While IonSense is a good way to get direct information on what's happening in the combustion chamber, there is another, more direct way - we can sense the pressure directly instead of indirectly with a pressure transducer fitted into the cylinder head. This is an *extraordinarily* powerful tool for research and tuning. The EFI computer would be able to auto-learn the proper timing, and get it right within a very small margin of error if one of these was employed.
There are a few pressure transducers commonly used in laboratory research for this purpose. The most common one is the Kistler 6061b. It's designed specifically for combustion chamber pressure measurements, and is water cooled.
Kistler makes other interesting sensors, specifically the 6043, which is very tiny (M8x0.75 thread pitch), and would be able to be fitted in a modern 4 valve/cyl engine without clearance issues, whereas the contemporary 6061 is quite large.
Incidentally, the cost of these things is unknown, although I'm sure you could find out. There does seem to be one source of a 6061b from a lab surplus place. For reference, if anyone REALLY wants one, it seems that it can be had with an amplifier for $199.
(search on the page for 6061b)
While these above transducers are all well and good, their integration into a normal automotive engine would be difficult at best. It seems Kistler has already developed a solution to this problem in the form of a spark plug with the pressure transducer integrated into it! This is a drop-in replacement for most engines, as long as the spark plug specs are close enough.
If anyone finds out how much these little beasties costs, please add it to the wikipage. :)
I called kistler here in the Netherlands. The one with the sparklug 6115A cost around 2800 euro. The 6043A60 cost around 2600 euro, too bad. Q.Straker
More information can be found in the links below:
Compression and leak-down testers
One can find useful information about his engine with these methods.