Low ignition timing (retarded) can cause a pop back as Charlie said , it's best to have a boost adjustable timing box (MSD or built into an EFI ecu) so that you can run a fair bit of initial timing advance - 32/36 degrees or so-this will give you good throttle response and idle ,and then pull timing as boost increases. It's usual to start with pulling 1 degree per pound of boost and then add a degree back at a time, checking your plugs carefully. I would expect you to end up around 22-28 degrees total timing at 16 psi,maybe quite a bit more given your low compression ratio (for E85.)
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