You wont get the teams to do the mileage on their race ...

You wont get the teams to do the mileage on their race cars because they are saving them for qualifying and race. Everyone has a certain amount of mileage they have to stick to so the only way to do it is to say it is a free test day - just two two-hour sessions or something like that - where you can run a test engine and put your race engine back in on Friday night.