| My desktop test platform is seen
above. I'm using the Megasquirt and Spark Extra to control both
fuel and spark. I'm using a crank fired ignition system using three twin
tower wasted spark coils for direct fired ignition without a distributor.
So this required me to modify the circuit inside a stock Spitfire 4
cylinder tachometer, to properly read the spark rate from a 6 cylinder
engine. Idle control is via a pulse width modulated signal fro mthe
Megasquirt, to control the idle air control valve that will be attached to
the Ford Mustang throttle body.
| I ripped out the guts of the
electronics contained in the base of the Oldsmobile coil module assembly.
I found this to be a convenient place to mount the three VB921 IBGT driver
transistors. The aluminum base plate will act as a heat sink, and keep the
power pulses away from the main computer. The drawing above is from ohming
out the gray connector (at left) to the various terminals that use to feed
the original Oldsmobile circuitry. If I had spent a little bit of time, I
probably could have required the original circuits here to use the power
transistors that were already here. But it would have entailed a bunch of
reverse engineering I didn't want to mess with as I wanted to 'get to it'
with the MS itself.
These transistors are what I was talking about with The Weird Pulse Problem . Since I didn't want to bring the main computer chip signal out into the 'real world' un-protected, I've since added a 74HCT125 driver chip to the proto area of the MS main board to drive the lines feeding this coil module. (see Bruce's post...second post at http://www.msefi.com/viewtopic.php?t=11502&highlight= )
As configured, this system will run up to about 9,000 rpm with no loss of the massive spark this coil setup is capable of. They have a primary resistance of only 0.35 ohms. So dwell times must be tightly controlled to minimize coil heating. I've now set the soft rev limit to cut the spark at 6800, and a hard rev limit setting of 7500. (see the Megatune Controls). I can open up the spark gap to nearly 0.25" (clip across the towers) and still not loose a spark. I've had to reduce my dwell settings to 1.7mS as this allows max power in the coils, without spending anytime fully charged, simply drawing power and heating up the coils. (see the weird pulse page link above)
Oh...you might be wondering about the tach modifications? To make a 4 cyl electric tach read as a 6 cylinder tach (or 8 cylinder tach)..a good read there is on Zach site... http://www.zaks.com/mgb/leonsmgb/Better_tach.htm
...more to come...
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