At 09:36 AM 7/24/2000 -0400, Geoffrey Gallaway wrote:
>I just bought a stock ignition coil from propermg for 19.95 although I
could have bought the Lucas sports coil at 40,000 volts for a few dollars
more. ....
>
>So, what voltage is the standard coil, 20,000 volts? How much does the
sports coil actually gain? ....
17,000 to 20,000 comes to mind as standard, although I think a stock engine
may actually run (marginally) with as little as 7,000 volts.
The voltage required to make a spark jump the gap at the plug is determined
by the size of the gap and by the pressure of the air in the cylinder.
Bigger gap = higher voltage required. Higher compression = higher voltage
required. Once the voltage in the coil builds up to the required level the
spark jumps the gap and the voltage goes away. The voltage will never go
any higher than what is required to make the spark jump the gap, regardless
of what coil you have. On the flip side, if the gap is too large, and you
don't have enough voltage available, then you will never get a spark. The
highest voltage requirement occurs at the point of highest dynamic pressure
in the cylinder, which for most engines is around 2500 rpm and full
throttle, which is the same condition where you might experience spark
knock with high compression and cheap gas.
HOWEVER, when you have a coil that is capable of putting up a higher
voltage, then you can open up the gap at the spark plug, which in turn
demands a higher voltage to affect the spark, which in turn will give you a
higher voltage (and probably longer duration) spark. A hotter coil may in
fact be necessary in a high compression race engine just to make the spark
jump a standard size gap. With normal compression ratio and a 40KV coil
you can open the spark gap from .025" to .035" or more and still get a
reliable spark. The hotter spark will then make it easier starting and
have much less probability of misfire under various running conditions.
You would also be less likely to ever have fouled spark plugs. Also the
system would be more tollerant of maintaince neglect, as it could fire a
spark across a badly worn plug with a much larger gap.
I happen to like my 40KV coil and .035" spark plug gap setting, finding
easier starting, more consistent smooth idle, and never a misfire with the
hammer down. I think it's a keeper.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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