Can you explain the cold and hot?

How do you know which direction to go?

Physically looking at them, how do you know?

Yes, physically looking at them.  I've found that the finer the tip the easier it is to spot overheated plugs because the electrode will erode fairly quickly because its being melted/burned away by the heat.  This assumes that the mixture is correct, because if you go too lean it will exacerbate overheating. 

Fouled plugs should be easy to see as well.  If the plug is black and sooty, or in a two stroke it will be oily, you know you need a hotter plug provided that the carb and timing are set correctly, and the fuel/oil mix is correct in a two stroke. 

In general, if the engine is stock, or only lightly modified you should be fine with the stock heat range.  If you have changed the cam, increased compression, added forced induction (or turned up the boost on an engine that was already force fed) you will want a colder plug.  If most of your driving is low speed and the plugs aren't getting hot enough to burn off the extra fuel from the warm up cycle you could benefit from a hotter plug.  In general though the more performance you wring out of the engine you will go colder. 

This page has a clear explanation: https://www.howmechanismworks.com/2018/09/difference-between-hot-spark-plug-and.html#:~:text=Hot%20%26%20Cold%20means%20the%20relative%20cooling%20capacity,spark%20plug%20tip%20and%20lower%20the%20tip%27s%20temperature.

From the page:

Hot & Cold means the relative cooling capacity of the plug while operating in your particular engine. If a spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature. Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug. The heat range of a spark plug is typically specified as a number, with some manufacturers using ascending numbers for hotter plugs and others doing the opposite, using ascending numbers for colder plugs."
 

val fulesday wrote:

question how do you read the numbers on the spark plug itself to go hotter or colder?

It depends on the brand.  Typically there is a part of the number that will go up or down, but that may indicate one direction in one manufacturer's numbering system and another in another's.  My advice would be to simply ask the counter guy at a good auto parts store.
 

val fulesday wrote:

back in the day that would work, today.... good luck, if it isn't on the computer they  don't have a clue as to even look in a catalog. off topic but true, I needed an axle seal went to auto zone, gave the guy all my information, he came back, and no kidding told me that seal is no longer made. I told him I guess it's just time to throw away the whole car then. how many seals do you think are made that are that size with just a different sealing area but still the same size, 50 ish?

It still works today, you just have to go to a parts store with people that actually know something other than what's on the computer.  My local NAPA has a couple guys that can still actually help you, and only one of them is an old curmudgeon. 
 

You can absolutely still figure out the heat range by the part number, no counterman needed. 

For example here is Autolite's explanation. https://www.autolitesparkplugs.com/how-do-i-find-a-colder-or-hotter-plug#:~:text=Autolite%20indicates%20the%20heat%20range,number%2C%20the%20colder%20the%20plug.

And here is NGK's charts https://www.ngk.com/ngk-spark-plug-numbering-systems

If you notice NGK, in their infinite wisdom get COLDER when the number goes up.

Using that you can compare it to the plug you have now.

I will say one thing, and this is probably equal to talking politics or religion, but I think the exotic metal plugs have no place in performance cars. I spent 10 years working at an auto supply (before Auto Zone's and Oreilley's) seeing all the BS with iridium, platinum, V-Power, etc, etc. Give me a good quality copper plug any day...

It's more about the engine and how it's tuned.

Here's my own example. When I very first bought my 1995 Lightning I did all the basic bolt on mods, like headers, a K&N, under drive pullies etc. I also bumped the timing up about 3 degrees over stock. I put in one range colder plug to ward off detonation.

Now that same truck has a completely different engine and a supercharger. So it has the coldest plug I could find from NGK, but STILL for the same reason, to ward off detonation.

If it were me and I had a mildly modified car I would run 1-2 heat ranges colder. If you add more modifications (alum heads for example) and go with more aggressive timing I'd drop the heat range one more to 3 colder.

In naturally aspirated engines with modern ignitions I've typically found that the stock heat range works fine.  In a nutshell I don't go messing with the plugs unless there's a problem, either a fouling issue, or a plug life issue. 

I agree with Raymond about exotic plug materials to a point.  In a naturally aspirated engine a standard copper plug is always going to work best.  The reason platinum plugs started wasn't better performance; it was longer life.  OEMs wanted to be able to have engines go 100k without needing plugs.  That's never going to happen with copper, so enter platinum.  The need for this extended life went up when they started firing the plug more than once per 4 stroke cycle.

Iridium is another animal because it was designed primarily for forced induction applications.  I would NEVER waste the money on iridium plugs in an NA engine.  In a turbo engine though they work really well.  Under those conditions the plug has to fire against 2x atmospheric pressure, or maybe more.  Chamber temps tend to be higher as well, so electrode erosion is a bigger concern.

One thing I'll add is that all the E3, Diamond Fire, etc. stuff is pure 100% marketing BS.  A spark is a spark.  All an engine needs is sufficient spark to initiate the burn.  If the spark is of sufficient strength you aren't going to improve anything with a different plug.  A fire is either lit, or its not.  In his book Smokey Yunick talks about the months he spent testing all manner of ways to initiate combustion in a spark ignited engine.  In the end he went back to regular old commercially available plugs, because he saw no benefit from anything else.  Apparently, even in the '50s the spark plug engineers were very on the ball, and old Smoke didn't praise many engineers, so it must have been true.