Octane and Power

[vintage42]

I asked Scion Customer Experience this question last week but have not received an answer.

The Owners Manual, Section 2, Page 114 says the engine will not run on lower than 87 octane:
OCTANE RATING
"Select Octane Rating 87... or higher.
Use of unleaded gasoline with an octane rating or research octane number lower than (87 octane) stated above will cause persistent heavy knocking. If it is severe, this will lead to engine damage."

The engine has a knock sensor that tells the engine's computer to retard the ignition the instant a knock is detected. This protects the engine against knocking which would otherwise occur on 87 octane under heavy throttle. However, as the manual states, the computer will not retard the ignition enough for the engine to accept heavy throttle on less than 87 octane without severe knocking:
DIAGNOSTICS - SFI SYST
DTC P0325 - KNOCK SENSOR 1 CIRCUIT - CIRCUIT DESCRIPTION
"Knock sensor is fitted on the cylinder block to detect the engine knocking. This sensor contains a piezoelectric element which generates a voltage when it becomes deformed, which occurs when the cylinder block vibrates due to knocking. If engine knocking occurs, the ignition timing is delayed to suppress it."

My question is:
Retarding the ignition reduces power. Does using 87 octane gas reduce the potential power of the engine during heavy throttle application, compared to using high octane gas?

[lelandstanford]

Your 1.5L engine is rated w/ 87 octane. Unlike 1GR-FE (4.0L V-6), it does not mention "For better performance.... premium fuel".

If the engine is like my 2TR-FE then use of premium fuel yields no increase in engine output.

[vintage42]

Thanks. That agrees with the answer Scion Customer Experience just emailed me:

From: Scion <scion_mail@scion.com>
Date: Wed, 06 Sep 2006 16:09:12 -0700 (PDT)
To: <...@insightbb.com>
Subject: Scion xB horsepower and torque [Incident: 060901-000071]
"… The vehicle is engineered and tested using 87 octane fuel. We recommend that fuel for optimum performance, reliability, and fuel economy on the vehicle. The vehicle does make the same power at maximum throttle position on 87 octane as it would on higher octane fuel."

[SuperBusa]

Originally Posted by vintage42

My question is:
Retarding the ignition reduces power. Does using 87 octane gas reduce the potential power of the engine during heavy throttle application, compared to using high octane gas?

Only if the lower octane and driving conditions make the engine knock. If the engine doesn't knock then the knock sensors do not detect anything and the computer doesn't retard the timing. You will usually hear a couple of knock before the computer reacts, so if you're not hearing anything chances are the timing is not being retarded.

[vintage42]

Originally Posted by SuperBusa

Only if the lower octane and driving conditions make the engine knock. If the engine doesn't knock then the knock sensors do not detect anything and the computer doesn't retard the timing. You will usually hear a couple of knock before the computer reacts, so if you're not hearing anything chances are the timing is not being retarded.

Knocking can be hard to hear. If you can hear it, it takes a second to recognize what it is. At high rpms, it sounds like pinging or grease frying, and hundreds of knocks can take place before you are aware of it. This is with old engines.

With our modern engine, a knock is sensed instantly and the ignition retarded. From the Scion xB Tech Literature:
DIAGNOSTICS - SFI SYST
DTC P0325 - KNOCK SENSOR 1 CIRCUIT - CIRCUIT DESCRIPTION
Knock sensor is fitted on the cylinder block to detect the engine knocking. This sensor contains a piezoelectric element which generates a voltage when it becomes deformed, which occurs when the cylinder block vibrates due to knocking. If engine knocking occurs, the ignition timing is delayed to suppress it.

Knock sensors sense a knock before we can:
"Most of today's engines have... knock sensors. Spark is automatically advanced until knock occurs and then is backed off slightly. The sensor detects engine knock long before your ears..."
--Trailer Boats, Oct 2006, Tech Letters, page 72.

The xB Owners Manual, Section 2, Page 114 says:
OCTANE RATING
Select Octane Rating 87... or higher.
Use of unleaded gasoline with an octane rating or research octane number lower than (87 octane) stated above will cause persistent heavy knocking. If it is severe, this will lead to engine damage.

So if we ever hear knocking from our xB engine, it means that either the octane is lower than 87 or the knock sensor has failed.

On octane lower than 87, the ECU receives the signal from the knock sensor to retard the timing, but will not retard the timing enough to prevent knocking. Evidently Toyota decided the engine will not run properly with that much retard. Too much retard makes engines run sluggish and hot and not reach high rpms.

[sak]

See this link for a previous discussion on Octane and performance.

More Power from Higher Octane Gas?