Spark Plug Heat Rating

[JerrryR]

Hi All,
I was hoping someone could provide a tutorial about the spark plug heat rating. I have had it explained to me in the past but never in a fashion that really explained it to me.
What is the purpose for the the different plug heat ratings?
What is different about the plug construction to accomplish this goal?
Thanks,
JerryR

Spark Plugs Cropped.jpg (129.84 KiB) Viewed 74912 times

[EKHKHK56]

Good topic Jerry. Different heat ranges are available to tune your engine combustion chamber temperatures after all else is done. Spark plugs have different heat ranges set by insulator length, style and thickness, center electrode metal type, and electrode length. A hot plug will generally have a long insulator and projected electrodes. Many things affect combustion chamber temperature. Compression ratio, timing and advance, purpose WOT?, engine load, fuel type, fuel mixture, spark gap, ignition type, ambient temp, engine temp, etc. The end of the spark plug needs to be at a temperature of about 900 to 1400 degrees F to be happy. Below 900° deposits build and don't self clean. Above 1400° the hot electrode will preignite the fuel charge causing pinging and detonation. So you take your super hot rod with deluxe everything that 5 different people built and do go do some spark plug color tests to see if you are close to in range of where your engine is happiest plug wise. Generally a heat range or 2 nearest the factory recommended spark plug is going to be the one unless you are severely modifying. It's amazing what a difference proper heat range can make. I tune all older Chevys and Fords with AP26 Autolites, the hottest one they sell. All those cold stock 24s they sell need to be shipped out of here to Arizona. They don't work in this cold climate even though stock. Single platinum no wear 4 year guarantee. Great unit! So different plug type different systems. Champion and Autolite plugs get hotter with larger number, NGK plugs get colder with larger number. The best plug is the one that works best! Erik K

[sportsterpaul]

Check out picture at this site:
http://mechanics.stackexchange.com/ques ... heat-range

It shows how the insulator in a cold plugs removes heat from the center electrode faster. Like the chard below that picture says-- too hot and it pre0-ignites the charge before the spark event. Too cold and the plug collects deposits that didn't get burned off.

[hayleyl]

Hello all - what everyone has posted is spot on - here is a stripped back explanation - 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, keeping the firing tip cooler. A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter. To choose the correct heat range for your scooter depends on multiple factors. Modifications or stock, type of fuel / quality, induction - forced or natural, engine speed / load, ignition timing, compression ratios etc. etc. Hope this helps. Cheers Hayley.

[Willis1955]

Hi, just thought I'd put my two cents in. On a Harley Davidson branded Sparkplug the smaller the number the hotter the plug. Such as: a number 4 is hotter than a number 5, 3 is hotter than a 4. This is the way Harley did it. Champion made the plugs for Harley but Harley had there on system for the plugs heat range. Hope this helps.

[LDB]

A spark plug, being cold or hot, refers to the physical temperature of the firing end of the plug when the engine is running. The heat range of a plug is the range that the temperature will vary.

A heat range test is the best way to determine the correct spark plug for a particular engine set up and a person's riding habits. When in doubt always choose a colder plug first until you determine what works best. The worst thing that can happen is the plug can foul out whereas too hot a plug can cause pre-ignition and possible damage.

Conduct a heat range test as follows:

Ride out someplace where you can run the bike at a sustained speed for a few miles without having to slow down or stop; very important. This allows the heat to build.

Run the bike up to the maximum speed in high gear that you would typically drive if you decided to drive fast, and I don't mean dangerously high speed. Hold the bike at that sustained speed and without letting off the throttle, turn off the ignition. Let off the throttle, disengage the clutch right quick, shift into neutral and coast over to the side of the road. Allow the engine to cool off a bit. Remove the spark plugs and examine the color and condition of the electrodes and insulators. The color will be a true indication of how hot or cold the plugs are running.

Now remember this .... if you run the engine and then let it slow down or idle and then check the plugs, you will not get a true indication of the heat.

Of course this is not all set in stone. A person who putts around at slower speeds and never faster may need a hotter plug. The lower the combustion chamber temperature, the hotter the plug should be. On the other hand, as conditions cause the combustion chamber temperature to rise, a colder plug may be necessary. Also, a fuel mixture that is too rich or too lean can confuse the readings; plus a poor ignition system, poor compression, an oil burner, etc., etc.; so the accuracy of the above depends on an engine with proper function.

In conclusion, the plugs should run hot enough to keep the deposits burned off and that's it.... This means the color should range anywhere from light brown to grayish tan. Sooty black is too cold and chalky white is too hot.

[hennesse]

Hi, just thought I'd put my two cents in. On a Harley Davidson branded Sparkplug the smaller the number the hotter the plug. Such as: a number 4 is hotter than a number 5, 3 is hotter than a 4. This is the way Harley did it. Champion made the plugs for Harley but Harley had there on system for the plugs heat range. Hope this helps.

The Sportster Service manual says "Plugs are labelled with the numbers 2, 3, 4, or 5, the lowest number indicating the hottest plug. Designations 3-4 and 7 are special purpose plugs. [...]if the number 4 plug is used on original equipment for normal service, the number 3 plug could be used for slow speed or short run operation, while the number 5 could be used for the higher speeds of highway travel or maximum throttle operation. It is not uncommon for best results to be obtained with plugs of different heat ranges in the front and rear cylinders with the front usually the colder.

My '54 KH (with KR cams, KR double valve springs, and KR heads) came with a #8 in the front and a #7 in the rear. I get that - coldest and penultimately cold.

But what the heck is a 3-4?

Anyone got a collection of old Champion plugs? I'm looking for a pair of NA-12 (or NA-10 or NA-14).

[EKHKHK56]

3.5 ? Maybe in between..have some I'll pull out and look at. I don't think I have any of the other ones you speak of but I'll look. Have quite a collection.

[sportsterpaul]

The 3-4 plugs have an extended tip. My theory was it would spark the mixture better, and get cleaned by combustion heat better. I think its heat range is closer to a 3 than a 4. I ran them for years until I got too cheap for factory plugs.

Here is a picture from an eBay auction selling them for 12 bucks a pair:

Harley_3-4_spark-plugs.jpg (122.95 KiB) Viewed 74807 times

Note how far the tips extend out compared to a 3 or a 4 or a 5. Make sure the piston does not hit the electrode however. They worked fine on my stock 900cc and 1000cc Ironheads.

Supposedly the Champion J12YC is the exact same plug as the Harley-Davidson 3-4 plug.

[mikeslemmon]

a HD 3=4 is the same as a champion J12Y or maybe a H12Y the H plug has a slighty longer thread both work in a sportster some say a colder plug in the front cylinder . .champion.J10 11 12 are all the same .. something to do with. dodge plymoth and de soto 318 motors (propaganda ) not heat range