Hi Guys,
Thanks for covering for me here in the cave...I've been on vacation. Horses for a week instead of horsepower
The more compression ratio and/or boost (or nitro which is liquid compression ratio), the more spark you need. More spark can be expressed in terms of either more voltage, current or spark duration. Voltage (or electrical potential) is what makes the spark jump the gap. Current is basically the SIZE of the spark. Spark duration is how long the spark lasts. The different brands of mags and coils all have different characteristics and the different characteristics benefit different applications. But that's another post.
The easiest and very best way to see if you need more ignition power:
Open the plug gaps up to something a bit larger than normal...say .025" or so. Make a few passes and note the ETs and speeds. Now close the gaps down to .015" or so and make a couple of passes like that. Did you gain any power with tighter gaps?
You're making the ignition work harder with the larger gap and making things much easier with the tight gap. Many times, a racer will see significant improvement with the tight gaps and more ignition power is needed.
The main issue with the box ignitions is that they use battery voltage...on a drag car without an alternator, it's impossible to suck 8-10 amps out of a battery and have it STAY at 12+ volts. Generally at high RPM, the voltage drops down to 10-11 volts. Remember, most guys have just FLOGGED their battery by starting the car at the line, so it's already in recovery mode and low on voltage before the race even starts.
With a magneto, the faster you spin it, the more juice it makes...period.
My own example...years ago I was running a 7AL-3 on an injected methanol SBC (14:1 CR). It twisted to 7400 RPM on each pass and the world was simple and wonderful. Then I started running nitro. At 25% I had a top end miss that I couldn't get rid of no matter what I did. A friend told me I needed a mag and loaned me a Super-Mag III setup. I dropped it in and not only was the top end miss gone, but I shed almost .05 off my ET and picked up almost 3 MPH consistently. SOLD!
You can still have bells and whistles with a magneto, but they are add-ons, not included in the box like with the 7AL-3 and others. Rev limiters, timing control, tachs, shift lights, etc. are all possible through various means. It really depends on what type of racing you do and what you need for functionality.
Spud