Bullet proof goes beyond that, describing something that can be pushed beyond what it's supposed to do and still hold up to the extremes.
I guess the key term here is "beyond what it's supposed to do." Would you agree that the testing Ford has put into Ecoboost V6 motor is "beyond" what people are supposed to use it for on the street? Indeed, policy duty is "beyond what it's supposed to do" and that testing would show that the engine is capable of "beyond what it's supposed to do."
It's "supposed to" get people from A to B with the ability to accelerate quickly and do so reliably over a long period of time (I don't know what auto mfr's target for the life of a vehicle), and in this case produce better fuel economy than a comparable V8 powered vehicle.
The testing shows that the engine will do that and much more, including having the absolute snot beat out of it with constant accelerations, decelerations, cold starts, hot starts and more.
By your own definition, that sounds "bullet proof" to me.
I understand where you're coming from, comparing it to modified vehicles, but most people use bulletproof to refer to a product (factory or not) that is highly reliable under most if not all circumstances, requiring not much more than regular maintenance.
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So, I wonder if Taurus owners will be able to order the police parts for their SHO? If I had one, I'd definitely want the better brakes...