Pertronix Second Strike
Race Director
Joined: Jul 2004
Posts: 12,691
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From: Scottsdale Arizona
Actually, it happens quite often. Not that most people would do it, but if you attach an oscilloscope and high voltage probe to a spark plug or coil terminal you will often see the coil voltage/plug gap voltage making several jumps in magnitude as the plug goes through several instances of hitting the high arc-over voltage, dropping down to the sustaining voltage, and then rising back up to the arc-over voltage as the mixture swirl velocity blows away the ionized (conductive) portion of the air-fuel mixture. This is a reason why high energy (versus high voltage) ignition systems are (theoretically) preferable as they have the "reserve" capacity stored in the coil to keep relighting the mixture if strong swirl currents keep interrupting the arc. (HEI energy systems are also good for long term emissions controlled vehicles, as we know, but that's a different topic for another day.)
So if I'm understanding you correctly, swirl velocity (agrees with Lars' point on high compression, I think) can cause the spark to be lost AND HEI has the reserve to reform the spark This sounds a little more plausible.
Tech Contributor
Joined: Jun 2004
Posts: 20,847
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Higher compression (with usually higher resulting cylinder pressures) requires a higher plug/coil voltage to arc over the plug gap. This higher voltage uses up energy from the coil faster than a low compression/low arc voltage engine. Simplifying things, the energy in the coil is used up as gap Voltage X gap Current X Time (Pretty much the same basic formula as the monthly electricity bill). If you increase the voltage requirement at the gap, you will automatically reduce the time that current flows across the gap. (The current across the gap is determined by several factors including the wiring impedance, gap impedance, and other factors too boring to deal with at the moment.) Essentially, with higher compression or wider gaps, the plug arc time is shorter.
Swirl in the cylinders is somewhat like the wind blowing (or perhaps a tornado) across the plug gap. It is independent of the compression ratio, and therefore, for all intents and purposes, does not require higher coil voltages to fire the plug. (Assuming that the ignition point is prior to the generation of significant swirl content.) A downside (there's always upsides and downsides) of strong swirl is that it can keep blowing out the arc. It's usually not a big deal if you have a homogeneous, combustible A/F mixture in the cylinder. After the first light off, the rest of the mixture will eventually be consumed by that first flame kernel. However, if the air-fuel mixture is not well distributed, there is always the chance that that first arc might not light things off sufficiently. This is where it is handy to have a reserve of energy in the coil to keep the arc regenerating to eventually light off the mixture.
Dry stuff, but that's kind of the basics.
Race Director
Joined: Jul 2005
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Not doubting the experiences of others, I have changed from points to electronic ignition on every "older" street machine I've had (and there have been several, since the 1970's)....and EVERY one of them had an increase in horsepower because of it. There was a 2/10 to 3/10 faster ET at the drag strip....every time. IMHO, ditch the points. But after installing the electronics, I have never gotten any more advantage by adding an MSD or other boxes.
Perhaps the stock points ignitions you replaced were well-used and tired. Under those conditions, new stock components would probably increase power by similar amounts. That's how Jacobs (for example) can advertise 15-20 hp increases, because what customer is going to pull fresh, new, just-installed components?
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Perhaps the stock points ignitions you replaced were well-used and tired. Under those conditions, new stock components would probably increase power by similar amounts. That's how Jacobs (for example) can advertise 15-20 hp increases, because what customer is going to pull fresh, new, just-installed components?
Safety Car
Joined: Oct 2004
Posts: 3,922
Likes: 733
From: Youngstown Ohio
Perhaps the stock points ignitions you replaced were well-used and tired. Under those conditions, new stock components would probably increase power by similar amounts. That's how Jacobs (for example) can advertise 15-20 hp increases, because what customer is going to pull fresh, new, just-installed components?
