4mulas
Fixem'up
Haven’t had a small block for a long time. What is the stock plug number for a 340?
71 10.5 compression, purple cam big valve J heads. thanks!
71 10.5 compression, purple cam big valve J heads. thanks!
What spark plugs for 340
- Thread starter 4mulas
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Yote
Well-Known Member
‘72 340 — I believe - Champion R12 YC is what manual calls for.
I am running the equivalent NGK GR4.
Yote
I am running the equivalent NGK GR4.
Yote
Professor Fate
Push the button, Max...
Stock plugs?
Autolite 65
Champion 38 or 44 (or the above posted numbers; different numbering system)
NGK 2635
AC R44XLS
.035 gap
Autolite 65
Champion 38 or 44 (or the above posted numbers; different numbering system)
NGK 2635
AC R44XLS
.035 gap
Scody21
Just send it
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This is what I run in my .030 over, 10.5-1, 2.02 J headed 340. And they are peanut plugs aka small socket plugs.
harrisonm
Well-Known Member
Autolite 65. They work great.
The original plugs were RN9YC
I realize asking for 'what plug' can lead to a lot of thoughts, so here is question1970 Swinger 340 4spd. I was told I had 10.5 compression, I use mostly,,,,, straight aviation fuel I get from the local airstrip. I tend to favor Champions, does it really matter, which one of these 3 ? RN7yc, RN9yc or R12YC , use with Taylor StreeThunder plug wires
Woodys_Cuda
Ontario, Canada
When I went to get plugs for my stock 68 340 a major Canadian auto store looked them up. R12YC. I liked the heat range R9YC better for the crappy gas we find here. There are other alternatives as you see here.
All of the big box stores show the RN12YC. I would be curious to see what a 68 service manual says. I think the 12 is more in line with the 73 low compression engine, not the 68, but I've been wrong before.
similar set up 10.5
heavy cam/ 3000 stall converter 3.23 sure grip /factory avs carb 91 octane /alcohol free with a bottle of 108 boost every tank
E3 .46 with pertronix converstion excellent throttle response in 2.02 J heads THEY WORK
heavy cam/ 3000 stall converter 3.23 sure grip /factory avs carb 91 octane /alcohol free with a bottle of 108 boost every tank
E3 .46 with pertronix converstion excellent throttle response in 2.02 J heads THEY WORK
Professor Fate
Push the button, Max...
The '68 FSM lists Champion N9Y gapped at .035" for both manual and automatic 340s.
Kinda what I thought, but since I don't have an early FSM I wasn't sure. That's just like the aftermarket to clump everything into one number. Thanks for posting that!
Professor Fate
Push the button, Max...
N14Y is listed for the 273 and 318- so you're probably right, the aftermarket most likely just lists N12Y as a catch-all compromise for all small blocks.
I don't have the 73 340 info, but the 12 heat range may even be incorrect for it as well.
lemondana
BlackDart
And the 273 4 barrel calls for N10-Y, same as the Hemi.
Woodys_Cuda
Ontario, Canada
I guess that is why my car runs better with the N9YC rather than the parts store recommended N12YC. Thanks.
aviation fuel is designed to atomize at low temps/altitudes and evaps even faster under a hot hood!. I am sticking with alcohol free 91/93 even that's bad compared to the gas they sold 50yrs ago for these engines!!!
Thanks for the info,,,,,the AV fuel I think is around 100 octane. Plus it has some lead in it, that should be good for the older engine,,, I think ,,,, The guys at the airport say it works good in old tractors, and lawn mowers. Also use it in your portable generator for power outages,, it will last forever, not go sour like today's pump fuels.
RN12YC are 318 , 360 plugs . They also work in 72 , 73 8.5 compression 340's but they too prefer RN9YC's. The 68,69 and early 70 10.5 compression and the late 70 and 71 10.3 compression take a colder plug RN9YC. A big difference in performance will be seen with the correct plug in the early 68-71 340 . The power will be greatly seen in the mid range and up. Years ago I threw some RN12YC plugs in my 69 swinger. I left them in for one ride and put the old RN9YC's back in.
lemondana
BlackDart
The first prefix R stands for resistor. I found out many years ago that my 68 340 GTS didn't run as good with the resistor plugs as the standard N9-Y. Might be that is just me. But then again I ran around with stainless solid core plug wires-they wrecked havoc with people's TV reception before cable.
I didn't use the resister plugs either . But that would just be more confusing for some members. The non resister plugs also interfere with some of today's later ignitions and Stereos.
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