The strange thing is that Part 1 implies the Delorean is completely electrical - both in the dialouge and the car itself. It's only the Foley sound effects which thow a monkey wrench into that idea.
Marty specifically asked if it ran on Unleaded. Doc replied that no, this sucker is electrical and runs on the Plutonium.
The Delorean itself didn't have tail pipes in the film. They were cut off and redirected by Filmtrix because the car was SUPPOSED to be electrical, therefore you wouldn't techincally have tailpipes since Doc replaced the engine.
As I mentioned earlier, the Sound Effects guys mess that all up by giving the car the V8/Turbine sound effects.
The "runs on oridinary gasoline" line is from Part III and both sequels has a TON of writing/plot cheats which I've always felt took away from the first film. This thing with the Delorean was one of them.
On the other hand, one could argue that Doc had created one of the first Gas/Electric Hybrid all the way back in 1985