dieselgeek wrote:it retards the timing to a "fixed 10 degrees total" not "retards by 10 degrees"
Scott, that is different than what barnym said. how would it (or could it) possibly know where 10* total would be?
The module assumes that you have the OEM distributor on an OEM application in the OEM position. So while it may have idled with 20-25 degrees total when the +5 signal is online (telling the HEI module to get it's timing info from the external computer). The 10 degrees was so that, if the computer or electronics or wiring failed, the HEI would operate in a limp mode and keep the engine running.
Ford TFI modules do the exact same thing, except their signalling is a different. Pull the "SPOUT" connector (which disconnects the spark-output of the computer from the transistor inside the module) and the unit defaults to 10 degrees advance, assuming the distributor is rotated in the correct position. On the Fords, it's also how you set the base timing.
Since both modules are reading the trigger signal (coil pickup inside the dist) they simply fire the transistor (thus the coil) a fixed spacing between the teeth that they're sensing. Which works out to 10 degrees BTDC on each firing event, if the distributor is in the right place. This 10 degrees stays fixed at ALL rpm's, that's why it's not quite accurate to say "the module retards timing 10 degrees" because, at 6000rpm the engine might have 34 degrees total, but in "limp" (or no-spout) modes, they'll run at 10 degrees BTDC all the time.
I agree with what's said about using an MSD or other CDI ignition, though. Dwell is a non-issue if you're using an external CDI box.