April 10, 1933: 423.82mph by an Italian Machii-Castold MC.72 seaplane
with two engines driving two contra-rotating props. The engines were
in-line, not side by side. The Italians considered the two engines a
single unit, the AS6.

Apparently there was some rule that said you could only have a single
engine?

In WW2, the British firm Napier developed the 24-cylinder Sabre engine
(effectively two in-line 12-cylinder engines!) for the Hawker Tempest.
On the bench, this engine easily developed 3,750hp. However,
Rolls-Royce persons were running the Brit war effort and the Sabre was
deliberately crippled so it only developed 3,055hp. Despite this
crippling, the Tempest/Sabre was the only piston-engined aircraft that
could chase down V1 missiles from behind without diving.

At the same time, the US Navy needed a fast fighter to chase down
Japanese kamikazis. They took the F4U Corsair, with a two-row R2800
round engine and put in an R-4360 three-row engine. The resulting
hot-rod was called the F2G. This engine, tuned for racing after WW2,
developed 4,500hp. It was twice the size of the liquid-cooled
24-cylinder Napier Sabre.

If you wanna fly fast, you need a _lot_ of horsepower! ;-)

And an in-line engine arrangement does wonders as well for drag!
Someone tell me those aren't surface radiators on the Italian
Machii-Castold MC.72 seaplane?!?!?!?

http://inline_twin.tripod.com/concept.html

Regards!