Subaru Ignition timing after CR change

2002 Subaru Impreza wrx 5 speed

Rebuilt the engine with CP pistons CR is now 8.5:1 . OEM is 8:1

Where should i retard timing to -4 degrees?

Car is a bit flat low down

It mostly depends on the fuel detonation threshold property... There is no fixed number that can be taken as a rule but needs to be found out by making some pulls and logging detonation and ignition data...

As Shota said, you need to try it and see - there CANNOT be any set number because there are a dozen, or more, variables to take into account.

If it's basically as it was, with just a piston change, a common error causing a loss of lower rpm torque is mis-timed camshafts and/or incorrectly set TDC datum.

Honestly, though, you give VERY little for anyone to comment on.

As an example: I was tuning an Evo on premium pump gas (98 octane) and couldn't get it to the power level it should have gotten because of knock. We changed the gas station brand and immediately picked up 30 whp at 1.7 bar of boost. So if the fuel you are using is strongly resisted to detonation you might be able to keep the ignition timing where it was before changing the CR. So I would suggest you making some logs and see the real picture...