This is not a trick or secret heat generator. It's simple thermodynamics. In A/C max, you are using air from the cabin recirculated. It is not as cold as air coming in from outside the car. It can then pass through the core (that has slowly heated back up) and you get what feels like warmer air for a few minutes. If you use the outside air, it will overpower the heat in the core and you'll barely get a minute of warmer air. Like adding cold and hot water and getting luke warm vs adding warm and hot water and getting warmer warm water.
In this scenario your core is restricted and the hot coolant coming in is not providing enough heat to "beat out" the cold air coming through the outside vent. You simply have less of a differential in temperatures with the A/C max setting.
If you are experiencing this ... where you get some heat, but not a lot and it fades quickly, your core, your hoses, your water pump, or your restrictor is clogged. Most often ... it's the core ... it has the tightest passages.
Again I thank you very much for all your thoughts and ideas. We can eliminate several things right off the top;
1. The engine is a brand new state of the art professionally built engine. Besides had the same problem when the last engine was in.
2. New NOS water pump.
3. New NOS heater core.
Note: Before I removed the old heater core I did flush it for several hours hoping this was the problem. I did get a lot of crap out it, both ways. I would have thought this would have cleaned out any junk in the restrictor on the 9C1 hoses? Problem is I didn't inspect the hoses for flow!
Tomorrow morning I driving the Impala about 3 hours NW so to have the new front suspension professionally installed (new tubular control arms, coil over shock, Flaming River steering shaft and HE Moog rebuild). I will return on Wednesday evening, once back I am going to once again drain the system, check all the hoses for flow and replace the thermostat. Might as well eliminate those two as it will get me closer to the actual problem.
One question; When my car was tuned I believe it was set for a 160 thermostat, I am running a 180, do you see any problem with this?
Closing note: This past weekend I buffed the Impala out then polished her to a beautiful shine! Being on the road with her Tuesday and Wednesday they are now calling for rain both days! Punks!
Thank you my friends for all the advise.