# 85 octane gas causing engine knocking and retarded CAM time?

I'm about to betray my ignorance of cars but here goes: I am a driver from Minnesota (where the lowest octane rating is 87) vacationing in Colorado and have noticed that after I put 85 octane gasoline in my tank I have been experiencing engine knocking and difficulty going up hills. An error message (code P1383 or P1381--can't remember) read from my car's computer mentioned retarded CAM timing. Would this be related? I'm a medical student so I have very little car knowledge; but from my Google searching this seems to be related. Can anyone speak to this?

• What make and model of vehicle is it? Engine size?
– Moab
Aug 9, 2016 at 21:52
• FYI you can check your owner's manual for the minimum require octane for your vehicle. Aug 10, 2016 at 14:36
• Thank you all so much! Apparently the problem was the timing belt and had nothing to do with my fuel. Aug 11, 2016 at 7:16

Two things:

• Running fuel with a lower octane rating than what your engine needs will cause "knocking" (premature detonation). Colorado has fuel with a lower octane rating because higher altitude reduces the need for octane, but this isn't fool-proof.
• Engine controllers generally deal with knocking by retarding the timing.

You fix this by either:

• returning to 87 octane, if your tank is almost out of 85 octane
• there are off-the-shelf "octane boosters," you may want to try some in your tank if you have a lot of that 85 octane left

You can mix fuels of different octanes (actually, it's not unusual for a gas station to have a blend valve to get the mid level octane fuel to eliminate the need for additional underground tanks). The new octane is a linear combination, you can calculate it as:

% fuel x * octane x + % fuel y * octane y

So if you've got a half tank of 85 and you fill up the rest with 89 you'll end up with:

0.5 * 85 + 0.5 * 89 = a full tank of 87.

If you've got 3/4 tank of 85 and you top it off with 93 you'll end up with:

0.75 * 85 + 0.25 * 93 = a full tank of 87.

So if you've still got a lot of 85 left in your tank just fill it up with something higher and it should correct the issue (presuming that's the issue, but it is a reasonable and likely explanation) once it runs through a bit. You don't really need additives, or to wait until your tank is empty.