This is totally obvious, but I'm always amazed a the size of aviation engines that have been installed in RVs. Here is a quick list of displacements in liters and cubic inches. I've got the two popular Subaru engine sizes followed by the Lycomings. I know this is not a "fair" comparison, since the RPMs are very different. Still, I am just amazed that the Lycoming 540 is almost 9 liters! The Hummer engine is 6.6 liters.
Code:
Engine Liters Cubic Inches
Sub 2.5 2.5 153
Sub H6 3.0 183
O-235 3.9 235
O-320 5.2 320
O-360 5.9 360
O-390 6.4 390
O-540 8.8 540