The engine of the MC72, theFiat AS6, was the union of two AS5, that were already used in the Macchi C52 and Fiat C29 in the 1929 edition of the Schneider Cup.
What's impressive of the AS-5 is not his power (1600 hp at 3500 rpm at the maximum), but it's measures, it displaced only 25 litres and weight 460 Kg (For comparison, the DB 605 displaced 35.7 litres and weight 745 kg). But this refinement is also it's limit. Think to use this engine into a WWII fighter is like to use a F1 engine to move a tank (mainly cause engines of italian fighters had to use an higly corrosive 87 octane fuel).
In fact, when Fiat decided to design an engine for the G55 (the unfortunate AS3
whose project was ready from 1939, they didn't start from the AS-5, but tried to do a completely new project.
The sturdier Isotta Fraschini engines were better for daily use.
At the 1929 edition of the Schneider Cup, the M-67 mounted an Isotta fraschini Asso 1000, that displaced 57 litres, and was capable of 1800 hp with a certain reliability and 2300 hp without it.
(in the third photo you can see both Asso 1000 and AS-6)
With the M-67, Mario de Bernardi was the first man flying faster than 500 Km/h
The Asso 1000 was the first of several good engines (of which I have already said in the "italian bombers" thread), and, ten years later, it was capable to develop 2000 hp continuously, but Regia Aeronautica requires radials, so, it was used as a naval engine.
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