i used to put 93/4 in my e30 except when gas prices spike (like now). you wont notice a diff either way (if your running stock). standard compression on an M20B25 is 8.8:1, so it works out fine. i think the B27 has a higher compression ratio tho, i think 9 something to 1. not sure exactly. but i dont think you should have probs with 89 either, i have never experienced knock, even using 87.