One heat grade colder spark plugs
 

If I understand correctly, the heat grade of a spark plug is a trade-off between keeping the plug hot to burn off deposits, and keeping it cool to avoid pre-detonation. Car manufacturers will bias this trade-off towards hot because it keeps the service interval longer. Whereas - we ecomodders - could bias this cooler because we don't want pre-detonation. Once the ECU detects pre-detonation (knock) the timing will be permanently retarded until the ECU is reset. Retarded timing means less power is extracted from the fuel, and so worse MPG.

The NGK website says Spark Plug Terms
"When making spark plug Heat Range changes, it is better to err on the side of too cold a plug. Running too cold a plug can only cause it to foul out, whereas running too hot a plug can cause severe engine damage."

So worst case, cooler plugs would just mean checking the plugs more often for fouling.

Will this work to improve MPG???

I don't know but your theory sounds good to me.

Not sure, but in my pickup the retard on the timing is only for a short period, basically once I lift up off of the throttle it resets.

If one is doing other things that tend to put an engine on the ragged edge of detonation like grille blocks, higher temp thermostats, nearly lugging it, etc., colder plugs may expand the envelope of what you can get away with.

This is done quite a bit in the tuner world, especially those who are tuning under boost and e85... typically they run chipped ECU's or complete FMU's so it's a lot easier to manage ignition timing.

That being said, those who don't have that option (due to emissions testing, budget, etc.) often "trick" the stock ECU in order to gain a few extra degrees of timing... myself included (emissions testing here requires OBD2 port plug in, so I can only run a piggyback)

If you're looking for that few degrees timing, the most common way is to retard the base timing, generally around 4 degrees from stock then reset the ecu (disconnect battery, turn ignition on, press brake pedal 10 times, let sit for a few min, works on most vehicles) and allow the ECU base-map to establish itself (usually around 100mi) and then set the timing to factory specs.

This will give you up to a 4 degree (ECU controlled) increase in timing throughout your load range without exceeding the max OEM timing advance... not sure what it is on your Toyota, but on my Acura it's around 45 degrees.

On the other hand, those of us who are good at DWL might need hotter plugs because we spend so much time at low power settings.

Just a thought....

...simple "verification" test is to pull a plug every so often and SEE if it's running hot (burnt) or cold (carboned up).

I guess I'll be the Guinea pig. My ranger is timed at 20*btdc with the stock plugs. What I can do is toss in 4 lower heat plugs, unplug the battery, and then get the timing with a timing light. Does that sound like a solid experiment? Or did I miss something? Ill do it this afternoon because now I'm curious. Also, should I go 1 heat range lower, or 2? Maybe 3? It's not my dd and I don't mind blowin it up (that would give me a reason to swap in a smaller motor)

maybe i missed it, but unplugging the battery won't do you much good. the computer constantly adjusts to whatever the conditions are. do you know if you have vacuum advance timing or electronic? with vacuum you may not see any benefit.

Hey BackroadBomber,
That's great that you are going to give it a try. I don't think you will see instant results with this, only under those limited set of circumstances when pre-detonation would occur. As has been pointed out, that is probably more likely if you have a grille block or other mods to keep the engine hot.

@KSA8907, disconnecting the battery is required after installing the new plugs to clear the ECU of those situations it has already learnt when pre-detonation occurred with the old plugs. i.e. to give it a fresh start and learn the new parameters.