spark plug heat range.

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AdamR

AdamR

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I need help picking plugs for my 340. I noticed today that it has 4 differant heat range plugs in it for some odd reason. You would think that it would have gotten a fresh set of mayching plugs before going on the dyno but it hac AC 43 44,45,46s in it. some are resistor plugs some arent. So my questoin is which is the correct set for the motor. Its a 11:1 compression 340 with iron heads.
 
Adam; I know this doesnt answer your question 100% , but in the Autolite line, 65 is the plug listed for a 340. I use them in my 360 PU and the 74 Duster, but they sure arent 11:1 engines.
 
Autolite 65s is what I all ways ran ib my small blocks put they were never high compression, big cam motors.

These cross over to autolite 24, 25 16 an 26 respectively which just confuses me more. Also some of them are TS plugs and some arent.
 
AdamR said:
These cross over to autolite 24, 25 16 an 26 respectively which just confuses me more. Also some of them are TS plugs and some arent.
Click to expand...


Wow, that is confusing. Hope someone chimes in later that can answer you question, Id like to know also.
 
Adam, I would start with the colder range on the stock plugs. You'll more likely not need more than a step colder. I would honestly be very surprised if you needed or the engine like a plug 2 steps colder.
 
I have autolite 64's in my 340.

The way I read the heat range is look at the electrode. The plating should be burned about half way or a little less from the tip to base. If it's only burned 1/3 of the way, too cold, more that half too hot. You'll see the definition line on the electrode.
 
I`m running a 9 heat range in Champion with a 10.8 - 1 CR. Here`s a Champion and NGK plug chart that may help you some. A lot of guys are running NGK now . I can`t believe they dynoed your car with those plugs and AC brand at that.( They seem to only work well in GM product). I would almost bet money it would have made more power with a new set of Champions or Autolites.

champion (Small).jpg


NGK chart (Small).jpg


NGK chartpg2 (Small).jpg


NGKsparkplugNum (Small).gif
 
I was surprised as well. If he asked I would have bought a new set for it. He probably didnt even look at them though. the motor sat in the corner of his shop for years before I bought it and he dynod it for free so I cant really complain.

I never had luck with Champions, all ways ran Autolites.
 
If you have headers you may want to take advantage of using 5/8" hex plugs instead of 13/16" hex.

Mopar switched to the smaller hex plug with the magnum engine, they have the same thread and reach as the bigger plug. The stock heat range for an Autolite plug is 3924, I have been running 3923 plugs in my 10.6:1 360.
 
Adam 43 would be the same as the old n10y that cam in a factory 10.5 340 in the day. 44's are good in about a 9.5 motor, and 45s are for later model 8.2-9.0 ranges. But it is actually how the plugs look after a hard run that counts. You don't want to melt them down. But for all around a decent part throttle steady drive on the highway and then color match is what you should look at too.

I think I would look in the 43 range if you stay with AC, or go with a 10 to 12 in a champ. These would be a good starting point.
 
AdamR said:
Autolite 65s is what I all ways ran ib my small blocks put they were never high compression, big cam motors.

These cross over to autolite 24, 25 16 an 26 respectively which just confuses me more. Also some of them are TS plugs and some arent.
Click to expand...

These are 5/8 GM plugs aren't they? It sounds like someone stuck them in there just to fill the holes! Do you have the stock or aftermarket heads ? I have the Edelbrock RPM heads and run 3926 Autolite, (really low compression).
 
Xstream_1 said:
These are 5/8 GM plugs aren't they? It sounds like someone stuck them in there just to fill the holes! Do you have the stock or aftermarket heads ? I have the Edelbrock RPM heads and run 3926 Autolite, (really low compression).
Click to expand...

Yes they are a 5/8" plug that was used in every GM ever made.
the heads are ported J heads,
 
None of this makes any sense to me and is making my head spin.

Who has a link for "spark plugs for dummies".:dontknow:
 
Adam how come you dont have a 3/4" plug in these heads? I worked on 1000s on GM cars, did tune wrk for those dummies for 15+ years. All GMs use like a r45ts, some truck motor homes use R44ts. Even IROC 350 and vettes used 45 heat range plugs. In the old days we used to see Nova's with the 300HP 350s running 10:5 pistons using 43 heat range plugs and vettes with 42s.

On AC plugs
R= Resistor
S= extended tip
T= 5/8 length taper seat plug type.

44 is one range cooler, 43 one lower yet and so on. A 45 is = to a champion in the old N14Y book, a 43 is is an N12 y, and a 42 would be the same an an N10Y which is what a standard high output 340 used from the factory.

Spark plugs are for some an emotional think. They all work fine, just nee the correct heat range, that said I will admit that most autolites have a harder shell.

I would think you would want the plug out in the flame travel and wnat to avoid carbon etc on the plug threads? I have seen heads modified for short plugs though. Is this the reason you have a short plug?
 
Dave, I have no clue why it has these plugs in it. My guess is he had them there so he put them in.
 
i run the ngk crossmatch for the stock champion 5/8" plugs in my 360. 10.8:1 eddy heads. i would start at 1 cooler for 11:1 iron heads. on msd, see how it likes .045 or .050 gap.
 
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