Toyota of Buemi and Davidson suffer electrical problems while in the lead.
Two entries will campaign at Bahrain.
Porsche lapped quicker than both Audi and Toyota and ended free practice two with the fastest time, setting up what should be a thrilling qualifying session.
After Toyota led most of the session, the opening practice at Spa ended with Audi on top.
Anthony Davidson is a difficult man to pin down as he combines his Toyota Racing duties with a role as a commentator and analyst for Sky TV’s F1 programming in the UK.