CR, if I even understand it a bit, if the amount that the air/fuel ratio is compressed.
"Compression
The piston moves up and compresses the trapped air fuel mixture that was brought in by the intake stroke. The amount that the mixture is compressed is determined by the compression ratio of the engine. The compression ratio on the average engine is in the range of 8:1 to 10:1.
This means that when the piston reaches the top of the cylinder, the air-fuel mixture is squeezed to about one tenth of its original volume."
<http://www.familycar.com/engine.htm>
Longer piston equals more compression. More compression equals more power amongst other things. Madza has been upping the power and decreasing emmisions by upping the compression. I also think they installed slightly weaker pistons on the 99+ Miatas (excluding the MS probaly)
Someone correct me if I am wrong.
Compression also equals an engine more realdily responsive. I've heard the BPT is such a pig with really low compression, but you can run 15 - 20 PSI reasonably safe.
Chris