.027" or .035"

[braol]

The Owner's manual says plugs should be at .027" gap but the Les Andrews book says .035". I have a 1931 Tudor...stock. Any thoughts from you pros out there?

[gustafson]

unfortunately there are many errors in the Andrews books

[George Miller]

The wider the gap the stronger the spark. But there is a point when it can no longer jump the gap. Pus it will take easies path just like we do. I say .032

[WHN]

Have used .35 for almost 60 years.

[J Franklin]

I use .035. and .027 with a high compression head

[Bob C]

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Page 454 of the Service Bulletins states "the spark plug gap has been changed
from .027" to .035"."


Bob

[braol]

Originally Posted by Bob C

Page 454 of the Service Bulletins states "the spark plug gap has been changed
from .027" to .035"."Bob

That settles it! Awesome.

[Corley]

Are you 6v or 12v? A good 12v system should easily support .035". If you are still 6v, you might want to stay around .030", though I've run .035" on 6v without trouble. Tighter gap, easier to fowel, bigger gap more chance of missfire, and hard start if on 6v system. Everything is a trade off, there is no one "right" answer.

[gustafson]

exactly
engineering has advanced since the days of the SBs

[MikeK]

The spark in a Kettering system is a function of coil impedance ratio and current density. Primary voltage, 3,6,12,24V is not related to the output energy or voltage at which the coil will discharge that energy across a plug gap.

The Minimum discharge voltage across a plug gap is determined by the atmosphere in the gap (compression ratio and water content of the air), condition of the electrodes (clean, sharp edges), and surrounding field ionization.

With the extremely low compression of an A, even with a 6:1 head, which is LOW, clean plugs will fire reliably on 6V Kettering system at much more than 0.035" gap. Thus the very conservative 35 in the service bulletins.

All the talk about smaller gaps is a carryover from non-inertia start magneto systems that produce a much lower output at cranking speed than a Kettering system.

[Purdy Swoft]

Originally Posted by WHN

Have used .35 for almost 60 years.

I agree!!! I have also used a gap of .035 for my model A's and other cars with points ignition for nearly 60 years with good results . Like Bob C said , Ford changed the gap from .027 to .035 . Too much gap with weak ignition can shorten coil life . I run the Pertronix flame thrower coil . An original model A coil can easily handle a gap of .035 or even more .

[redmodelt]

Bob C has it, we just talked about this today at the shop.

[pgerhardt]

The coil type can make a difference. I switched from the unknown brand 6V coil that was on my car, to a original slant pole coil that a friend spotted (I sent him picture of one, so he knew what to look for.) and picked up for me at Hersey, because it was more appropriate for my 29 tudor. Well you could practically hear my car sigh when I first started it with the slant pole! It started quicker, idled better, and generally had more pep. Like Model A Viagra!

[gustafson]

The fact remains that it requires a higher voltage to fire a lean mixture, and A Model A does not have a lean mixture compared to a modern vehicle


https://books.google.com/books?id=UP...ixture&f=false

[Corley]

Originally Posted by MikeK

The spark in a Kettering system is a function of coil impedance ratio and current density. Primary voltage, 3,6,12,24V is not related to the output energy or voltage at which the coil will discharge that energy across a plug gap.

The Minimum discharge voltage across a plug gap is determined by the atmosphere in the gap (compression ratio and water content of the air), condition of the electrodes (clean, sharp edges), and surrounding field ionization.

With the extremely low compression of an A, even with a 6:1 head, which is LOW, clean plugs will fire reliably on 6V Kettering system at much more than 0.035" gap. Thus the very conservative 35 in the service bulletins.

All the talk about smaller gaps is a carryover from non-inertia start magneto systems that produce a much lower output at cranking speed than a Kettering system.

I would agree with this but... You are not taking into account a couple of items. First, a 6v system, with the high current required for starter motor, and if not pretty recently cleaned connection points throughout the electrical system, including grounds, often has little voltage left over for the ignition when starting, and can produce a less than desireable spark output. Second, many 6v coils in model As are eons old, and on their last legs, whereas most 12v systems will sport newer parts in better condition. Third, ANY resistance anywhere in the ignition wiring has a greater detrimental effect on a 6v system than it will have on a 12v system. And lastly, the resistance buildup of arcing points affects the higher current requirement of the 6v system more than that of a 12v system. This ain't rocket science, it's simple electrical facts.

However, if everything is kept up to snuff, .035 should not be a problem on a 6v car, and it should even be capable of wider gaps.

You are absolutely correct in saying that a 6v model A ignition system should work fine at .035 gap, but in my opinion, as an electrical engineer, a good condition 12v system can do even better. Much better.

Lean burn? Yes, that is one important reason high energy ignition systems were implimented in all regular cars, nothing to do with racing. Not in our scope of discussion, bringing that in is just adding confusion to a simple topic.

[DHZIEMAN]

Had some ignition troubles with my purchased Model A. Got the plugs out, and the electrode ends were kind of worn and checked the gap and they varied from 045 to 050!

Replaced the plugs and read someplace a gap of 032 to 035! Set them there, found some other ignition stuff to fix and the car has been running good since then! Plugs are gapped at 032! (or were)

[old31]

Originally Posted by J Franklin

I use .035. and .027 with a high compression head

J, i am curious. With a HC head wouldn't you use 35? Any reason to go to 27? I am asking because I have a hc head with 35.

[Corley]

Pretty sure he is just saying a higher compression ratio requires a hotter spark since the spark does not like to jump as well when under compression. Frankly, any head on a streetable model A engine is low enough CR for this to be a rather moot point. (Streetable infering not a super high performance racing engine.). If everything is sixes, Bob's your uncle with .035" on your model A.

[BILL WILLIAMSON]

Has everyone "MADE UP" & back to figuring out, "WHAT'S FOR BREAKFAST"???----Who cares what kind of coil you're running??? as long as it runs GOOD!
IF it doesn't run well, try painting the coil a DIFFERENT COLOR.
Bill & Dog Hungry

[J Franklin]

The smaller gap with higher compression heads is just something I read and adopted. It works fine for me. Smooth running and never fouling.