Oscilloscope from Arduino!

I know about a software suite such as Xoscillo to turn my arduinos into a oscilloscope and i wanted to test CDI(http://en.wikipedia.org/wiki/Capacitor_discharge_ignition) for fault with it , i wanted to know that whether the 7khz speed with 4 input channets makes it useful for the purpose, also i haven’t used oscilloscopes before so may be it will help educate me.

Arduino-based oscilloscope projects are generally meant for low voltage projects (<5V).

What voltages would the CDI be operating at?

If the scope is connected to the low voltage side (i.e. the capacitive part) then the voltage would be nominally up to 12V. But, since it's inductively coupled the HT side there will be inductive spikes of tens of volts.

On the HT side the voltage will be in the tens of Kilovolts.

You're talking motorcycle engine ignition? Don't they run at thousands of RPMs?
I would think that using pulseIn() to capture your control signals would be sufficient, whule the hundreds or thousands of voltls needed to fire a spark plug would be beyond the reach of inexpensive oscilloscopes without special probes.

@ Sir Robert , The ignitions CDI units here are for three wheelers and four wheelers.

and about the voltages needed for operation the CDI will operate on 12volts, while the thousands of volts needed to fire the spark is real thing i just need to care for the inner circuit that whether its performing as it should , actually i look forward to define a benchmark (like a particular make of sine wave for a block of cicuitry in the CDI that is certified working) and then match that wave form or stats with other CDI units that i would be checking.

Okay, you can generate the 12V switching with a transistor, and read if back with a voltage divider to keep the level under 5V.

4 cylinder engine at 5000 RPM fires the plugs at 20,000 RPM,
20000 pulses in 60 seconds, so 333 pulses per second, pulse every 3mS, Arduino can keep up with that.

It should be half that rate surely, giving even more time.
1 cyl 4 stroke - down up bang down up = 1 spark per 2 revolutions.
4 cyl 4 stroke = 1 spark per 2/4 revolutions

Don't forget that the discharge cycle only occupies a tiny fraction of the ignition cycle. The basic design of CDI (capacitive discharge) is to use time to incrementally build a charge and then let it all go in a blast. In order to capture a decent discharge waveform, you will need a sample rate that is at least 10 times faster than the discharge pulse width.

I would say that 1 million samples per second is a minimum and that is pretty far from 7kHz.

NI$HANT:
i look forward to define a benchmark (like a particular make of sine wave for a block of cicuitry in the CDI that is certified working) and then match that wave form or stats with other CDI units that i would be checking.

If you're doing this professionally, get the proper test tools.

If you only want an informal check that the unit is working like other standard units, I suggest you test it with a passive HT strobe costing a couple of quid.

The CDI's i develop are having a capacitive discharge of about 18,000 Volts that is a Big amount.

If you're doing this professionally, get the proper test tools

what are the proper test tools according to you to test this?

The cdi's have something between 400-600volts that depends over the capacitors i have to achieve that 18kvolt to 40kvolt current level the ignition coil is there ,so majorly any testing needed needs to cope up with this 400-600 volts.

any ideas?