it works well. I'm currently testing AC R43S since I live in the country and rarely drive city driving stop and go speed limits. I'm always 50+ mph.
 

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Since 5/8 hex, these are called 'peanut' plugs for cylinder heads '72 and newer. Autolite 25
By design, the reason for different heat ranges is the heat burns off combustion by-products. Low rpm engines, aka, taxicabs, idle speed or below 1500 rpm, a hotter plug is prescribed.
More rpm's in the operating range, a colder plug is prescribed, aka, 302, high rpm and high compression and other designed high rpm engines, more heat generated, a colder plug needed.
A daily driver, meddle heat range.
AC used to market a AC 46, and the 'R' means a resistor is installed between the wire terminal and electrode.
Also, on gap, ign coils will develop spark regardless of gap and provide a set amount of field for spark when field collapses.
A small gap, 0.020" will give a long duration of spark at a lower high tension voltage.
A larger gap will produce a shorter duration of spark at a higher voltage with the same field created from same coil.
Change coil to more windings creates a larger field, thus more duration energy for spark, need a larger gap.
 

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Discussion Starter #18
Well, after going through three different NGK plugs I finally got ones that should work. After getting different suggestions I initially chose the NGK 7938. However, once I pulled the plug I had in there I could definitely see that it was much longer reach. What I pulled out was an Accel 576S and it has a reach of like .375 where the NGK 7938 was .750 so I decided to return it and go with a different NGK but when I compared it to the one I pulled out it was an ever so slightly longer of reach. I was going to live with that but then looked at installing and the 13/16" hex was going to a super PITA to get to because of my headers. I then went back to researching and was about to pull the trigger on an another identical set of the Accel 576S that came out but they are $50 a set and I really wanted to go to NGK brand. I then stumbled across a post that lead me to the NGK UR5 which had pretty much the exact specs as the Accel I pulled out. I installed them today and so far they seem to be what I needed. I'm still having a slightly high AFR with my fuel injection but that's a hole other issue I'm working through.

I have pictures of the different plugs together but I'm out of room for pictures on the forum so I'll have to think of another way to show them.

Here is a link to the NGK UR5 that I used.
https://www.summitracing.com/parts/ngk-2771
 

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