1976-1981 LM GTP Rules/Specs?

This is a class that I know little about (aside from several of the cars that participated), and it was a LM24 exclusive class. Beyond that, about all I know about it is that it was a predecessor to IMSA GTP and Group C in concept, and it required closed cockpit cars and had some type of fuel consumption limit, and was biased towards the Index of Thermal Efficiency trophy rather than overall wins.


Can anyone give me further info about this class of LM racing cars?