Hi,running a 302 in a "Pop,flip front,no wings,no inners,so fairly open.....runs 13s on the strip,been running fine,temp reaches 90 slowly then at 94 ish number one fan comes on and brings temp down to 86 where it will stay until I hit traffic then the procedure starts again.I have fitted a small alloy expansion tank and fairly recent custom built 5 core rad has 15lb cap.There are NO visible leaks anywhere but the expansion tank has been going dry for a while now,I renewed head gaskets last month,just in case but it has nt solved the problem oh thermostat was. Recently fitted and I checked it. Before putting it in ,a160 degree.Then went on a 13 mile run yesterday and every time I stopped, steaming water gushed out of expansion tank overflow. got home and radiator was short of about 2 pints!Thinking I had a blocked thermostat I removed it,,did the same run today,average speed 45 with occasional bursts up to75 plus.This time no gushing steam BUT again radiator needed 2 to 2.5 pints,motor showed no signs of overheating and temp was ok.TOok rocker covers off. ,NO mayonnaise anywhere,oil dipstick fine,water in rad looks fine too,so what's causing the waterloss???I took bottom hose off yesterday and ran water through,no problem.IT seems water is being lost when system is under pressure,when I'm driving? Any ideas?