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MG MGA - coil needed

I need a new coil for my '60 A. IS there that much difference between the regular coils and the high performance ones. Also, what about the ones from Advanced Auto or Auto Zone? How do those compare with the high performance ones compare to the HP ones from Moss, VB or Scarborough Faire?
Thanks,
Jack
Jack

Jack,
I use a Lucas Sports coil that I bought from MOSS about 10 years ago. It does supply a higher voltage at the spark plug that helps at start up. If I recall, the voltage is some where around 40K, where the original is about 37k.
You call MOSS and ask one of their reps they surely could tell you the voltage and the original coils voltage. That info might even be in their catalogs.
If you need a coil, then I would buy the Lucas Sports coil.

Ray
Ray Ammeter

When I first got my 1500 it had a bad coil, and I replaced it with a cheap replacement coil from Advance Auto. It worked fine. I later replaced it with a Lucas coil and could tell no difference in starting or performance. My opinion on the "High Performance" coils is that unless you have a highly modified engine, with increased compression, they really won't do anything for you. A well tuned stock engine will start and perform just as well on a standard coil.
Jeff Schultz

Jack. The MGA needs a coil taking a straight 12V input. The "12 Volt, use with external resistor" coils are actually a 6V coil and, when used in an unaltered MGA, will cause problems. The standard Lucas coil is capable of generating 20,000 volts when required. The Lucas Sports Coil is capable of generating 40K volts when required. Normally, however, the system is operating at about the 11K volt level as sparking voltage only goes high enough to generate a spark. Hence, the use of a Sports Coil is, for most people, a waste of money--the normal coil will generate more than sufficient voltage is the operating system is in good order. If the operating system is not in good order the Sports Coil is marginal--it may help in some cases but will simply cause the system to quit working completely in other cases. (I might point out that the voltages mentioned are based on my observations testing cars with both standard coils and sports coils during tuning sessions on my engine analyzer.)

So, a standard Lucas coil is correct and will work fine. If you are willing to pay extra for a pretty gold plated case, invest in a Sports Coil. Les
Les Bengtson


One more question: I have a positive grounded car. Does that make any difference with the coil, like it does with the fuel pump?
Thanks,
Jack
Jack Weiss

Jack - The coil can be hooked up to accommodate any polarity that you have. You just have to hook positive to positive and negative to negative (for a positive ground car the wire to the points in the distributor is hooked to the terminal marked +). This only becomes problematical when the coil has markings such as SW (switch) and CB (contact breaker). If the coil is marked like that, then it is necessary to know which polarity the coil was intended. Even then, the car will run fine unless everything is marginal to start with, since a incorrect polarity will only cause about a 15% degradation in performance.

Unless you have a new, all electronic fuel pump or it has been modified by the addition of a diode, the original fuel pump for an MGA is not polarity sensitive.
David DuBois

The "sport coil" will not change the voltage that your plugs fire at, unless you increase the gap of your plugs. With a larger gap, it must build up a higher voltage to jump the gap, thus, a hotter spark. I set my plugs at .035". This setting would cause undo stress on a standard coil, and might lead to premature coil failure. The "sport coil" is made to handle the increased load of a larger gap,(higher voltage) and will hold up under these conditions.
Ed Bell

Ed:

What is the performance benefit of a hotter spark? This is not a loaded question; I am investing in a new engine soon, with higher compression, and was wondering whether a sport coil might be of benefit.

M.D.
'57 Coupe
M. D.

Ed. B.S. Have you checked your theories on an engine analyzer? I run .035" gaps on my spark plugs with both the standard and Sports Coils. In each case, the voltage build up at time of spark is about 11K volts. Even opening up the gaps to .040" does not cause a build up above 13K volts on my engines. Since the standard coil is capable of operating at 20K volts, it is not going to be damaged by the 11K-13K volt build up. Les
Les Bengtson

Some people may not be aware of the problem that Les mentions "the 12 Volt coil with external resistor". Some cars, the most common seem to be Triumph cars, have a 12 Volt system and a 6 Volt coil.
The way it works is there is a resistor in series with the 12 Volt supply to the coil, which is a 6 volt coil. When 12 Volts is applied through the resistor the effect (current flow) is the same as if 6 Volts is applied.
Why?
When the starter motor is activated a separate lead takes 12 Volts directly to the coil. This passes double the normal operating current through the coil. This compensates for battery voltage drop during starter operation. Makes starting easy.
But, during running the voltage must be passed through the resistor or the coil will overheat and the contact breaker points will burn out quickly.
Must not be used on a standard MGA.

Mick
Mick Anderson

Following on the comment by David about coil polarity. A quote from my Bosch factory issued catalog is as follows, "Wrong polarity will cause a weaker spark resulting in loss of performance, misfiring and hard starting".
In addition to the + and - markings or the SW and CB that David mentions, Bosch coils made in Germany may have the terminals marked 15 and 1. For a negative ground car the switch wire goes to 15 and the 1 wire goes to the Distributor (opposite for positive ground cars).

Mick
Mick Anderson

All

I am finding this thread very informative as I have never understood the difference between the various types of coil. Furthermore, its not written in prose!

Now for my question. What is a ballasted coil system? Is it the external resistance referred to in some of the previous comments?

Steve
Steve Gyles

Yes, the external resistor is often called a balast resistor.
Jeff Schultz

Steve it is interesting and there was I believing I had a very good understanding, being an electrical engineer.
Mick I can not understand the comments by Bosch? re the problems caused by wrong polarity? Looking clinically at the situation , wrong polarity will ONLY cause the spark to jump from the tip of the spark plug to the centre, instead of the norm which is from centre to tip. The result of this would be premature wear/burning of spark plugs! Is that assumption wrong?
Les, whilst I understand the voltages involved short of calculation your figures may or may not be correct. What I feel is correct is that a normal coil is designed to fire a spark across a 25 thou gap at the plug, naturally if you want to fire a spark across a larger gap then a higher voltage coil will be useful, hence a sports coil and a potential gap of up to 40 thou.
As a post scrip, I had a coil fail on my midget when many miles from home. I could only get a ballasted coil and hence I used that fed directly with 12 volts(13.5v when running). The coil successfully ran for 500 miles and got me safely home.
Bob (robert)

Bob,

My unfortunate experience with a ballasted coil, running without the resistor, caused the plastic insulator in the points to melt after about 10-15 miles.

I expect you are correct in the wearing of the spark plug. I would expect that the materials used are selected for a particular current flow. Reversing it should increase the wear on the gap.

Also being an electrical engineer, I believe that the performance improvement is more in the area of 1)the higher energy storage in the coil at high revs, and 2) a higher energy spark, (not to be confused with higher voltage). I have seen this years ago on an engine analyzer in running a side by side comparison between a std coil and a sport coil.

In addition, using a coil in the "wrong" polarity should get the user about a 2% reduction in voltage. Hardly enough to really notice on an analyzer or in performance.

Chuck
Chuck Schaefer

Bob. My figures are based on the scope reading provided by my Sun engine analyzer and cross checked with a newer unit owned by a friend who has a shop. Here in the US, the .035" spark plug gap was common back in the mid 60's onwards, using a standard 20K volt coil. Why the Brits were still using a .025" gap, I do not know. I have read that the petrol available in the UK during that time period was not of the best quality. Something referred to as "pool petrol" and still being used into the mid 50's when the cars were being produced. I do remember reading that several cars were tested in Belgium, using Belgique petrol, with better results than could be obtained in the UK.

Back in my younger days, I believed what I read in the advertising. Today, I tend to believe what I observe through scientific tests using proper test equipment. While a higher voltage coil may be of benefit with a racing car or professional rally car, something operating at higher revs constantly, I do not see any benefit from such a coil for normal driving.

Another point, seldom addressed, is that, if the high output coil is actually being required to deliver the higher output, the rest of the system needs to be upgraded along with it and hte system life will be shortened. Specifically, the rotor, distributor cap and high tension leads need to be in perfect conditon and will deteriorate at a somewhat more rapid rate.

As to the "12V coil for use with an external ballast resistor", there are two common forms of resistance. Some cars use an actual resistor in the circuit and some, like the MGB, use a resistor wire to drop the voltage. Not all of the vehicles using a 6V coil will have the two leads going into it that Mick mentions. My 77 Scout does not. My later model MGBs had the 12V input, from the starter when the starter circuit was engaged, due to the problems created by the emissions controls. Supposedly. When I switched both cars over to a points type ignition system, I rewired them to straight 12V input using a new 12V coil. Both start as easily as they did before. Les
Les Bengtson

Les you are quite right I was not questioning your figures but just suggesting the machine might not be 100%. I too will only try to comment upon things I have personally tested and I get very annoyed when things I have gone to a great deal of trouble to prove or disprove are questioned by someone quoting from a book. Thats another story though.
Regarding the dual wiring, if the ballasted coil is not for help when starting, ie connected to 12v direct when the engine is cranked and therefore having a good spark when battery voltage is low, then what is it for?
Bob (robert)

Pool Petrol (unbranded, 75 octane) was introduced in 1939 and lasted until early '53, when branded fuels were reintroduced. FWIW.

A. W. Risk

I've just looked in a 1974 Volkswagen Scirocco workshop manual and the plug gap is 25 thou so it wasn't just a British thing. In 1982 Ford were recommending a 30 thou gap on the Sierra.
malcolm asquith

All

As I intimated above I know very little about this subject so please shout me down if I am going off down the wrong track re the Brit 25thou and the US 35thou plug gaps.

Am I right in saying that lower RON fuel needs a hotter spark to ignite the mix, hence the 35thou spark plug gap?

As you in the US have been on lower RON (95?) fuel for a lot longer than ourselves you have also been on larger plug gaps for a lot longer.

There is a saying over here that we follow the US about 10 years later!

Steve
Steve Gyles

I'm about to install a pertronix ignition system on my 1500 A. It is advertised alongside upgraded coil etc. Is there any need to upgrade my coil?
Dan Barton

Many people swear at Pertonix systems, especially in old Fords. They say generator voltage spikes can kill one in a heartbeat.
Another thing about my 6 volt, positive ground '48 Ford is that the original ignition system has a resistor between the ignition switch and the coil, which reduces the voltage to 4.3. I use a moden, Standard brand, UC-14 '6 volt coil' with no internal resistor. I don't think the Ford external resistor is needed, but I'd like to hear your opinions.
S.R. Barrow

I am now an ignition addict! I am reading an old British book:'Practical Automobile Engineering' It is dated 1959. It reads:

"...some manufacturers tend to specify a smaller (plug) gap than can be used satisfactorily in practice, probably in order to allow for neglect on the part of the owner and for abnormal periods between cleaning and regapping."

"As a general rule....the setting is 0.025 inches."

"...a wider gap than normal is specified when the carburettor incorporates an economy device which provides a lean mixture at cruising speeds, necessitating a higher voltage spark to ensure satisfactory combustion."

"On high compression engines, in which the greater compression pressure increases the resistance to the passage of the spark, a gap of 0.20 to 0.22 inches is usually suitable. ....Too small a gap will cause difficult starting, poor tickover and, sometimes explosions in the silencer on over-run. Too large a gap results in hesitation when accelerating and possibly misfiring at high speeds."

Here ends the reading. Food for thought and comment? Also, does the cubic size of each cylinder make a difference? Typical post war British cars are 250cc to 400cc a cylinder, occasionally up to 500cc whilst most US cars are probably treble that!

Steve

Steve Gyles

This thread was discussed between 17/01/2005 and 18/01/2005

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