10:1 vs 8.6:1 compression pistons for Mazda Miata turbo application

Hello all,

I have a Mazda MX5 Miata (NB2, 1.8 VVT) that is currently turbocharged via a Borg Warner EFR6258 and Kraken manifold, full 3" exhaust and ME221 ECU.

I acquired a spare 1.8 VVT engine which I have torn down pending imminent rebuild. The plan is to run forged rods but I'm in two minds whether to re-use the stock 10:1 pistons or replace them with forged supertech 8.6:1 pistons and a +1mm overbore.

Research suggests that my 300whp power goal for the new engine is safe and reliable on the stock 10:1 pistons, in the absence of detonation of course.

Seems the main advantage of the forged pistons are;

1) 8.6:1 compression is more ideal for a pump gas car in that it won't be as knock limited and able to achieve more ignition advance (torque!) everywhere.

2) +1mm overbore gives a slight displacement bump from 1.8 to 1.9 litres and therefore slightly better turbo spool.

3) In the unfortunate event of detonation, forged slugs are more likely to resist damage.

Does anybody here know how much extra ignition advance is likely to be achieved moving from 10:1 compression down to 8.6:1?

Thank you

The ignition timing change will mostly depend on Octane rating of particular pump gas... If you'll go with smaller compression ratio it will allow you to use lower octane ratio pump gas and the opposite - higher compression ratio will require high octane rating.