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I've contemplated multiple times brazing soft copper tubing to my exhaust manifolds and plumb coolant to them to help speed up warm up times. I think on short commutes you could definitely see up to a 20% increase in fuel economy from faster warm up times. However, then you have to handle the constant input of that additional heat into the cooling system. That means a larger grill opening, larger radiator, and thus larger aero penalty. Overall, I definitely think its a win, but it would be so nice if we were able to find a way to turn it off like Toyota does with its exhaust heat exchanger on the Prius.
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