Depends on the box as there are various features they can perform. Essentially they provide a multiple spark for each time the plug fires. This provides improved throttle response, easier cold starts and better fuel mileage. Boxes can also perform functions like: RPM limiting, RPM based switching (useful for nitrous applications) and timing control. Just be sure you do enough research before buying one as some may not be compatible with your distributor and coil.
- MSD and other multi-spart ignitions switch to a single spark once you get to over 4000rpm. There just isn't time for multiple sparks above that.
- CDI (Capacitive discharge ignition) charges your coil with approx 400volts instead of 12V. This allows the coil to charge faster, which in turn means better spark at high RPM.
- CDI ignitions has a very short spark burn time. If you are trying to ignite a lean mixture (better fuel econom), the older style systems may work better.
All of the above posts were right. Personally, I'm ditching all of the stock set up, and going with teh following:
MSD 6AL box
MSD Blaster2 Coil
MSD Mechanical advance distributor.
The box will allow rev limiting, as well as provide a better spark/multiple spark. Watch when ordering the coils, as some are made to work with the stock ignition set up, while others only with the boxes. If you are using a stock/near stock engine set up, then I think the gains from the box wouldnt be that great. BUT, I feel that if you pair the box up with a coil that is designed to be compatible, along with a strong distributor and a good set of plug wires..then you will feel gains. In race/performance motors, definitly invest in a quality ignition.