i have used the MSD one, but they both work by intercepting the signal from the distributor to the ignition box, thereby effecting the timing. worked great on spray. you can hook them up to a MAP sensor also, IIRC, and have it vary the retard for boost.


the MSD one i have dealt with, though, has had some issues (7 out of the 9 times he took the car out, it died, and wouldnt restart.) wound up being a issue internal to the retard unit.

installation is straight forward, all depending on the ignition system you have. the MSD is plug-n-play with the 6AL box.

and it works great (this one was a 68 Nova, 427 11:1cr, worked heads, 296 duration cam, not to mention a 1150 dominator and it was spraying a 175shot on top of that... )