This is the series of posts that prompted this:


Originally Posted By: Bob_Spidell
However, as soon as the secondary voltage has risen sufficiently for a spark to jump the plug gap--or find another path to ground--the coil will discharge its energy and the voltage will rise no higher.

Hi Bob,
That is not how I understand the process. The coil doesn't discharge when it reaches any particular voltage. It discharges when the points open, causing a drop in voltage in the primary wiring of the coil. This change causes the secondary field to collapse-causing the high voltage discharge.


The "new source's" explanation seems to indicate the spark fires when the gap is sufficiently ionized--after the points open--and the voltage across the points varies; i.e. it's not fixed at the coil's theoretical output (which I assumed the reply to my post implied--I may have been mistaken). As I re-read these posts, there may not have that much disagreement. I didn't state it explicitly, but I certainly meant the process happens after the points open. I was actually convinced by the second post that the voltage across the gap would be fixed at the coil's rated output--e.g. 40KV (depending on gap)--but that's apparently not the case.

My main point was that a 'sports' coil won't buy you much in a stock Healey (Austin) engine--unless you're running at high RPM and/or have higher than stock compression--a 'sports' coil will fire at the same voltage as a stock coil all else being equal (the sports coil will support a larger gap, however).

The voltage across the points will always be around 12, give or take a volt or two depending on the charging system . The output of the coil is usually fixed by the ratio of the primary windings to the secondary windings. It is odd that the "new source" interchanges amps with time. I understand what he is saying, but I think it can be misleading and/or confusing. I gotta' get to bed, I have to be up for work in about 5 hours. I'll write more about the time/ amp thing maybe tomorrow.

Cheers

Oops ... meant "voltage across the gaps," not "points." The "source" was trying to explain how the excess power gets dissipated; i.e. with a spark of longer duration.

The scope doesn't lie, but I'm still having a hard time understanding why a coil--a transformer, essentially--that is mathematically able to boost 12V to, say, 40K, will only fire an 18KV spark. Where does the other 28KV "go?"

My 2 cents;

Any coil has a maximum voltage " potential " of x amount of KV.
As Greg has said the ratio between the input side number of turns to the output side number of turns determines the potential maximum voltage that the coil could produce assuming that 12 volts is available as the input voltage.
`
A quick and dirty way to determine the ballpark potential maximum voltage of a coil is to multiply 12 volts time the ratio of the output resistance to the input resistance of the coil. The input resistance is three ohms.
I have not checked my coil but if I did the math right the output side of a 40 kv coil should be in the neighborhood of 1111 ohms. A 20 kv coil 555 ohms. That should be true as long as the coil is not old or damaged.
.
The spark plug gap and the electrical characteristics of the fuel air mixture in the gap ( how easy it can be ionized by the spark ) determine that actual amount of voltage needed to cross the gap. Once that voltage is present at the spark plug gap, the spark happens and the voltage doesn't go up any more.
That high voltage becomes present at the spark plug gap right after the points open or the petronix hall effect transistor stops conducting. That is when the magnetic field of primary side of the coil collapses and that is what induces the voltage on the 2ndary side of the coil.
.
It is good to have as wide of a spark plug gap that can be used by the potential available voltage. The wider the gap, the larger area of hot, ionized material is made to start the ignition of the charge of air/fuel in the cylinder. Thus the wider .035 gap is recommended when a coil with a higher voltage potential or rating is used rather than a standard coil.


Ed

Ed, using points and standard coil, what gap would you suggest? A bit far fetched question, but would the engine then run hotter? And why wouldn't the manufacture (DH) suggest wider gaps?

Hi Bob,
Sorry this took so long, life has been busy for me.
The 28KV still gets used. If you go back to that first section
The energy produced by the secondary windings will remain relatively constant because of the ratio to the primary winding. At the point in time where the dizzy points open, the coil doesn't know how much voltage it will take to bridge the gap at the plug. It is dumb and only wants to bring itself back into equilibrium. As long as the coil's primary windings have enough time to reach saturation, they will produce full output in the secondary when the magnetic field is collapsed. Once the plug gap has been bridged, maintaining the arc takes less voltage so there is an increase in amperage as the voltage drops. So the remainder of the energy is dissipated in this state. You can see where that occurs in the scope graph just to the right of the spike as was pointed out.

Where replacing amps with time is confusing is; all things being equal, adding a higher voltage coil could lengthen the arc time without a change in amperage. So the two variables aren't interchangeable.


I still don't get what he is saying is false? Is it about plug gap? No one in this thread was saying increase the gap for a stock setup. Only that Roger has some higher output parts that would handle the increase without misfiring.

This is a misunderstanding/miscommunication that is entirely my fault. I was convinced the voltage across the gap would be greater with a 'hotter' coil--no one here said that but I took it that way. Anyway, I now have a better understanding; the voltage produced by the coil secondary will be fixed--more for a hotter coil--but won't necessarily be seen across the gap (unless it's required by a larger gap or greater compression, etc.). The extra energy--per the power equation--will be dissipated as amperage across the gaps. IOW, if the coil is capable of producing 40KV per the winding ratio, you won't necessarily see that across the plug gaps; you'll see, say, 18KV at idle and the rest of the energy will be current.

Right?

Yes, that is my understanding of how it works. There is a book you may be interested in reading called "Performance Ignition Systems". Here at Amazon. The author holds several patents and has his own line of systems. I'm actually using one of them in my Healey.

Hi Tahoe,

A standard coil will work best with the standard recommended spark plug gap.
...
D.H. made his recommendation based on his available coil at the time. He didn't have a 40 kv coil to play with or if he did, he knew the factory wouldn't agree to the increase in cost per car.
That would be my guess.

Ed

Bob,

What you have stated is right...
Now comes the advanced part of the situation.
How much current flow do we want to allow after the spark happens ?
Once all of the fuel/air is ionized in the spark plug gap,
how much more current is really needed or desired ?
We have plain wire spark plug wires with little resistance to current flow .
Then we have wires that are designed to limit current flow Immediately AFTER the spark happens such as carbon fiber stranded conductor wire and wound conductor wires like Magnacor wires.
Limiting the unnecessary extra current flow is desirable for
prolonging the life of the spark plugs, wires, rotor, points or electronic trigger, cap, and coil plus there is less of a drain on the electrical system of the car. It also prevents the car's ignition system from acting like a radio station and transmitting signals ( pops and whining noise ) to your car's music system or the computer system controls in a modern car.

Ed

Hey guys,

Being my springs are old and don't have any color on them to tell me if they are the correct ones, I've ordered a set (green on one end, red on the other end of the spring) from Moss. That should eliminate the spring question.

The next question would be if I have a vacuum leak. I've tested the vacuum unit and it's fine. No leak in the copper tube to the carburetor either.

How do I test for this:
The SU carburetor is highly vacuum dependent for its operation. Vacuum leaks at the throttle shafts are a common problem on these carburetors, these leaks allow unmetered air into the mixture and prevent the piston from raising properly due to the lack of vacuum in the system.

Something about spraying carb cleaner there?

It is a rough test but yes if you spray carb cleaner or even WD 40 around the shafts, etc, with the engine running and the rpm's increase or smooth out it is an indication you may have some air leaking around them which can be fixed with a rebush.

Also try timing the drop of the two pistons from all the way up to when they land on the floor of the carb--the time should be about the same and differences can indicate wear on carb bore or piston or both.

Yeah, you just need to make sure how you spray doesn't allow the mist to go in through the air filters giving you a false reading. Were your carbs re-bushed when they were rebuilt? Be careful with high flash-point cleaners around the exhaust. Non-flammables can show a leak by lowering the engine speed. You might want to hit the carb-to-manifold stack also.

Did you check the vacuum line to the brake booster?

Greg,
First time hearing about that.
Would you explain what I check for and how that affects things.
Thanks a lot,
Roger

Check for crack along the hose, maybe a loose connection at either end. If it's old, might as well replace it for cheap insurance. The hose taps into the manifold vacuum so it would have a similar effect as leaky gaskets on the carb spacers.

Hi Roger,

I am getting 11 mpg in moderately spirited urban driving. FYI.

Thanks Tom for putting that out there.
I don't jack rabbit start and I'm an easy driver. I get between 9-12 on fill ups.

Are Tom and I the only guys that get 9-12mpg urban driving in a BJ8?

Others say they average 15-19 ???
Anyone else want to post their average mpg?

Cheers,
Roger

"Anyone else want to post their average mpg?"
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My average mpg is 22-24, perhaps 1-2 more for sustained two-lane highway travel at around 65-70 mph average. This is with a five-speed Smitty box and a 3.55 diff.

Going for bragging rights Michael?
I was hoping for an apples to apples (BJ8) comparison.
Thanks anyway Mr Healeyota.....Healey/Toyota.

Cheers!

I get 15-16mph around-town driving, 18-19 hard driving (mountain roads, freeway @ 75+, etc.) and 23-24 on the highway @ 55-60.