Fox Spark plug heat range

skiwesser11 · Mar 21, 2019

Looking for part #’s for a “stock” and a plug 1 heat range “colder” for a good non protruding tipped plug.. or a non protruding plug equivalent in heat range to an autolite 3924

Anyone have a link to a good NGK heat range chart that contains the 5820 if I’m correct that the 5820 is “stock” heat range??
If not, anyone know which number is? Also which number would be 1 heat range colder?
I’ve been running Autolites 3924 but am going to a 100-150 shot of wet nitrous TB plate again and my understanding is the NGK isn’t a protruded tip plug like the autolite, and is preferred w/no2.. I have AFR 165/58cc heads and N/A have had best results with the 3924’s over NGK’s and Mo craft....I may not even need a “colder” plug but would rather start with one to be safe... I just don’t know what stock heat range is in an NGK... charts I find are for race plugs that cross reference to 5820 etc but I don’t trust what I’m finding... lower number is “hotter” but no actual part #’... what are you guys using??

Gear grabber · Mar 21, 2019

I used 3924 n/a,NGK R5671A-8 up to a 185 shot. Just changed to R5671A-9.

RangerJoe · Mar 21, 2019

skiwesser11 said:
Looking for part #’s for a “stock” and a plug 1 heat range “colder” for a good non protruding tipped plug.. or a non protruding plug equivalent in heat range to an autolite 3924

Anyone have a link to a good NGK heat range chart that contains the 5820 if I’m correct that the 5820 is “stock” heat range??
If not, anyone know which number is? Also which number would be 1 heat range colder?
I’ve been running Autolites 3924 but am going to a 100-150 shot of wet nitrous TB plate again and my understanding is the NGK isn’t a protruded tip plug like the autolite, and is preferred w/no2.. I have AFR 165/58cc heads and N/A have had best results with the 3924’s over NGK’s and Mo craft....I may not even need a “colder” plug but would rather start with one to be safe... I just don’t know what stock heat range is in an NGK... charts I find are for race plugs that cross reference to 5820 etc but I don’t trust what I’m finding... lower number is “hotter” but no actual part #’... what are you guys using??

Just bought another set of R5671A-7's, which are 2 steps colder I believe. For a 10.8:1 331 with a 100-125 shot. Believe these may still be a little hot and might bump to an 8 next. I think NKG in a stock heat range is around a 5.

Here are the NKG 7's vs 3924's.

Joe

skiwesser11 · Mar 21, 2019

Do you guys think I should at least go to the non protruding tip?? Joe your pic is exactly what I’m trying to change FROM and TO but want to be right on understanding heat range..
so R5671-(heat range).. is what I want yes? If I’m right about that I’ll probably start with a 5671-5..?

Thanks again guys... always get more help than criticism here... getting rare on these forums lol

RangerJoe · Mar 21, 2019

skiwesser11 said:
Do you guys think I should at least go to the non protruding tip?? Joe your pic is exactly what I’m trying to change FROM and TO but want to be right on understanding heat range..
so R5671-(heat range).. is what I want yes? If I’m right about that I’ll probably start with a 5671-5..?

Thanks again guys... always get more help than criticism here... getting rare on these forums lol

The R5671 is the plug type, the number on the end is heat range. Stockish should be a 5, so if you are going to spray it, I would go at least one colder for starters, so a R5671-6. From my research, the non protruding plugs were better for nitrous use, so yes, i would recommend it. Basically the more heat in the chamber, the colder plug needed. Cam design, compression, base hp level of the motor can all play a part in your decision in addition to the amount of nitrous you spray.

There are a lot of things wrong with these plugs, but these are the NKG's I pulled and replaced last night.

skiwesser11 · Mar 21, 2019

RangerJoe said:
The R5671 is the plug type, the number on the end is heat range. Stockish should be a 5, so if you are going to spray it, I would go at least one colder for starters, so a R5671-6. From my research, the non protruding plugs were better for nitrous use, so yes, i would recommend it. Basically the more heat in the chamber, the colder plug needed. Cam design, compression, base hp level of the motor can all play a part in your decision in addition to the amount of nitrous you spray.

There are a lot of things wrong with these plugs, but these are the NKG's I pulled and replaced last night.

Ok thanks, that’s what I was trying to get st that the R5671 part is what I need shank wise to fit head and replace the autolite 3924’s... so I can safely assume 5671 -5 is stock(ish) heat range?

RangerJoe · Mar 21, 2019

That's correct.

skiwesser11 · Mar 21, 2019

Ok I’m still confused.. can’t find a r5671-5 but a bkr5e or bkr6e... 5820 seems to cross reference as a r5671-10?? Obviously different type of plug but I need shallow(er) electrode and possibly colder than autolite 3934 I believe....
sound correct?

skiwesser11 · Mar 21, 2019

Are the BKR(x)E race plugs??

RangerJoe · Mar 21, 2019

skiwesser11 said:
Are the BKR(x)E race plugs??

Those are a projected tip, not what you want for nitrous use. Not sure if it would be considered a "race" plug or not. They may not make a stock heat range plug in the r5671 style, which would make sense considering they are a race, power added oriented plug. If you are going to spray the engine, why do you want to stick with a stock range plug?

skiwesser11 · Mar 21, 2019

RangerJoe said:
Those are a projected tip, not what you want for nitrous use. Not sure if it would be considered a "race" plug or not. They may not make a stock heat range plug in the r5671 style, which would make sense considering they are a race, power added oriented plug. If you are going to spray the engine, why do you want to stick with a stock range plug?

Honestly, I wasn’t sure I should go to a “colder” and would like to know which NGK plug has similar heat range to the autolite 3924’s... that said you are correct however when searching eBay for a R5671-5 it only showed the -10 which I would think would be super cold yes??
Bottom line.. for a 125-150 maximum shot through a TB plate or single nozzle should I start with the R5671A-7? I don’t think I want 2 ranges “colder” but I don’t think I saw a 5671-6 range either and had 5671-10 coming up when 5671-5 was entered in search has me too puzzled to pull the trigger... I could go to local part store and have them pull every NGK plug they have... about only way to get them locally... idiot kids at autozone etc...

skiwesser11 · Mar 21, 2019

https://www.sparkplugs.com/learning-center/article/116/how-do-i-find-a-colder-or-hotter-plug

RangerJoe · Mar 22, 2019

I think you are right, they don't make that plug any hotter than a 7. I am sure you could get away with another style plug in the 6 heat range on such a small shot. In my opinion, the worst that could happen from a too cold plug is it could foul out quickly. Too hot though could lead to a melt down. Like I was saying on mine, I think a 7 may be too hot. I am going to pull timing this weekend and try it again with a fresh set of plugs.

Joe

Gear grabber · Mar 22, 2019

The -7 NGK will be fine. On my car with,tall single plane intake and 950 carb.,i have no problem keeping -9 plugs
clean. A.F.R.'s are 13.5 to 13.7 at idle and 14+ at cruise. This is much richer then your EFI car will ever be.

skiwesser11 · Mar 22, 2019

Ok looks like the NGK R5671A-(X) is considered a “race plug” perhaps because of the non protruding electrode?? At any rate I ordered some -7’s online but at parts store today I got confused with remembering the fact that the BKR6E which I BELIEVE to be one range colder than an autolite 3924 is still a protruded type as Joe informed me of above... instead of idiot kid I got the aggravated old guy who didn’t know squat nor cared to help me try and find out what part number is not protruded but in heat range of around-6 (which doesn’t appear to exist) I bought them as I was afraid of the consequences so I suppose I can at least compare to the autolites and have fresh plugs to “try” and read... thanks again guys.... and if anyone knows a plug part number that is 1 heat range colder than a 3924 please let me know no matter the brand..
Good night interweb

RangerJoe · Mar 23, 2019

I think you already mentioned these, but I think this is as close as you are going to get.
https://www.summitracing.com/parts/ngk-4922/

If you stick to your 100-125 goal, you will probably be fine, but you could always call Ngk and ask.

Joe

HotFox · Mar 23, 2019

Have had really good luck by calling the NGK tech line for advise. I have built some odd ball stuff in the past and they are always good to work with if you know compression ratio etc.

skiwesser11 · May 11, 2019

Well I had guy at speed shop tell me this week that a “6” heat range is too hot for anything.. you guys agree? I’m running the non protruding 6 heat range mentioned above but I do have a set on -7’s which guy (who is someone I’m sure knows what he’s talking about) said is absolute “hottest” I should run with any spray...recommended -8’s for 125 or moreover a 52n jet with a 19-23 split for fuel and an extra degree less timing so I’m going to start with a 53n 31f and 6 degrees pulled.. with the .7’s at .032... thoughts?? AFR called for an autolite 3924 NA which I believed to be equivalent to an NGK 5 heat range wise so I THOUGHT a 6 was one colder..? I believe guy at shop as he does mostly big power mustangs but internet can’t be wrong..?? Thank you guys!

Gear grabber · May 11, 2019

His plug recommendations are spot on. Pulling 6 degrees is a bit much for a .053.
I am spraying near double that (.050n and .052n) and pulling 6. (24 * total timing)

skiwesser11 · May 21, 2019

Gear grabber said:
His plug recommendations are spot on. Pulling 6 degrees is a bit much for a .053.
I am spraying near double that (.050n and .052n) and pulling 6. (24 * total timing)

2 “stages” ? What fuel jet with each? Or are you carb?

Fox Spark plug heat range

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