383 overheating problems

I have a freshly built 383 stroker with a holley 770 carb, alum. radiator, racing water pump, electric fan and a 160 degree thermostat.  The heads are cast iron and are flowing just fine but the engine still overheats after running about twenty mins.  Temp gets to about 220 and I shut it down so I don't hurt anything.  There is no blockage with the cooling system and the exhaust is wraped so now I'm lost.  Any ideas??  Thanks!  

what percentage of coolant are you running.. or straight water.... straight water is not a good idea...     does your radiator cap hold pressure?  

will your electric fan hold a shop rag to the front of the radiator when its running..   is the electric fan thermostaticly controlled..????

remember..   C4 corvettes dont call for the cooling fan to come on until 228F    most gm gars call for them at 215F to 223F..

swap in a 195F thermostat...   the larger temp difference will help  the problem...   make sure that you are running enough antifreeze that the boiling point is close to 265F  when checked with an antifreeze tester... 10 bucks..  make sure the button on the middle of the rubber gasket of our radiator is spring loaded ...

do you have an overflow bottle so the coolant can expand into the bottle and pull back in when the engine cools off to keep the radiator totally full????

after you change the thermostat.. and verify the concentration... se if it still over heats...   test it at idle... stopped..   and cruising it down the highway..  with the boiling point of the coolant at 265F and 15 pounds of pressure ..   and a 195F thermostat..  expect the system to stay at 215 to 218... if your electric fan can keep up...

with a 160 stat...  the stat just opens wider and wider and wider.. until the water is pumping around so fast the airflow cannot cool it..       with a 195.. the coolant will cycle.. stopping in the radiator....

and i hope that you have some kind of bypass on the system..  so the thermostat is not totally blocking the flow.. remember.. the thermostat is over a foot away from the nearest heat source...     if you don't have a bypass hose.. drill a 1/8 " hole in the thermostat flange  to allow some coolant to bypass the thermostat...   this is usually enough to keep water circulating through the system and keeping the thermostat close to the engines actual operating temp.. so it can cycle properly..  

yeah listen to waynep. there is another mod you can do. at the bottom of the block there are 2 drain holes, 1 on each side. plumb into that and run your hoses to the radiator. if your good and have the space, you can plumb out of the water pump instead of the block.

what this does is it bypasses the water from going into the block and heating up. instead goes back to the radiator to cool down even more. do not plumb it to or around the t-stat, you will get incorrect or incosistant temps.

hope this helps.  

I have a rough 50 50 mix of coolant and water and the elect fan can hold the shop rag and is on at all times (turns on when ignition is on) I'm going to try drilling the hole in a 195 thermostat and see if that helps, thank you and I'll let you know if it works.  Thanks!