TOPIC: Spark plug gapping and some limitations

Ron had posted a video awhile back about the effects of increasing the sparkplug gap.

We already know that increasing the gap assists in increased fuel efficiency and Ron's test proved that out even more so while testing the lawnmower's emissions output at different plug gap settings.

One question that popped was the effects at higher rpm's.

With electronic ignitions I cannot speculate but with magneto and coil ignitions
we are limited to electromagnetic induction and dare I say it... physics.

With the lawnmower running at a fixed rpm the ignition coil will output a spark at a specific voltage each time it fires, by increasing the engine rpm's it changes the voltage ramp up time of the ignition coil, hence changing the output voltage of the coil and its effects at a given gap size.

On a magneto, at a given rmp the magnet passes the coil at a constant rate creating an inductive field that repeats, by increasing the rpm, the magnet spends less time in the area of the coil thereby reducing the effects of the magnetic field in the coil, resulting in a slightly lower spark discharge. The higher the rpm, less influence of the magnet on the coil resulting in a slightly lower voltage.

An ignition coil fires each time the input voltage is removed from the coil primary causing the magnetic field to collapse, creating a electromagnetic discharge on the secondary, ie: the spark plug wire. At a fixed rpm or repetition rate the output will be constant, as the rpm's increase the time allowed for the field collapse reduces affecting the output of the secondary on the coil thus reducing the effective voltage constant to the spark plug.

What I found with my 2007 Avalanche at idle
the stock spark plug gap was .035", Longterm fuel trim was at -12.5
adjusted the spark plug gap to .060" Longterm fuel trim jumped to -14.8

After a 180mile round trip, I again find the computer confused as it was reporting 18.4mpg and in fact at fill-up was 16.2mpg.

I've regapped the plugs to .050" and now the Longterm fuel trim is up to -18.8.

I'm not declaring the Long fuel trim to be a guage for the plug gap but this is what I see changing when the plug gap is changed.

These are the plugs at .060" gap



Notice the difference in coloring where the ignition flame was blocked


My conclusion here is that on engines that run constant speed, ie: lawnmowers, generators and the like, the efficiency won't be effected by having the plug gap at it's limits, the spark will still be constant.

With automotive engines, ie: variable rpm, it might be a good idea to determine what plug gap will work the best when operating through the rpm range.

Input, comments... bring it on, if anyone can include this perception in their monitoring and note the differences they get that would be good info to share.

Karl Fortner
Tacoma, Washington

Good on ya, Mate!
This is an excellent point. This is why I suggest finding the max value, then driving the vehicle to check mileage. If it drops, then it's too wide. Reduce the gap by the 20% increments used to increase it.

That should take care of the issue, while delivering maximum charge to the plugs.

Thanks for bringing this out, Karl!

Most all modern engines use Coil on Plug ignitions. Going back some years there are coil pack and then finally electronic ignitions with distributors.

So this applies to any engine using Spark Plug wires. Many Gadgetvolk will be aware of and use this knowledge, some less so.

A gasoline engine needs Fuel(and air) Compression and Spark to run.

Whether or not you are changing the plug gap to optimise the Groove effect, the Ignition must be in top condition, else efficiency and engine life are compromised.

I've found many car people who don't completely understand plug wires and how to tell their condition/efficiency.

IMHO, most auto manufacturers allow stupid high resistance spec's for their plug wires (maybe they dont want those engines to last so long? I dunno) often up to 25,000 ohms. I don't like anything over 2,000 ohms. As an independant mechanic I did scores of tune-ups, I was/am picky about it. I would do a tune-up, people liked the result, and I had lots of repeat business. But almost NO come-backs on my tune-ups 'cause they would LAST!

Everything between the ignition coil output terminal and the firing tip of the plug is a conductive path for that spark. Everything adds resistance(ohms) in the way of that spark robbing energy from that spark. We can look at a dist. cap and rotor and judge their condition. But to really know how good the plug wires are you must check the resistance measured in Ohms.



I do it by using a digital multi-meter (DVOM). I use leads w. alligator clips on the ends. Take the plug or coil wire, and connect the leads to the terminals as shown. Be sure the leads are in the correct sockets then set the dial to 20K (20,000) Ohms. Look at the readout, and switch down to 2K and then 200ohms until one of the settings shows a blank readout. Now go up one setting on the scale(for this short coil wire 200ohms was blank so I went up to 2K) to get the readout in ohms. Now GENTLY pull/twist the wire and watch the readout-does it jump up/down, or does the readout go blank? ensure the leads are still firmly connected. If the readout jumps by more than 50 ohms, i'm suspicious. if it goes blank switch up one click on the dial scale and continue gently pulling/twisting. If the readout jumps around more than 50 ohms or if it goes blank for a second, either the internal conductor or terminals are going South.

If I find any wire in a set to be flaky or much over 2000 ohms, I reinstall that wire and check another. Typically if one wire is bad they ALL are or soon will be. Any cracks anywhere in outer insulation, oily soft or otherwise degraded condition also tips the scales here. There are MANY vehicles out there still running the OEM plug wires w/ over 100K miles or ten years of service.

Always check the Plug wires, cap/rotor if equipped! The flip side of this is how well/long the computer will compensate for the deterioration of these parts. It blows me away how an engine w/ 140,000 miles on it, maybe the spark plugs have been changed, but nothing else, and it still runs smooth! (well pretty smooth).

Old/ worn ignition parts stress the coil/coil packs and can fry them as well.
Your Ignition Can't Be TOO GOOD!

If you are increasing the spark Plug Gaps to optimize the Groove effect, then all of this just applies that much more.

And Guess what, Batman? Try hooking the meter leads to the top and lower center tip of your spark plugs, check to find the lowest value scale that reads and do comparisons...there ARE differences!!

If I go to buy a new wire set I bring the DVOM along and test the whole set before I buy. Don;t worry about the parts person--you may wind up teaching them as well! If I buy a cut-to-fit set I Check my Work as I go. A problem is uaually from a poor terminal connection.

Much like driving over a freeway bridge we take these parts for granted for years, until there's a big pothole or worse! Fix/optimise the ignition parts as needed; get good low-resistance well-built plug wires, and then you and your customers really can forget about em---well for awhile anyway!

"Shure is cold out here,eh, KId? Your wife is shiverin' and Junior 'dere he don'na looka so hot wit' da blue runny nose, hey is dat frostbite startin on youse fingahs? So ya gots like a one mebbe twose chances to start 'dis fire ya got built here, right Kid--Now I dunno, itsa uppa ta youse, kid, Ya wanna takes a chance on all tree o' youse dyin' o' exposure out heah, ya wanta try usin' dose last two matches from dat matchbook--who knows how old dey is, right? Or mebbe you woulda rather try 'dis here nice new Road Flare I gots right heah in my kit bag, eh, Kid-- I betch'a some nice hot coffee and cocoa for da Missus and Junior 'dere would like-a help out some, or my name ain't Benny da Torch, Kid, I'm an Ignition Expert!"