subaru has an interesting engine construction, ej18, ej20 and even ej25 have the same geometry, so the cylinder heads can be easily installed between the mentioned types. So I installed the ej18 engine heads on the ej20 and replaced the original 1.6 mm thick gasket with a 0.6 mm gasket so the mathematical compression ratio is somewhere around 1:13. in the original it has 1:10