before i install the eat chip

[1991 525i Novs]

should i completely drain it, like how far down will be safe, until light is on, or what, need some advice boys, thanks

Yes definitely, running on 89 with a chip could be harmful to the engine

[ryan roopnarine]

if you are in the united states, you should have already been using 91 octane fuel, the damage has already likely been done, as you have a 10:1 compression ratio and no knock sensor (it says on your gas door to use premium unleaded gasoline). if you live in the US/canada, and are in an area with 93, just mix your remaining gas with 93, and, so long as you have 1/2 or less of a tank left, the average will be 91 octane,which is fine for the eat. i'm not trying to be a dick to you, but i don't know why people with m50s think that the presence of a knock sensor makes it OK to run less than premium. i'll occasionally run a tank of 87 (because somebody attempts to fill the tank for me), and no additional knock compared to 93 octane occurs.

[1991 525i Novs]

what kind of damage is done to the engine??

if you are in the united states, you should have already been using 91 octane fuel, the damage has already likely been done, as you have a 10:1 compression ratio and no knock sensor (it says on your gas door to use premium unleaded gasoline). if you live in the US/canada, and are in an area with 93, just mix your remaining gas with 93, and, so long as you have 1/2 or less of a tank left, the average will be 91 octane,which is fine for the eat. i'm not trying to be a dick to you, but i don't know why people with m50s think that the presence of a knock sensor makes it OK to run less than premium. i'll occasionally run a tank of 87 (because somebody attempts to fill the tank for me), and no additional knock compared to 93 octane occurs.