Spark plug testing

[Bruce B.]

I have an old Champion spark plug tester and it works great but I have a question concerning testing the plugs.
The tester has a fitting for hooking up a air pressure line, a pressure gauge and a pressure adjusting setup and a high voltage wire which attached to the plug.
You can observe the plug while applying voltage to it.
The question is, by pressurizing the plug chamber does that have affect on the spark characteristics?
Thanks.

plug tester 001.jpgplug tester 006.jpg
I have added a picture of the tester and a plug being tested.

[Duke W.]

Re: Spark plug testing

The higher the pressure (and gap) the higher the voltage required at the electrode to initiate the spark. Typically this is around 5-7 KV for a properly gapped plug that is in good operating condtion.

Once the spark forms, voltage drops way off.

This is why "50,000 volt" coils are BS. If a coil really will develop this voltage and you pulled a plug wire off it would probably burn through the wire insulation.

The most important parameter is per spark energy. The more energy, the longer the spark lasts, and a long lasting spark is more likely to light the fire, particularly with a relatively lean (stoichiometric or less) mixture.

The TI and HEI develop about double the per spark energy of the single point system, which is why the former will light the fire with a wide gap and significant fouling when the latter may not.

Duke

[William F.]

Re: Spark plug testing

But I believe you've said you don't like the Pertronix3 or MSD ignitions which multi spark, therefore longer lasting spark, at least up to 3000rpm, which would seem to lessen chance of plug fouling. Seems contradictory. Wher am I going wrong?On an L79 I previously owned, was no comparison between single point ignition and an MSD 6AL with Pertronic trigger regarding plug life and pulling to redline without ignition breakdown.If better, why not go for it ?Please note I consider ignition different from changing manifold, carb and other original components.

[Duke W.]

Re: Spark plug testing

CD ignitions generate a very high energy, but short duration spark, which is good for engines that run rich/high density mixtures - like racing engines.

Lean/low density mixtures, which is typical most of the time for road engines, have more ignition lag and require a long duration spark for reliable ignition. This is why all OEMs use inductive rather than CD ignitions.

Sure, the MSD has multiple sparks at low revs, but the subsequent sparks are considerably retarded from the first. IMO it's not worth anything. It's just a marketing gimmick.

Most single point distributors were set up sloppy from the plant, and high rev ignition breakup was a common problem. I had so many problems with mine in the early years, I bought a TI system (off the "12-mile" '67 L-88), but it failed twice without warning in the next seven years.

By the mid-seventies when the TI failed a second time, I had learned how to set up a single point distributor including proper point selection for the engine rev range and hotter than OE plugs, and now it runs reliably to 7000+ revs. I repaired the TI system and put it in a box for the next fifteen years and sold it to David Burroughs in 1988 for reinstallation on the L-88 that it originally came from.

I documented how to make the single point system work consistently and reliably in my San Diego presentration, in A Corvette Restorer article about ignition points last year, and in a December thread for a L-79.

It's less expensive to do a blueprint/overhaul on a single point distributor, which includes optimizing the spark advance map than buying an aftermarket system or electronic switch, some of which don't appear to be very reliable.

There is nothing wrong with the single point distributor design, but the sloppy assembly made a lot of people (including me in the early days) think it was junk. Once I found out it was just a matter of tightening up assembly tolerances, which was easy to do, and proper point and plug selection, it became reliable and durable with no need to re-engineer it or replace it with a different system.

Duke

[Joe C.]

Re: Spark plug testing

I have an old Champion spark plug tester and it works great but I have a question concerning testing the plugs.
The tester has a fitting for hooking up a air pressure line, a pressure gauge and a pressure adjusting setup and a high voltage wire which attached to the plug.
You can observe the plug while applying voltage to it.
The question is, by pressurizing the plug chamber does that have affect on the spark characteristics?
Thanks.

[ATTACH=CONFIG]44610[/ATTACH][ATTACH=CONFIG]44611[/ATTACH]
I have added a picture of the tester and a plug being tested.

The pressurization provision of the testers is a gimmick. It doesn't come close to emulating the conditions in the bore at plug firing. Actual chamber pressures are an order of magnitude higher than the capability of your air compressor. Air-fuel ratio has an effect on voltage required, as well, with leaner mixtures requiring higher voltages.

The above criticism of high voltage ignition systems is, predictably, myopic. High voltage systems, like old fashioned 12 KV systems only supply sufficient voltage to enable the spark to jump the gap. The balance is called "reserve voltage", and is available to compensate for: large rotor air gaps, lean mixtures, very high combustion chamber pressures as found in high VE engines, leaky primary wires, carbon tracked distributor caps, grossly rich mixtures, large spark plug gaps. This bears repeating. High voltage systems, like old fashioned 12 KV systems only supply sufficient voltage to enable the spark to jump the gap. The balance is called "reserve voltage", and is available to compensate for: large rotor air gaps, lean mixtures, very high combustion chamber pressures as found in high VE engines, leaky primary wires, carbon tracked distributor caps, grossly rich mixtures, large spark plug gaps.

This last item "large spark plug gaps" are desirable, but only if the system has enough reserve voltage. Why is it desirable? Because it provides a much hotter spark, and larger "kernel" which promotes much more consistent and complete burn. It virtually eliminates misfiring, especially in old design engines which run rich mixtures and low combustion chamber pressures.

The above criticism of MSD is also, predictably, myopic. At engine revs above about 3500 RPM, the MSD function gives way to single spark, but the trade off comes because the reserve voltages above 3500 are extremely high.........more than 45 KV and in some cases 50 KV. The MSD functions to keep spark plugs clean at low RPMs. Although not scientific, the fact that every single high performance and race engine since the mid seventies is equipped with MSD, and used by virtually all of the top engine builders, most of whom are engineers like myself and the so-called engine expert here.

[Duke W.]

Re: Spark plug testing

Most who lack a solid foundation in physics don't understand the difference between voltage and energy.

Duke

[William F.]

Re: Spark plug testing

I surely don't see any reason not to replace points, and their attending maintenance, dwell variation,etc., with an electronic trigger inside distributor. and like Joe said and in my practical experience, the MSD box provided reserve to fire plugs to redline consistently, a big improvement over stock setup.

[Joe C.]

Re: Spark plug testing

Most who lack a solid foundation in physics don't understand the difference between voltage and energy.

Duke

I understand it, but maybe you should explain it for those who do not.
I graduated Summa Cum Laude from a top engineering school with a BSME, and had plenty of grounding in physics.

[Gary C.]

Re: Spark plug testing

Please limit comments to "technical" and do NOT include any comments that are disparaging about another member's technical comments. In other words respond with a technical difference, not a subjective comment.

The TDB is not a forum for innuendos and negative comments about other member's technical findings.

Please read the FAQ User Agreement!

With that being said, respectfully continue with a good technical discussion on spark plugs and high energy ignition systems.

Gary
....

[Bruce B.]

Re: Spark plug testing

I think a simple answer to a simple question will suffice in most cases.

[Ken A.]

Re: Spark plug testing

I have an old Champion spark plug tester and it works great but I have a question concerning testing the plugs.
The tester has a fitting for hooking up a air pressure line, a pressure gauge and a pressure adjusting setup and a high voltage wire which attached to the plug.
You can observe the plug while applying voltage to it.
The question is, by pressurizing the plug chamber does that have affect on the spark characteristics?
Thanks.

[ATTACH=CONFIG]44610[/ATTACH][ATTACH=CONFIG]44611[/ATTACH]
I have added a picture of the tester and a plug being tested.

Simple answer is no, as the air pressure does not come close to combustion pressure. HOWEVER, if the spark fails under air pressure, then pitch it.

[Bruce B.]

Re: Spark plug testing

OK, I understand, but what would the cylinder pressure be at compression and then at the time of detonation?
Actually what would the max pressure be after detonation?
Thanks.

[William C.]

Re: Spark plug testing

Max at compression, about 185, after ignition several thousand...

[Joe C.]

Re: Spark plug testing

OK, I understand, but what would the cylinder pressure be at compression and then at the time of detonation?
Actually what would the max pressure be after detonation?
Thanks.

Bruce,

I don't think your question was concerning "cranking compression", and I'm not sure if you're confusing "detonation" with "combustion". A normally operating engine should not be experiencing detonation, which can be thought of as an explosion......................a sudden and violent buildup of pressure which causes the cylinder walls and piston to resonate, thereby causing a "pinging" sound. The normal combustion process should be complete, proceeding along one flame front from the point of ignition (the spark plug, not a hot spot in the chamber) to the terminus.................and THE FASTER THE BETTER!!!!!! When an engine detonates, it rapidly builds pressure in the combustion chamber many times that of the typical 700-1000 psi of a mild performance SBC at max torque. This is why severe detonation causes broken rings, melted spark plug electrodes, and holes in piston crowns.

Ideally, you want ignition timing to be such that max cylinder pressure occurs at about 15 degrees ATDC, and for most vintage smallblocks with compact iron wedge heads ("closed" chambered, generally 58 - 64cc), that's why WOT spark advance should be around 36 degrees BTDC plus/minus a couple degrees! Modern "kidney" shaped chambers having more compact form and better swirl action as well as more centralized spark plug location requires as little as 28-30 degrees WOT spark advance. Smog heads ("open chambered") which were used on SBC after 1970, require more spark advance because of the larger volume. Why 15 degrees ATDC? Because if pressure starts to build at a point too early, then some of that building pressure would occur while the piston is still traveling upward. Fifteen degrees extracts the most mechanical work out of the combustion event as possible.

Maximum cylinder pressures are always attained when the engine is developing maximum torque. An unmolested 327 engine is 4.000" x 3.250", and force is pressure times area, with the maximum torque arm being 1/2 the stroke, and the effective torque arm varying as crank angle changes. Connecting rod length and compression height also have a large effect on this calc. Each of the 8 cylinders should be delivering pulses that, more or less, seamlessly overlap each other if the engine is firing evenly. The calculation of torque imparted to the crankpin as each piston rises and falls in its cylinder requires integral calculus to figure, since the pressure and effective torque arm is constantly changing as a function of time.

Any calculation of the above is more than I want to attempt, and more than you want to know. I can tell you that my 327 makes better than 455 ft-lbs of torque at its torque peak of 5500 RPM. The max cylinder pressure at 5500 RPM, according to Engine Analyzer Pro, is 1361 psi.