The main reason for using the MSD ignition system is the
incorporated boost retard circuit. Essentially what this does is retard
the timing by a predetermined amount per psi of boost.
Anytime you add a turbo or supercharger, one must take into consideration ignition timing under boost. As cylinder pressures increase, timing must be lowered. For optimum performance, ignition timing should be fully advanced in off-boost conditions and timing should be pulled only under boost.
The MSD 6BTM does this very job. It will allow you to run standard ignition timing thereby regaining low end power (rather than setting the timing at a fixed point) and the 6BTM will pull timing only when under boost. The amount of timing that is pulled is fully adjustable via a knob installed on the dashboard.
Not only does the 6BTM retard ignition timing automatically under boost, it also increases the spark energy with the patented MSD multiple spark technology. .
There is a better way to control the ignition timing when using any sort of forced induction system, this would be to use a fully mappable ECU based ignition system. Most modern cars have such ECU based systems. They control the fuel management and the ignition timing and will usually have knock sensors to help control detonation. Whilst I could have ripped the entire standard 14CUX ECU unit out and replaced it with a custom aftermarket unit that controls both ignition and fuelling, I decided to remain with the standard ECU and use the MSD.
This was a more cost effective option although on reflection for not too much more I could have had a mappable system, but at the time I was looking for something there wasn't anything around that had been used and was proven.
Because the standard 14 CUX ECU uses the 12 volt signal from the coil for timing there is a little more to do than just wire the MSD Unit in as per the standard instructions. This is because the MSD actually outputs about 450 Volts to the coil primary rather than 12Volts. Therefore you have to install a step-down transformer to reduce the voltage back down to what e ECU expects.
This is detailed in the MSD documents but it is not totally clear that it is required for the Rover V8. It was only by looking at the Rover schematic that I noticed there was the signal from the ignition coil.