Quote:
Originally Posted by oil pan 4
My belief is that exhaust heat is better.
It takes about 20 seconds for the exhaust manifold to get hot and it's true waste heat.
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The problem with hot air from exhaust manifold is that small heat exchange surface area on the one hand and huge airflow on the other do not allow efficient heat exchange and appreciable increase in intake air temperature. I tried drawing hot air through a hole in exhaust manifold shield. It accelerated engine warm-up a little bit, but did not improve fuel economy.
Heat from radiator is as true waste heat as that from exhaust.