Is a PC Sound Card Recording nearly as good as a DSO?

I have used various PC sound cards as a accurate Digital storage scope even though the sample rate is 48 kHz both channels.

The Frequency's I look at are between 10 to 250 Hz Stereo and I need to tell the phase difference between the channels eg the ignition controller for my car running in QuickBASIC off an old Toshiba notebook.

An example of a recording is here http://soundcloud.com/smokingwheels/1500-rpm-60-9-d-1-04-set-2-92

To calculate the Degrees advance for my engine controller and double check my program is doing the right thing I look at the difference in the input and output signal.

In the pic the Blue channel is the trigger and the red is the firing signal from the computer.

There are 2 measurements involved to calculate the firing angle of the wave forms.
1. Find the previous trigger cycle width BLUE in number of samples =952
2. Measure from Blue falling edge to Red falling edge in number of samples = 324

3. 952 / 180 Degrees between events eg 4 cylinder 4 stroke engine. = 5.288888
4. 324 / 5.288888 = 61.26 Degrees But there is an offset of 62 Degrees BTDC
5. 62 - 61.26 = 0.739 Degrees BTDC

Here are the 2 pics
Step 1



Step 2


Is my method accurate enough to rely on?
Does the sound card come into question?

Ps I have a program that tells me the difference in degrees but I used to do this method for years to get QuickBasic to run in real time.
Also I have driven the line input of many sound cards with 5 volt digital signal and not one has died yet. touch wood..