Last month in Part I, we examined why some Porsche values accelerate like an early 911S, while others lag behind like a 924 with a bad head gasket. Except for a few celebrity connections—such as Speedsters driven in the movies by the likes of Paul Newman and Steve McQueen—probable values can be established by answers to some basic questions.

There are two value patterns of interest to car collectors. One, the “strong value” pattern, shows a price that rises but not very fast and not very far. Another pattern shows a more rapid rise and to a far higher level. “Collectible cars” exhibit a price performance that usually guarantees their value will remain high, even in severe market downturns. So, how to tell if you have a collectible Porsche?

Is it a race car?

Start by asking if your car is a built-by-the-factory race...