Mark,
As you probably know, compression ratio is a volume ratio and it's an important cycle parameter for any IC engine. It has a strong influence on the ammount of work done per unit air flow. In general, the higher the compression ratio, the higher the peak gas tempertaures and the higher the thermal efficiency. The min threshhold of diesel compression ratio is probably around 13-14 and the max is somewhere like 24-25. The lower end is dictated by the ability of the compression process to ignite the fuel and upper end by the material limits of the piston crown and the crown cooling. For tractors, weight is not a draw back in an engine but it is for over the road vehicles so engine weight is also a factor that influences the compression ratio. With increasingcompression ratios the loads on the pistons and sleeves, rods, crankshaft, and bearing increase with compression ratio also and that drives more weight into the engine. Thanks to the lastest EPA rules, emissions will also play a factor in compression ratios. the higher gas temperatures mean the more NOx is produce and the EPA doesn't like that.
Pre-ignition is the very method that a diesel uses to ignite the fuel. It's a good thing in diesel. It's just the opposite of a spark ignition engine. That's what the cetane number is all about. The higher the cetane number the easier it is to initiate combustion of the fuel.
Throw it all together and the 17.5+/- compression ratio is what you normally see in larger tractors(>~50 hp). In some of the smaller tractors (<~50 hp) you'll see higher compression ratios most likely because they have to get more work out of a smaller displacement and it's inherently harder to do on a smaller scale efficiently so the compression ratio needs to be higher.
Does this answer your question?