Spark Plug Heat Range

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ramcharger

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I know that is something that is not discussed much here so I thought I'd bring it up. I'd like to hear everyone's opinion and practical experience as well as share my own.

My first true experience with the wrong heat range was when I added a 150 shot of N2O to a Ford I built when I was 18 (OK fine, Flame on peckerheads, but this is how experience is gained, lol!). Anyway, I couldn't understand what the little "specks" were on the center electrode insulator when I'd pull a plug after a nitrous run. needless to say, I blew a head gasket shortly thereafter and had to pull the heads. When I did, I noticed that one edge of a few of the pistons looked like it was chewed by a small mouse.

This being the early 80's and widespread use of nitrous was fairly rare and there being no internet as we know it now, I figured it was due to too much advance and backed off 4 degrees. Went back to the strip and promplty comprimised another head gasket. I limped back home with a couple jugs of water and changed the gasket again. The chewing was worse and I had a three or four (my memory fails me, lol!) cracked plug insulators.

I called NOS, and they said "Drop your heat range 2 numbers". End of problems but I still had to re-piston the ford.

So, this being my experience I thought I'd also add what Autolite says about heat range:

Heat range

The term spark plug heat range refers to the speed with which the plug can transfer heat from the combustion chamber to the engine head. Whether the plug is to be installed in a boat, lawnmower or racecar, it has been found the optimum combustion chamber temperature for gasoline engines is between 500°C–850°C. When it is within that range it is cool enough to avoid pre-ignition and plug tip overheating (which can cause engine damage), while still hot enough to burn off combustion deposits which cause fouling.

The spark plug can help maintain the optimum combustion chamber temperature. The primary method used to do this is by altering the internal length of the core nose, in addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges. When a spark plug is referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into the engine head, which keeps the firing tip cooler. A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter.

An unaltered engine will run within the optimum operating range straight from the manufacturer, but if you make modifications such as a turbo, supercharger, increase compression, timing changes, use of alternate racing fuels, or sustained use of nitrous oxide, these can alter the plug tip temperature and may necessitate a colder plug. A rule of thumb is, one heat range colder per modification or one heat range colder for every 75–100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.

The heat range numbers used by spark plug manufacturers are not universal, by that we mean, a 10 heat range in Champion is not the same as a 10 heat range in NGK nor the same in Autolite. Some manufacturers numbering systems are opposite the other, for domestic manufacturers (Champion, Autolite, Splitfire), the higher the number, the hotter the plug. For Japanese manufacturers (NGK, Denso), the higher the number, the colder the plug.

Do not make spark plug changes at the same time as another engine modification such as injection, carburetion or timing changes as in the event of poor results, it can lead to misleading and inaccurate conclusions (an exception would be when the alternate plugs came as part of a single precalibrated upgrade kit). When making spark plug heat range changes, it is better to err on the side of too cold a plug. The worst thing that can happen from too cold a plug is a fouled spark plug, too hot a spark plug can cause severe engine damage

In conculsion, what brought this up is that I had installed stock heat range plugs in my latest engine without thinking (DUH)! I'm out tomorrow to grab some plugs one range cooler due to my added compression and relatively short duration cam.
 
HUmmmmmmm, for the most part, I use stock heat ranges for the rides. 1 or 2 steps cooler if the ride is hot and it is an experiment to see how it works, Nothin like a track to test and tune though.
 
My biggest question is how are we supposed to know if the plug tip is at 500 to 850 centigrade? lol!

I say run on is a big indicator, but that would be seen most often in a street driven car, not a strip car with N2O. Watch out for the "specks" at the strip though!
 
Great post Joe. I have tried to research what plugs should be used and how to correctly read them and came across some conflicting methods even by big name racers so I'm still in the dark as to whats exactly right other than I do look for specs as you mentioned cause I know that is definitely an indicator of a problem. As soon as I read the part about how to keep the temp between 500-850C I thought the same as you, how would we ever know that because 99% of us average guys wouldn't have a way to measure that.

One thing I'm kinda unclear on is the part where they said a cold plug dissipates heat faster than a hot plug which makes sense but from what I've read about determining the heat range you should use is indicated by 2-3 of the threads nearest the firing tip should show heating. More threads than that and your plug is too hot. Less threads and your plug it too cold. That seems to be backwards from what Autolite is saying if I'm understanding them right but they didn't mention any way to indicate the correct heat range except for the temp method.

I read the above mentioned indicator of the correct heat range on FBO's site and a couple other places. What do you guys think? Am I understanding Autolite right or have it backwards? If I had $20 extra to blow I'd just go out and buy a set of colder plugs and throw in and run it and see what happens. I may do that sometime anyway.
 
10-4 Fishy (Pardon the 70's CB reference, lol!). All Autolite really gives you is the 100 hp "Rule Of Thumb". I'll keep Googling to see what I can come up with. I'm sure that the pro drag and road race guys have found some answers but they make their living with this knowledge and I don't blame them for not sharing.

BTW, I have not installed the cooler plugs yet due to the heat caused carb boiling problem. One change at a time, right? LOL!

As Hot Rodders we are all in essence mad scientists treading where the OEM's don't go or won't tell us what they know. These engines were never designed to produce the HP that we're pushing them to, but damn, it is fun! lol! Unless we're running NASCAR or maybe the new Challenger Drag Pack, they are not going to give us the info we need, so it's up to us to gather data through empirical experience and share with out Mopar Bretheren.
 
the 100 hp "Rule Of Thumb".
?????????? OK, fish bites .............

Never mind, I found it again..

A rule of thumb is, one heat range colder per modification or one heat range colder for every 75–100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.
 
10-4 Fishy (Pardon the 70's CB reference, lol!). All Autolite really gives you is the 100 hp "Rule Of Thumb". I'll keep Googling to see what I can come up with. I'm sure that the pro drag and road race guys have found some answers but they make their living with this knowledge and I don't blame them for not sharing.

BTW, I have not installed the cooler plugs yet due to the heat caused carb boiling problem. One change at a time, right? LOL!

As Hot Rodders we are all in essence mad scientists treading where the OEM's don't go or won't tell us what they know. These engines were never designed to produce the HP that we're pushing them to, but damn, it is fun! lol! Unless we're running NASCAR or maybe the new Challenger Drag Pack, they are not going to give us the info we need, so it's up to us to gather data through empirical experience and share with out Mopar Bretheren.

That 100 hp rule is interesting. That's the first time I'd heard of that. If that's the case I probably have the right plugs in my 360 since I have 68 340 heads and run one step colder than stock 340's ran and my engine is about 80-85 hp more than they were stock.

That's funny about us being "Mad Scientists" cause when I first got Desktop Dyno I was constantly playing with it and my wife called me a "Mad Scientist" because of it.
 
That's funny about us being "Mad Scientists" cause when I first got Desktop Dyno I was constantly playing with it and my wife called me a "Mad Scientist" because of it.
You oo huh? My wife says such things about me, cars aint the end all of the matter ethier. You know the drill, it works fine, bring it here, let me see if I just can't make this work better.
 
In 1965 I had a 63 dodge 330. We changed the pistons to the 13.5:1 ratio ones and had spark plug problems. We got fuel from the airport and the problem persisted, little beads on the porcelain. Also porcelain very white and glazed. The only solution we could get then was J2J Champion spark plugs which worked great. The originals were J9Y Champions. I don't remember exactly what J2J's fit, a 2 stroke lawn mower I think. We just couldn't find any of the "racing" plugs, different numbering system, where I lived. Anchient history huh?
 
Very interesting thread. I never even thought about heat range of the plugs going to zero deck and higher comp pistons......may have to try a colder plug and see what happens.
 
not to sure about that 100 hp rule , i have done many motors that have doubled the hp. and ran the stock plug just fine, that does change with high comp, and power adders tho...
if i can find it ,there is a great link to a plug reading site. there is a lot more to it .maybe some of the other guys have that site ?
here is the link
http://www.4secondsflat.com/Spark_plug_reading.html
 
Good site 68gts340. Lot's of great info there.

That's the one I use also. Don's a member here on the board but I can't remember his handle.

BTW: I shopped around for a spark plug magnified reading light. Found them at a couple places for $50-60. I was just ready to order one then one day we were in Walgreens and I noticed an ear and throat inspection magnifier light for $9 so I thought what the heck might as well try it. If it didn't work to read plugs I could always throw it in the first aid kit. It actually works pretty decent to read plugs. You can see all the way down to the bottom of the insulator.
 
I found a website sometime ago that had a PDF file on 'Speed Tweaks and Tricks' and for Magnum engine (Since I retrofitted Magnum heads to my 360 LA). And it mentioned using AutoLite 3923 plugs...

"While the factory and factory replacement spark plugs work, the Magnum engines have shown slightly better performance with the Autolite 3923 Spark Plugs. THe Autolite 3923's are a standard tip spark plug that is one heat range colder than stock, as compared to the "Special Power Tip" plugs that the engine came with. Why the Difference? A standard tip is less susceptible to detonation, is a good replacement for modified magnum engines, and work very well with modified engines as well...especially when using nitrous or a supercharger. In some cases, the Autolite 3923's also showed a 4-5 HP difference at the rear wheels on the Dyno. (The test was performed with new OEM Spark plugs vs. the Autolite 3923's. No changes to the engine and the plugs were tested on the same day under the same conditions.)"

I've been using them with MSD 6A and wires and running fine.
 
that brings another point thats worth mentioning,,, when you are using a msd, plug reading becomes much more difficult becuase of the multi spark .. plugs tend to burn so clean there is little to read..so we really have to be careful in this case.,,,at least this has been my experiance....
 
I found a website sometime ago that had a PDF file on 'Speed Tweaks and Tricks' and for Magnum engine (Since I retrofitted Magnum heads to my 360 LA). And it mentioned using AutoLite 3923 plugs...

"While the factory and factory replacement spark plugs work, the Magnum engines have shown slightly better performance with the Autolite 3923 Spark Plugs. The Autolite 3923's are a standard tip spark plug that is one heat range colder than stock, as compared to the "Special Power Tip" plugs that the engine came with. Why the Difference? A standard tip is less susceptible to detonation, is a good replacement for modified magnum engines, and work very well with modified engines as well...especially when using nitrous or a supercharger. In some cases, the Autolite 3923's also showed a 4-5 HP difference at the rear wheels on the Dyno. (The test was performed with new OEM Spark plugs vs. the Autolite 3923's. No changes to the engine and the plugs were tested on the same day under the same conditions.)"

I've been using them with MSD 6A and wires and running fine.

I read that article also. I used to have a 93 Dakota with a 5.2 magnum and over the yrs. I tried a few different plugs in it to see if I could tell any difference. I didn't have a dyno to go by or drag strip time slips, just seat of the pants and mpg but when your used to driving something everyday you can get a feel for changes. I found 2 plugs that did seem to perform a little better than the original Champions and the Autolite was one. Equally as good was a NGK V-power and they seemed to last longer than the Autolites. The first aftermarket plug I tried was the Split Fires. That's right when they first came out and I let the hype get to me and wasted $40 on a set of spark plugs. They didn't run any better if as good as stock plugs. I think it's because the dual electrode shrouds the flame front but that's just my theory. Knowing that voltage takes the path of least resistance to ground it makes sense that these plugs are nothing but an over priced gimmick. I later added a MSD and found it ran better with the plugs gapped at .045 and also got a little better MPG.
 
I just picked up some 3923's today. haven't had a chance to install them with all the swimming and barbequeing going on:)
 
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