Greetings All,

What would be a good suggested spark plug gap for my GT? Ngk plugs, Lucas sport coil, Jeff's dizzy?

Cheers
Larry

If it's points type application, go stock setting 0. 025"
RS

---

"It is a mistake to try to look too far ahead. The chain of destiny can only be grasped one link at a time."
Sir Winston Churchill
---------------------------------------------------------------
"It all starts in your mind's eye, then it goes to your heart
and finally to your very soul."
G.S.George PHD

And you can certainly ask Jeff. He may tell you to use something slightly bigger.

---

David J. Kinsey
Virginia Beach
Tidewater MG Classics
1980 MGB "Runaway"

Hi from Miami Larry! I have that same setup (but NGK iridium plugs) and Jeff told me try .032" which works GREAT . When he returned my formerly Pertronix - fired 25D dizzy he also asked me give his points a fair trial and I'm sold on em - instant starts, great performance.

---

~ Marco and Viajero ~

Gap depends on several factors, compression, current potential, ignition timing, fuel mix, and style of plug to name a few.

The wider the gap the fatter the spark. The more dense the mix the higher the potential should be for any given gap. But without a strong coil you run the risk of the spark not occurring across a wide gap.

Given a normal compression ratio, say 9 ( 8.8 ) to one, a "normal" coil, say a "Lucas" coil ( actually made in Asia 'cause someone bought the name ) and a mix that is sufficiently wet, and have BP6ES NGKs or N9Y Champions stuck in the holes, you should stick to the book at 0.025".

If you use noble metal plugs ( I have had very limited success with them ) and a hotter coil ( say a genuine Bosch Blue coil for non-ballasted systems ) Marco's scenario is not out of line, between 30 and 32 thou will work just fine.

Going to be just puttering around on Saturday with your grandchild in the car and a "normal" coil out front? Try a one stage hotter plug ( say BP5ES ) at 25 thou.

Milled the head a little and gonna drive the do-do out of the thing on extended runs every chance you get? Go to a BP7ES at thirty thou. Wanna spend three times the price on your spark plugs and have a hot coil so that you have braggin' rights and you can feel your oats? Use the Iridium plugs set at 32 to 33 thou.

Here is a five page primer on plugs. http://www.ngksparkplugs.com/tech_support/spark_plugs/index.asp?mode=nml

Heat ranges have zero to do with gap, but they do have a relationship to the other factors that make your engine run. The idea is to choose the correct heat range and then assess the rest of the tune elements in order to choose a gap that makes it all work.

Jack

---

---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

Hey Marco,can you send me a set of those points,I'll test them out over here.

One thing that is not well understood by many people is the effect that plug gap has on the rest of the ignition system. Jack is correct that wider gaps have the potential to give a better ignition BUT to strike the arc requires a higher voltage. Every part of the secondary circuit is therefore placed under more stress. This includes the distributor cap and rotor. Given the quality of some of the distributor caps and rotors we are seeing these days, it is easy for the voltage to "puncture" or create a leakage point in the plastic. At that point you start to get misfires or worse.

The pointy plugs (which I use and prefer) require less voltage to strike a gap compared to a conventional plug with the same gap. So, when you move to pointy plugs it is probably acceptable to increase the gap *slightly* (say 0.030".

---

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Terry - "pointy"??

---

David J. Kinsey
Virginia Beach
Tidewater MG Classics
1980 MGB "Runaway"

I don't know how else to describe the plugs whose center electrode is as fine as a needle and almost as sharp (and typically made of platinum or iridium).

---

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Points ignition, .025", electronic igniton, .035". The difference being the eelctronic ignition systems provide about 1-1/2 to 2 times the voltage of the conventional ignition systems, and can support the wider gap more reliably.

Not sure that you are correct Jack.

The voltage is controlled by the windings in the coil and the available voltage from the battery. All the points or electronic stuff does is trigger the coil. Increased point dwell will allow the potential to build up, but I am not certain that the coil itself would or wouldn't have built-in limitations.

The electronic thing might give more precise timing of the triggered event, but I don't think that there would be a measurable difference in the coil output based on just that.

Want more voltage to the plugs? Use a coil containing more turns and make sure that there are no resistance issues with the cap, rotor or cables.

Jack

---

---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

This is the issue I was referring to that most people don't understand!

The available voltage is controlled by the ratio of the windings in the coil and battery voltage. That is, if you disconnected the spark plug wires from the plugs, hooked a scope up to the wires and turned the engine over you would observe a certain (high) voltage. Changing the coil for a higher ratio coil might increase that voltage.

HOWEVER - if you reconnect the wires to the spark plugs, at some voltage, hopefully less than the unloaded voltage, an arc will form (spark) and this will cap or limit the voltage. The wider the gap, the higher the voltage required to form the arc.

As an example, suppose the unloaded maximum voltage is 20 Kvolts. With a 0.025" gap, you might find that the puncture voltage (where the arc will form) is 6 KV. (The exact voltage depends on many factors including compression, temperature). If you increase the gap to 0.035", you might find that the arc forms at 10 KV. But, once the arc forms you won't see much of an increase in voltage no matter how hard you drive it (although the spark will get stronger and hotter).

So, there are advantages to "hot" coils and wider gaps. The hot coil can deliver more *energy* to the spark and the wider gap increases the odds that the spark will ignite the air-fuel mixture.

However, past a certain point, an ever hotter spark doesn't really help. It is kind of like being pregnant - you are or you aren't. The spark either ignites the mixture or it doesn't. The hot spark is most advantageous when starting in cold weather or at very high RPM (also most ignition systems lose their pep at high RPM). But if your engine is running well with a conventional coil then it is highly unlikely it will run any better with a hotter coil (but it may start easier at -20).

There are also disadvantages to the wider gap. As was pointed out above, the wider gap means that there is a higher voltage required to fire the plug. This voltage is applied to the rotor, the distributor cap, the plug wires. If there are any weaknesses in the secondary system, it may show up at the higher voltage (wider gap). So, instead of getting more energy to the spark plugs, you actually get less (because it is shorting out in the rotor or cap).

Does that make sense?

---

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

For grins, when I installed the Jeff-installed Crane ignition (which adds nothing to spark intensity, merely a timing device) with the newly rebuilt 25D in my 73, using a just-installed 6-year old (NOS) Pertronix epoxy filled coil I gapped my std NGKs at 35 thou.

"Anything worth doing is worth doing to excess!" (--Ovid Bolus, nee' Bill Jeanes)

Runs really well. Might be fooling myself, but it seems to run smoother at idle (800-850) than I've heard it run before. And I reckon the B is one of the worst running cars at idle that I've ever heard...apart from a SAAB 93 or a DKW 1000 (which I would LOVE to have!).

In the past I've tried different gaps (25, 28, 32, etc) and heard no difference. Not in sound, not in mileage, not in performance. Even tried a set of pointy plugs...NGK somethings, a click down from Iridium, and saw or heard no difference between them and the stockers. Just in price (many times the stock price). Even on a gas analyzer smog tester: same nasty HC content (misfire) at idle that diminishes quickly when the throttle opens. Same rasty vacuum needle until the throttle begins to open.

I need to learn more about using my newly acquired Diagnostic machine w/scope (and to replace flash tubes, at $65 each!) in the timing lights). A first run verified that the plugs were firing at a high 30K+ voltage. Almost off the scope.

At this point, my personal bottom line is "if you have a high voltage coil, use it". (Lucas Sport Coil does not qualify in this category. We used to get a "Corvette" coil as the hot set up decades ago, not the Lucas).

Gap the plugs over what you might otherwise use. See what the idle is like. No change, screw it...crank the gap open some more and see what happens. If you get to where it doesn't want to start/run and have seen nothing interesting, back to 25 thou and be happy. You've learned something, something useful. And YOU did it! Not some idiot on the 'net.

Now on the road this might not be wise, but in the driveway...??

---

1973 Roadster. A nice 50-footer!
SUs, Datsun 5-speed
MGB Tips and Tricks: www.mgrescue.com

I run a DUI system on my supercharged engine. The distributor is based on a '80 4 cylinder Jeep unit that uses the '70s GM design HEI coil in cap design. The company makes their own 50,000 volt coils, distributor caps, rotors and pickup modules. With NGK BP7ES plugs, set at the company's recommended .055" gap, my engine starts right up from cold and never complains. It's a great improvement over the stock ignition, but it is one huge ugly beast. Just another one of life's trade offs. RAY

And for your ignition system, the 0.055" gap is probably ideal. You have modern cap, rotor, etc and they will withstand the voltage stress. Not as recommended for a more stock ignition system with a high output coil. I'm not sure a stock ignition could actually fire a 0.055" gap plug!

But, for what its worth, for many years I ran a capacitive discharge ignition system with standard cap and rotor and it had higher than stock voltage so I gapped my plugs at about 0.040". I never had any problems - but it still isn't a wise idea.

---

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Hi,

My 2 bob's worth.

Like Terry I ran CDI for many years. Started gap at 40 thou. The difference in cold running was very noticable. Could push choke in after about 1/2 mile, with standard ign at least 3 miles. Being lazy, one of the least done maintenance things I did was to check the plugs. When I did they were at around 80 thou. No noticable problems. This was on a BMC Morris 1500, east west engine.

With the Bs IIRC the 25D was specified as 25 thou, whilst the 45D was 35thou. As previously mentioned the gap is largely determined by what voltage stresses the rest of the ign system can stand. A larger gap statistically gives a more reliable ignition.

Herb

LOL hey mate I'm sure Jeff will for a modest fee; I have no spares presently I do have a new 25D Pertronix - fitted dizzy I'll be posting for sale soon though.

---

~ Marco and Viajero ~