Chris White

A commonly agreed upon speed for the ‘speed’ of an electric arc is 1/3 the speed of light, round numbers makes that 60,000 miles and hour. So figuring a stock gap of 1/16 of an inch that means the time to arc the stock gap is .00001515 seconds.
Using an assumption of a 1” distance from the rotor center to the point of arc we can calculate that the rotor tip moves .004363 inches per degree (crank degrees) of revolution (or “timing”).

So – basically the error introduced by the off center position of the distributor cap / rotor is .00001515 for each 1/16 of an inch it varies.

The total range of ignition timing is about 30 degrees, or about .065448 inches. (just a tad over 1/16 of an inch – remember that the rotor is rotating at ½ the crank speed) so the total hysteresis of the mechanical system is approximately ¼ of a degree.

I think I could live with that…..but then again I don’t have a distributor anyway….!

Chris White

Tomas L

Chris, I think you have an error in your calculations.
The speed of light is approximately 300000 km/s or 186500 miles/s. This would mean that the time to jump the stock gap is 5.2 E-12 seconds.
If the rotor is 30 crank degrees away from the pole in the cap then the jump time would be. 2.2E-11 or at 6000 rpm 7.9E-7° or 0.7°/1000000.
There is still one flaw in this calculation though, as long as the cap pole is within the rotor width the arc distance doesn't get any longer when the rotor rotates.
My guess is that we have other more significant ignition timing issues....
IIRC this thread was about EGT in the begining...

You were right to begin with. The reason for the wide tip is to allow the DME to change ignition timing. The side effect is that it will also be tolerant for variations caused by flex in the timing belt and other parts. Note that in this case tolerant means that it will not missfire due to the belt flex, as the above calculations show the rotor and cap cannot be that missaligned so that there will be any measurable ignition timing deviations. Long before that point the spark will fail to jump the gap.

TurboTommy

Tomas,
"My guess is that we have other more significant ignition timing issues"

So, what's your best guess as to what these issues may be.

Trucho-951

Your calculations are very nice, however, you guys are calculating the travel time for electrical current, recall from physics that, normal convention has defined current as “positive holes”. However, electrons are negatively charged, therefore, the electrons will jump the sparkplug gap first, travel up the ignition wire, then jump the distributor/rotor gap and finally arrive at the ignition coil. My point is that the time between the DME timing signal and spark at sparkplug = approx. zero.

All calculations aside, this weekend I will place a whiteout timing mark on my balancer and rev. the motor through various rpm ranges to see for myself just how stable my dizzy timing is. Not very scientific, but can be very gratifying so see a nice stable timing mark.

Chris White

Ah, yes. I must have spilled my beer on the cocktail napkin as I was doing the calcs…either way the answer is still a really small number!

Chris White

Trucho-951

Trying to steer this back to topic...

here is a great 2 in. dia. combination egt and cht guage, too bad EGT scale only goes upto 1500 deg F
entire kit including both probes for only $149.

http://www.pro-flo.com/proflo_2in_si...auge%20kit.htm

DrRon

Plenty of gauges (EGT and CHT) here:

www.westach.com

Tomas L

The time it takes for the current to go from the coil through the distributor and plugs are for all practical aspect zero. Our calculations were not really that seriously meant, just a short exercise to show how ridiculously small this delay is.

However when you state that the time from the DME timing signal to the spark at the sparkplug is zero I have to disagree. There is a time delay of significant amount from the DME signal to the time the coil has built up enough voltage to jump the plug gap. This delay is not large enough so that it will effect spark timing in a significant way. It still does matter though.
If we have a spark plug with some deposits on it (which we always have more or less) so that there is a conductive path (with quite high resistance) from the center electrode to ground then some current will use this path and bleed of energy stored in the coil before it could be used to produce a spark.
The advantage with a CDI type ignition is that the voltage rise time (=delay) is shorter than that if an inductive ignition therefore there is less time to loose energy before the time that the voltage has risen to a level capable of jumping the spark plug gap. A CDI has other disadvantages though, like to short spark time.

Tomas L

If I were to rank the three most important factors they would be.
1. Correctly tuned DME ignition maps.
2. Correctly tuned DME ignition maps.
3. Correctly tuned DME ignition maps.

After that there are a couple of areas were I would like to see improvements, these are without any ranking against each other.

• It would be an advantage to be able to change ignition timing as a function of intake manifold air temperature. The DME has a map for timing as a function of air temp in the AFM. This makes it possible to take ambient temp in consideration but not MAT changes due to turbo efficiency or IC cooling efficiency.
• A better knock control system. A self learning system would be nice...
• Load (MAP) based ignition maps at WOT to better optimize timing during spool up.
• A better timing resolution than the 1.36° the DME has.

These are the things that pop up in my tired head right now, I'm sure there are other things that I haven't listed.

Campeck

or a matching pair...

http://cgi.ebay.com/ebaymotors/Aircr...QQcmdZViewItem