Car in sig loses 1 quart of coolant and no more in about 1 hour of driving...

here are the FACTS:

Ebay aluminum radiator
New cap
New thermostat
Coolant jug was filled on bench and held water over night with no leaks
No water in oil
No oil in water
No combustion in cooling system (chemical block test)
Coolant jug is empty all the time after the system loses the quart
NO external coolant leaks of any kind
No coolant burning in exhaust
Car does not overheat
Compression test = fine
Leakdown test= fine

Just to clarify, if I fill the system to the proper amount, and check it again after an hour of run time or 3000 miles the radiator will be exactly a quart low, and with empty coolant jug. It loses a quart and only a quart, no more, and no less, and is accurate pretty much to the drop! I am completely stumped, I have no effing clue what to do next. Any ideas? Thanks in advance!

 

ARP suggests re-torquing your head bolts after 1000 miles... DO IT...

 

have you pressurized the radiator ( whole coolant system) and left if overnight? Use the one that bolts on to the neck of the radiator in place of the cap and then let it sit for 24 hours.... Check for leaks.