Spark plug electrode gap.

Hey ho, I think this thread could run and run so I'll get my few pennyworth in first!

With decent modern electronic ignition you can usually get a nice big fat spark which usually gives a better burn/better start with a nice big gap.

There are limits to this and there is info out there on the web about careful 'tuning' of gap to performance.

My old Hyundai developed a really bad missfire and poor starting...I checked all the usual stuff, and rather than throwing money at it, I just took the plug gaps down to 0.018" like I used years ago on old magneto ignitions.... result? ...

Perfect starting & running.

( I think the actual problem was EHT breakdown/leakage from the coil assembly).

This just illustrates that a bit of tweaking can make a big difference. I shall now sit back and watch the posts flood in....

(usual disclaimers apply, no animals were hurt in the writing of this post)

Okay here's my reply flooding in...

I agree with Del.

However, the spark gap can effect performance slightly as the energy in the coil isn't just pulsed across the spark gap it is a resonant damped sine wave pulse, if the gap is to small the pulse is heavily damped and too short, if the gap is too large the opposite is true. So the timing and dwell is changed....

Disclaimer: as its been decades since I worked on automotive engine control systems I might be totally and absolutely WRONG!!!

John.

John and Del,

Yes, but.... Provided the gap was within EPA sticker range (for factory computer calibration for the engine), and static timing was okay, wouldn't the ignition module and ECA (computer) automatically compensate spark timing, dwell angle and other inputs/outputs to keep the engine calibrated to its profile? Assuming there is some "slack" in the system profile (including fuel mixture monitoring), and that changes in plug gap can have any appreciable affect on computer output and ignition module response, you'd want to discover (?) the gap that would result in most complete burning while avoiding too-high combustion temp...over the dynamic range of tailpipe sampling, that is, idle and moderate speed on dynamometer. Would these objectives be better met with higher energy spark (larger gap) or vice versa, or at the happy median? Another point to consider is how accurate, how precise, the state's test equipment is over its ambient operating range. (Thinking about the flood, I probably should not have mentioned this.) I have actually observed a station pass my car on one test and fail it on the next--on separate days or even on the same day.

BTW, my engine is ECA IV (Ford) with TFI module. And yes, the module is the replacement stipulated under the California class-action lawsuit affecting all such Ford modules nationwide.

GuestS

exactly, it affects the timing.

the size of and heat of the spark is manufacture by the material used in the plugs construction.

computerised systems always have a set of parammeters, so I guess they'd have to opperate within them. set anthing outside that range, and you'll have over compenstation without efficiency. and may even blow circuit components if it continues, depending on the tolerances used in components of computerised unit.

I think you are over estimating the 'intelligence' of a car's engine managenent cumputer system..it's a bunch of lookup tables linked to some dodgy sensors...

See post #1 taking the gaps down to 18 thou compensated for a dodgy coil unit.

My previous post makes no sense at all....And was posted by an imposter!.

Dodgy is right on...as I and learned all too painfully, while my pockets were lightened substantially, during my car's last test cycle. The repair facility kept insisting my car's HEGO sensor was "not the reason," for the best test result--barely pass--they were able to obtain; a result which, not unexpectedly, was not be repeated at an official, state-sanctioned test station. So I decided to replace the sensor myself, anyway...and the car passed well below the Fail limits (and I got my registration renewal plate tabs) on the very next test! I was told that any well trained emissions tech should have known about swishy O2 sensors (I forget the trades term they used) and should have recommended it be changed even if it couldn't be diagnosed as bad.

Yeh, the post by my imposter was true..but not an actual sensible reply to the post it was meant to address.

You are so right about the dubious analysis... 'Computer says .. no' mentality.

There are hardly any decent mechanics left they just swap out parts until, if they are lucky, you get a fix ... no re-fund for all the incorrect bit that have been swapped.

Please don't start on me...it's not their fault as such it's pressure of time & poor training .

A while back I bought an ex company car...the rep who owned it warned me it would keep dying at tickover. The main dealer couldn't fix it, despite plugging in analysers and all that cr*p.

After much messing about over a week or so I took off the thottle butterfly assembly and cleaned it in a tin of petrol. Cured.... there were a couple of fine airway bleed holes which were blocked with crud which were critical at tickover....the 'intell'gent' engine management couldn't compensate to keep the revs up, and the 'analyser' couldn't analyse it.

I went in and told the garage....

Do you think they were interested? Send your answers on a post card to

No12 We Don't Give a Damn Street'

'We have your money' Town.

That business on 'No 12 We dont give a damn street', was that - Suckers taken in and whitewashed Unlimited. Ahh yes, I've often heard about them.

Fortunately, now, when I dont have the time to sort it out myself even after years of self experience, I go to a little experienced garage around the corner run by father and son who just use common sense. When my oil cooler leaked oil (would you believe, it's IN the cyl block, good old GM ggrrrr!!!!!!) and the emulsion blocked the radiator, he spent two weeks repeatedly flushing through and recovered it. I do believe in recycling, big chain garages dont. Sorry for going off topic.

I thought about this for a long while. I decided "what the heck".

Here in Austin, Texas there is a garage on Pond Springs Rd, called pond Springs Garage (very clever). No credit cards. No charge for diagnosis. No BS.

Not exactly what I call a media plug. Not exactly on topic. Just felt a fair oppurtunity to give credit where credit is due.

If the gap is too large... say +50% then you will lose horsepower as some cylinders will not fire. A small difference will have little effect. There will be more consequence with the humidity in the air then a 10% difference in the gap. The gap will not likely cause a smog failure.

Smog failures are more likely from worn rings,.. small hint if there is smoke coming out of the muffler and it is white or bluish white, you are burning oil and need an overhaul. If it is black, you have un-burned fuel and need the injectors (or carburetor) cleaned.

See http://www.smogsearch.com/index.php?page=faq (bottom of page) for a general list of causes.

Techno,

As said, I am seeking any advantage I can--before the fact--to hopefully avoid a barely fail situation when I might have gotten a barely pass... Fortunately, the engine does not have any of the mechanical faults (indications) you describe, and the ignition components are in good shape judging by performance and reliable starting. Thanks for the link to tail pipe emissions causes...which could come in handy.

S.

Oh, I forgot. How does ambient humidity affect combustion inside the chambers--or, more specifically, tail pipe emissions? I've often noticed that high humidity and cooler temps will seem to yield better overall engine performance: quieter run, more power,... But I never considered how it might affect smog test results. Since this could influence time of day and weather selected for testing, I'd be interested in learning more....

Humidity will do a few things.

1. It will displace some air so there is less air for combustion. The air/water mixture will also be heavier (have more substance) , therefore, will draw more gasoline into the combustion chamber. The fuel mixture will be slightly richer then a low humidity condition. If a vehicle was tuned near the UEL of gasoline, the higher humidity can actually stop the engine from running. If a vehicle was tuned near the LEL, the higher humidity will make it run better.

2. It will cool the cylinder. This also will reduce detonations since fuel detonations is temperature dependant. The higher the temperature, the more complete is the combustion, however, the more likely oil and any dirt will burn off and create pollution. In a dirty engine where burning crud can cause run-on, it can temporarily reduce the run-on.

3. In summary there are two ways a engine can fail. Either it is too rich, or burning oil. If it is too rich a low humidity environment is best. If it is burning oil a high humidity environment is best.

Electrodes wear with age thats why the preset them to the lower side of the range.

This part of my query was one of those (ya know?) "I remember that I knew the answer, but I don't remember what I knew" kinds of things. So, trying to jar the old brain cells logically, I considered two parts of two aspects: (1) the gap will either expand by electrode erosion (your answer) or contract by deposition which can't be removed; and (2) the mfr will factory set the gap at minimum either to elicit quicker replacement (sales)--because it knows many will find it easier to simply replace rather than clean and re-gap once they've gone to the trouble of pulling plugs--or to make sure their plug "lives up" to its (and its competitor's) no-maintenance/durability claims. (My guess is, that since the bi-annual clean & re-gap thing became a thing of the past, most people (nowadays) simply replace and never touch a plug gap gauge tool.)

Anyway, thanks for clearing up the matter--and now I seem to recall, that I always fudged by re-gapping to the middle of the EPA label's specified gap range...which is the same as remembering that I really never did know the true answer...at least after precise gapping (before electronic ignition) was replaced by more (ignition-systems-) tolerant gapping.

BTW, is that Rosemont in south Balmer (County)? or Rosemont in Frederick County?

and we flood.

To accurately measure gap, or more accurately measure, use the leaf type as opposed to the ramp type. Much to my dismay I cannot recall the name..oh there it is... feeler gage. Thats the one.

(thought I was going to have to reduce my corpse value there for a minute. whew)

C,

Yes, that would probably do in a pinch, but I'm not so sure it's a proper, or accurate, method--that it wouldn't be using the right tool for the wrong application. Consider:

• For precise measuring, there could be difficulty (as opposed to ramp-type coin gauge) selecting the desired gauge blade thickness--since feeler blades, too, come in standard gauge increments. And if blade stacking was resorted to, one or more blades would probably be very thin...and subject to buckling or damage under tension while measuring (or attempting to measure); or even (although it seems unlikely) to being grooved while being pulled while "clamped" between electrodes. (Unlike plug gap measuring, the method for feeler gauge use is to bring the "contact points" slowly into contact with the blade until the point of blade clamping (blade immobility) is just reached. This approach is not possible [in a practical sense] with spark plug electrodes.)
• To measure (but not to set) a plug's gap is to gauge the minimum distance (the minimum air gap resistance) between the two most proximate "points" from plug anode to cathode. In contrast, air gap measuring (and setting) between moving moving components for timing purposes would be more like gauging (not the minimum so much but) the average distance between flat or roller contact surfaces.

That said, I can't disagree that feeler blades could be used in the absence of the correct tool...provided that the air gap dimension selected was not at either limit of the acceptable gap range. Keeping the gap near the mid point of the range, there would be more tolerance for measuring error, without going out of range...

I've been gapping plugs for 25 years using feeler gauges as the "correct" tool as I was taught by my father befor me. Often blade stacking is required to achieve the proper value. Only a novice would stack and use a very thin gauge. When I don't have a large enough blade for the application, I just split the value evenly. For example, my Ford pickup requires an 0.044 gap. My gauge set only goes to 0.035. Split 0.044 and you get 0.022. Obviously, I don't have two 0.022 blades so I use the 0.021 and 0.023. They're right near each other and neither is so thin as to buckle when used. While I am not arguing the use of ramp or wire type gappers as wrong, they're just not precise enough for my taste. Happy wrenching!

If you need to be that accurate you also need first to get a file out to square up the electrodes ends. This gives a more powerful spark as it tends to come off the circumference of the electrode and not the centre.

Feeler gauges have been used for years, this isn't rocket science. After setting gaps for a number of years you can get it pretty Ok by opening it up high, squinting through the gap and knocking it on your vice to reduce it until it looks right?!*?!* Larger gaps need higher voltage, more energy, to fire across them, better fuel consumption through quicker combustion, which slighty retards ignition timing.

!!!!!!!!!!!!!!!!!!!!!!!!!! ???????????????????????????

Feeler gauges are fine...that's what they are designed for, measuring gaps!

Obviously you use as few as possible to get to the correct size.

We aren't talking high precision here anyhow! Years ago I would simply use my thumb nail ( it was tougher in those days and spot calibrated at ..018" )

Any how, weren't we supposed to be discussing the quality of the combustion?

And this is definitely effected by the spark duration timing and fatness (that's a technical term!)

If the Gap becomes too wide the spark will not stay between the electrodes but can go sideways.this will cause combustion too start at the wrong place and it will spread wrong hence emmissions will be affected. Bigger gap means more power needed to generate a spark therefore slight delay in timing or no spark at all. Smaller gap makes a weak spark and less complete combustion. (both situations can create hard starting and incomlpete combustion) Cold weather will help the combustion because there is more O2 per liter of air (that is why Intercoolers are used)

Good points...as related to performance that is. If only they could be related to some statement (especially the air temp thing--which a person might have some control over when selecting test time of day) about emissions test results.

For example, many people, car repair techs included, will say to drive the car until the engine is heated thoroughly--and keep idling while paying and waiting in line--before testing is started. But this could be nothing more than urban myth--I've never heard any "scientific" or non-anecdotal explanation why this should be so. I have also been told that test equipment is sophisticated enough so that such factors as ambient temp and humidity--even exhaust system leakage!--are factored in--so they give "true" readings almost no matter what.

Del and I are really on the same wavelength! Magnetos and thumb nails! Once went to a vintage car meet, had to strip and wash out the goo that was the insulating shellac from the mag' before we could drive home again! Probably gapped the plugs with thumbnail while I was at it. OK there's a huge amount of science around where we are today with emissions but I can't say all of it impresses me. What percentage of a vehilce s life is it's catalytic convertor working positively for the environment?? How much CO2 would have been saved if we in Europe had kept 'lean-burn' these last 20 years???

Hugh Price

Yes, I totally agree. Too many manufacturers gave up to early on lean burn a few years back. The big manufacturers couldnt get the power they wanted and I suppose in a commercial world where spending a cent too many is one cent too much for them the decision was inevitable. If they had all persevered we would be a long way down on a better technical path with better fuel economy and without the repeated cost of catalysts, and today, the carbon footprint they leave behind, besides wasting exotic materials.

Thanks, it so helps to sometimes know it's not just me :( :)