I am aware of the "heat riser" system and chose not go that route because I tested mine out and found I was only getting 40 degrees above ambient, I wanted more heat. I also wanted to get more radiator capcity so I can run more grill block, so why not use my motor to pull air thru a heater core
The commute to work this morning confirmed that it is only very slightly better than underhood air, I don't think any real heat trasfer is happening in that extra heater core. The only thing I noticed is that the intake temps slowly increased, after an hour they were around 30-40 degrees above ambient. I'm going to have to think of a way to seal this off to use the intake to pull air thru...