If you have changed your ecu for any reason, or you bought the car used and dont know if the previous owner(s) have changed it, then this may not help you.
Here is how you are able to check if your have 8.6:1 compression ratio or 9.1:1. On the ecu, there will be a number usually like (A11-*** ***) once you have that number, find it on this list:
----KA24E S13 240sx------
A11-A42 G41 89 240SX A F S13-KA24E 9.1:1 23710-40F60
A11-B03 G49 89 240SX A F S13-KA24E 8.6:1 23710-40F61
C11-B08 G54 89 240SX A F S13-KA24E 9.1:1 23710-40F62
A11-B16 G03 89 240SX A F S13-KA24E 8.6:1 23710-40F64
A11-B16 G12 89 240SX A F S13-KA24E 8.6:1 23710-40F65
A11-B16 G17 89 240SX A F S13-KA24E 8.6:1 23710-40F66
A11-B99 G54 89 240SX A F S13-KA24E 8.6:1 23710-40F68
A11-A44 G43 89 240SX A C S13-KA24E 9.1:1 23710-42F60
A11-B05 G51 89 240SX A C S13-KA24E 8.6:1 23710-42F61
C11-B10 G56 89 240SX A C S13-KA24E 9.1:1 23710-42F62
C11-B13 G59 89 240SX A C S13-KA24E 8.6:1 23710-42F63
A11-B18 G05 89 240SX A C S13-KA24E 8.6:1 23710-42F64
A11-B18 G14 89 240SX A C S13-KA24E 8.6:1 23710-42F65
A11-B18 G19 89 240SX A C S13-KA24E 8.6:1 23710-42F66
A11-C01 G56 89 240SX A C S13-KA24E 8.6:1 23710-42F68
A11-A41 G40 89 240SX M F S13-KA24E 9.1:1 23710-40F00
A11-B02 G48 89 240SX M F S13-KA24E 8.6:1 23710-40F01
C11-B07 G53 89 240SX M F S13-KA24E 9.1:1 23710-40F02
A11-B15 G02 89 240SX M F S13-KA24E 8.6:1 23710-40F04
A11-B15 G11 89 240SX M F S13-KA24E 8.6:1 23710-40F05
A11-B15 G16 89 240SX M F S13-KA24E 8.6:1 23710-40F06
A11-B98 G53 89 240SX M F S13-KA24E 8.6:1 23710-40F08
A11-A43 G42 89 240SX M C S13-KA24E 9.1:1 23710-42F00
A11-B04 G50 89 240SX M C S13-KA24E 8.6:1 23710-42F01
C11-B09 G55 89 240SX M C S13-KA24E 9.1:1 23710-42F02
C11-B12 G58 89 240SX M C S13-KA24E 8.6:1 23710-42F03
A11-B17 G04 89 240SX M C S13-KA24E 8.6:1 23710-42F04
A11-B17 G13 89 240SX M C S13-KA24E 8.6:1 23710-42F05
A11-B17 G18 89 240SX M C S13-KA24E 8.6:1 23710-42F06
A11-C00 G55 89 240SX M C S13-KA24E 8.6:1 23710-42F08
If it's not on this list, your ecu is from 90+ 240sx sohc ka or stanza ka...
