Recently I read where Vader might add an engine upgrade (engine boost?) to his Advanced TIE. There is no wrench & screwdriver icon on the Advanced TIE bar. Is this allowed? Thanks, Rich

Yes it is.. all ships are allowed one (1) upgrade of this kind. No ship has that icon, just so you know.

The rules for that particular upgrade can be found with the Firespray and YT-1300 rules insert, where those cards were introduced in the wave 2 release.

New Upgrade Cards (top left of rules insert)

Modifications and Titles

"Modifications and titles are special upgrades that do not appear in any ship's upgrade bar. Any ship may equip a modification or title unless the card is restricted to a specific type of ship. Each ship is limited to one modification and one title."

Each ship is limited to one modification and one title."

Except for Royal Guard Interceptors, whose one title allows them to take two modifications.

Each ship is limited to one modification and one title."

Except for Royal Guard Interceptors, whose one title allows them to take two modifications.

Which is explicit to that cards explanation. But in general the OP was for the original rule covering modifications and how many can be applied to a ship as their is no wrench/screwdriver upgrade symbol on any ship card.

Each ship is limited to one modification and one title."

Except for Royal Guard Interceptors, whose one title allows them to take two modifications.

That falls under the old "Any card that contradicts the rules overrides the rules" rule.

Rulerulerulerule.