Yamaha had no part in the makeing of the 3.4l V8... I don't know about it being based on the 2.5 I've never heard of that. I do know that Ford was impressed by the Yamaha V6 but they wanted more power for less money in 96, that's why they dropped Yamaha and designed there own v8 that would fit in the Taurus, that saved alot in shipping costs alone.
