European engines are as small as 1.0 litre and top out at about 2.0, except for the really BIG engines like a 2.8 litre. (To compare this to cubic inches, a 1.0 l engine is 61 cubic inches. The 250 OHC six is a whopping 4.1 l!) They use very, very high compression to get enough power out of them. SAAB even has an engine where the head is on a rocker and can vary the compression ratio from low 8's to high 10's. That's some cool technology. They almost always have a much longer stroke than bore diameter. For these reasons they really need high octane. Unleaded gas was just becoming available in the early 90's...