Sorry, not the most articulate or concise title.

The issue:
At $5+ a gallon, I’ve switched to regular 87 octane from 91, and had an occasional dieseling after also leaning out the primary rods on my Eddy. AFR was perfect at cruise around 60*F (about 15 to 15.5).

It stumbled slightly if I was light footed on acceleration and kept the vacuum just high enough to not allow the primaries to jump into the fatter (thinner) part of the rod.
I suspect this contributed to some hot spots and potentially heating up some carbon deposits causing dieseling at shut off.

I put the “winter” rods back in, and while it dieseled a few more times right after at shut off, it hasn’t done it the last few days.

The question:
With a 9.5:1 FE, HEI with .055 gap on the factory heat range plugs, would shrinking the gap to .045 effectively split the difference between factory spec gap and what Davis Unified Ignition suggests, so that I could run a little lean and squeeze a little economy while not completely giving up a benefit of a hotter ignition?

Lets hear what y’all think about spark plug gaps and how much effect they can have.

 

Once the mixture is ignited, I don't see a plug gap having any further impact on temperature in the chamber. Chamber temps are controlled by fuel/air ratios and other facets like timing, compression, etc.

Maybe I mis-understood the question.

 

I think that the HEI was originally designed to be able to fire plug gaps up to about .100. I know some Olds 307 engines ran .080 gaps. The thing about trying to run the engine very lean to save gas is that as you lean it out the torque output drops like a rock so you end up having to give the engine more throttle to move the vehicle and then you use more fuel. Also you tend to get more cycles where the cylinder fills but does not fire fully or at all and that also wastes fuel.

 

You got it.
I’d read some “literature” that stated the larger the gap, the hotter the spark, because of the additional energy required to overcome a larger gap. Most everything I was able to find related to race cars.

The back of my head was telling me the additional heat that could build up from a hotter arc, combined with a lower octane and/or leaner mix could add up to higher potential for pre-spark ignition.

 

That torque output was one of the other things I noticed, hence having to be very deliberate about accelerating so the vacuum dropped to the point that the eddy primary rod springs pushed the rods up to richen the mixture. It made me question the (il)logic of running that lean where output drops so much. Might as well pull plug wires off one bank, lol.

If it didn’t weigh 6500# might help too.

Just trying to ask a more general question regarding the combustion relationship in the chamber, but also a very specific perspective and slant to it, and probably a little too theoretical for a 50+ year old engine.

Still, some folks like to nerd out so figured I’d toss it out there.

 

I always understood the term "hotter spark" as referring more to the mix being exposed to the longer spark. No doubt that it takes more energy to jump, but the gap doesn't add the energy, that's supplied by the coil, etc.

You can jump a gap with a 9v transistor radio battery, but not a long gap. Much of a gap and there simply is no "potential" there.

I know that lean mixtures burn hotter than rich mixtures, and I've read that spark plug electrodes firing mixes that are lean can run hot enough at times that combined with heat of compression, will cause uncontrolled ignition of fuel mixes. Water injection can cool such, as well as recirculated exhaust gases.

 

Well and that’s kinda what I’m wondering too.

My background in AC systems also includes how an available fault current can effectively create a bomb with a massive explosion of energy released when something goes terribly wrong in a distribution system. Everything in the upstream of the system contributes or detracts from the theoretical potential of energy that could be released.

In the engines case, we’re looking at a 12volt DC system, but with different characteristics depending on the ignition. For example, a stock style points system or duraspark is only operating on around 7 volts dc (resistor between switch and coil sucks up around 5 volts). An HEI should be operating on full voltage, but depending on design it could be limited in other ways.

It requires more energy to jump a larger gap, so could that translate into more retained heat after combustion is complete? I wouldn’t think it would be enough to make a huge difference, but a small granule of carbon can sometimes be enough to cause pre ignition detonation when heated hot enough so….?

This is probably getting too far into the weeds, and would most likely be more appropriate on a site like AllAboutCircuits, but figured it was worth asking just for “edutainment” value.

 

We don't create energy, we just use it, control it, convert it, change it's form, but it's presence in the universe is constant. Even after we use it, it remains as heat, so we just changed it's form by our usage. An ignition coil takes low voltage at a higher amperage and steps it up to a much higher voltage at a much lower amperage. High amps at low volts will not jump a spark plug gap, but high volts at low amps will.

 

Well, the entertainment I received from "Leaning-Out" the air / fuel ratio resulted in burning a hole thru the top of the piston.

Other times the result of lack of oil in a two-stroke Waverunner engine was overheating and piston seizure with terrible scarring.

When I purchased my new Husky 395xp timber faller's saw a few years back, I told the salesman / owner of the saw shop that I'll be running a 32:1 two-stroke mix _ _ _
he piped-up and said "you just VOIDED your warranty." _ _ that was such comforting public relations....._ _ thanks guy.
I'm on my fourth year running that saw, and it cuts 34"-diameter pine for hours on end with No Protest _ _ and yes, now, _ _ _ no WARRANTY. _

OMG - Just yesterday, I took my wife's '07 Chevy Suburban 2500 with the 6.0L engine in to the shop for a Physical Checkup.
As an experiment, I deliberately ran a set of ACDelco irridium spark plugs 130K miles and nine-years to see how they would hold up.
Well, all plugs looked a slight bit on the LEAN side in my opinion, but GM has those Vortec engines running on the lean side for Fuel Efficiency.
I went out to the barn / shop and measured the gaps to be .082". Wow, about double what the factory spec of .040" gap.
I'm surprised that I hadn't over-worked and lost a coil before now.
In the '02 Subaru WRX I ran for 14-years, the NGK plugs looked new after 100K miles, and the gaps were within spec, but, I changed them out anyways.

Now, regarding Air Fuel ratio - If I am towing a 7,500 lb trailer and climbing into the Sierra Nevada Mountains on a 100-degree day, I run a Rich Mixture so the cylinder temps stay within a reasonable degree, and more fuel in my opinion will cool the mixture so not to run hot and chance scoring a piston or burning a hole thru the top.
As an example, on my 429 engine, 385-series, I am running a 750-cfm Holley single pumper 3310. I keep the ignition purposely retarded to avoid Pinging on long-hard pulls where I've got the secondaries Wide Open and rpms between 3,800 to 4,400. No overheating, and Cool-Running.

When tuning my '77 Ford F350 CrewCab pickup, with 300 Six gasser in 2024, I discovered that having tuned the engine by "ear" resulted in too lean of the IDLE CIRCUIT mixture.
I decided to follow my dad's suggestion of Tuning with a Vacuum Guage. That resulted in Richening the Idle Mixture by slightly more than one-full turn out from previous.
The enrichened mixture got rid of a slight hesitation and stumble while taking off from a stop sign. I also advanced the timing just to point of no-pinging on accel.

In conclusion - my engines are tuned for hard work climbing into the surrounding mountains and not for squeaking fuel mileage on flat ground.

Ah, the joys of Fuel / Compression / Spark

 

This is true in that you never want to tune the engine without regard to the duty cycle. I've got some good stories on that topic.

 

Speaking of holes in piston(s), way back in the early '80s was at the area office doing reports, early AM hours, cold night. Dispatcher called on phone, wanted me to head south NOW and so I hopped in my Plymouth that was cold from sitting, and south I went WOT. Not hardly any traffic so once I punched it, I never cracked the throttle. Suddenly she lost power, still running, but it wasn't happy. Dispatcher called on radio, told me to 10-22. Only burnt holes in two pistons.

 

I guess in that case I’m currently performing tests to see what it takes to burn holes in forged pistons? FOR SCIENCE!!

I’ve had the same thought on pulling a load (richer mix), and ultimately I’ll probably end up putting fuel injection back on and keeping the numbers in the safe zones. It’s just with the carb I end up seeing so much variation throughout the day; 45* this morning running perfect and 75* on the drive home running rich.

I’ll probably reduce the gaps slightly, also in an effort to extend plug life a little, and keep running regular from here out while maintaining a slightly rich mixture.
It’s been kind of amazing to do the math and see the minute difference in area (or cross sectional area for you trades people and engineers) from one rod to another, and yet it makes such a huge difference spread out over 75cc x 8. Then trying to factor for air temp changes and the effect on AFR is kinda amazing.

I should probably get rid of the AFR gauge; that would fix it.

 

Figure the area of the rod at various places, subtract from the area of the hole in the needle jet .... eye opening.

 

Yeah, we do the same thing (electricians) for calculating conduit fill with wires, and are required to limit it to a certain % based on the install parameters.

After looking at the tables Edelbrock includes, and getting frustrated trying to “divine” by guesswork, I knew some simple math was the answer.
—————
For anybody else reading this who ain’t a math whiz; you subtract the area of a smaller circle (the rod) from the area of the bigger circle (the jet), keeping in mind the rods have a step. For cruise you have a smaller difference in area, and part throttle a larger difference (smaller space for gas then bigger space for gas).

Thankfully Holleys are easier cause there’s no rods, only jets.

3.14159(pi) x radius (or half the diameter) x squared
——————
example:
.095 jet area = 3.14159 x (.095/2)squared = 0.007088….sq in
.065 rod area = 3.14159 x (.065/2)squared = 0.003318…sq in

So difference is .007088-.003318=0.00377

If determining decrease in area in percent to a .067 rod: 0.003318/0.003526=0.941 or 6% leaner

Ive also used the difference in areas to compare larger jets with larger rods to smaller jets with smaller rods so I could fine tune. A lot of times you can get closer to ideal if you know where the overall area should be and your using an AFR gauge to hunt a 1/2 or full ratio difference (it’s tricky).
——————
Theoretically you can use this coupled with changes in temperature or elevation for changes in air density to approximate a needed change for seasonal changes or mountain passes. Realistically it’ll give you an idea, but there’s so many real world factors to account for, you’ll still have to test….. or get a machine that does all this for you (EFI).


Side note on maths: if you try to use just the diameters to calculate your percent difference, it won’t be accurate; .065/.067=0.97 or shows 3% difference in diameter, not area which is what the actual effect is.

 

A long time ago a guy who worked at the local auto parts store was working on a degree in computer software development and he needed a simple applied project. I had him write me a quick and easy program for figuring jet/metering rod area combinations where you could input your existing values and then go + or - whatever percentage that you wanted and it would give you a new size for either the jet or the rod to make the change. It worked slick and he got an A on his project because it included graphics and everything.