87 octane in 93 octane required car

[ZachE84]

Hey all. My buddy borrowed my car last night and replaced the gas he used with 87, instead of 93. The manual warns in anything below 92 is used, heavy knocking, poor acceleration, and possible engine damage will occcur. I can testify that the previous 2 are currently happening.

Whats options do I have? try to burn all the 87 out, and refill with 93 when it is almost empty. Fill it up with 93 so it mixes causes a higher octane. Anything else?

All help is appreciated! Thanks!

[bjdm151]

What year is your car?

[ZachE84]

Its an 05' Legacy GT. Its turbocharged with 250HP...and I think that makes the compression high, hence the 93...but im no expert!

[CBFryman]

its high octane beause its turbocharged. syphen the gas out of your tank and replace it with 93 octane and then get some octane booster start your car up and let it idle for a while...this is to burn off any 87 in the fule line...then drive it around untill yo uare almost out of gas then fill up and use the octane booster agian and once that gas is all burned up just put in 93 as usual. lesson learned. dont let stupid friends barrow your car.

[SaabJohan]

Take a fuel line off and use your fuel pump to pump out the fuel that is left in the tank.

[Evil Result]

THat might be alot of gas to pump out. With how much velocity do you think the gas will be forced at? might cause a big mess/hazard.

[CBFryman]

that is why syphening it is faster and easyer...

[benchtest]

There is no need to drain the fuel. Simply reconnect your brain to your right foot and drive the gas out of it. It's a normal mild-compression engine that will run fine on 87 UNLESS you stick your foot in it. It very likely has a knock sensor anyway, so with no/light boost it should have no problem backing out any knock. Take a nice highway drive and at 1/4 tank fill it with 93. Add octane booster if you want. Perhaps your friend would like a nice cruise...since he should be buying!

[SaabJohan]

Originally Posted by Evil Result

THat might be alot of gas to pump out. With how much velocity do you think the gas will be forced at? might cause a big mess/hazard.

The fuel pump can probably deliver 100 to 200 litres per hour, so it won't go fast but there is on the other hand no risk that one damage the tank while trying to use a hose down the filler (newer cars are equipped with systems to take up hydrocarbon vapor, these can be damaged by a hose).

The manual usually list the lowest octane rating that can be used, lower than that and the ECU might no longer be able to supress engine knock.

[Igovert500]

Yeah, but that lowest octane rating is for high boost, say your stock boost goes up to 14psi, then you need that 91 or 93 octane for when you are boosting at 14psi, not when you are cruising at 2,000rpms at 0psi. So what benchtest said was right, just cruise, don't boost and use up the gas.

[curtis73]

Originally Posted by Igovert500

Yeah, but that lowest octane rating is for high boost, say your stock boost goes up to 14psi, then you need that 91 or 93 octane for when you are boosting at 14psi, not when you are cruising at 2,000rpms at 0psi. So what benchtest said was right, just cruise, don't boost and use up the gas.

I completely agree. Just drive that tank with a really light foot. The reason higher octane is required is because of higher combustion chamber pressures. That can come from cam timing, compression ratio, or (in your case) a turbo. The nice thing about your case is that you don't need to use the turbo. The turbo only supplies boost at a certain level of "foot" and if you stay under that you'll be fine. If you have a turbo boost gauge, use it. Don't go to full boost and listen for detonation. You'll find a happy medium.

The main reason (There are many) that detonation or knocking is bad for a car is that it causes extreme spikes in combustion pressures, which is bad for the bearings and pistons. Think of it like a hitting the tops of your pistons with a hammer 2000 times per minute. The spiking pressures can also shorten head gasket life.

[RandomTask]

Originally Posted by benchtest

There is no need to drain the fuel. Simply reconnect your brain to your right foot and drive the gas out of it. It's a normal mild-compression engine that will run fine on 87 UNLESS you stick your foot in it. It very likely has a knock sensor anyway, so with no/light boost it should have no problem backing out any knock. Take a nice highway drive and at 1/4 tank fill it with 93. Add octane booster if you want. Perhaps your friend would like a nice cruise...since he should be buying!