Since gas keeps going up in price, but the price offset between 87 and 89 octane fuel has been staying fairly constant, I decided to do some research. For the $0.10 difference between the two grades, if I could improve my mileage by 2.5% then I'd be ahead of the game. At 32 to 34 MPG on the highway that would be a 1 MPG improvement. I started a steady diet of 89 octane for two weeks and then took a long HWY trip. For three successive fills, I got back 38.1, 34.1, and 36.3. I know that I had a tail wind on the first tank, but the other figures indicate that I likely exceeded 2.5%. This was on a Honda '04 Accord, 4 cylinder, 5 speed. This would lead me to believe that this engine can automatically add spark advance before falling over the mean best torque point by increasing octane. Do the readers know how the algorithm on the K24 engine works?

Kevin

Here's my take on high octane and Honda J30A4 V6:

J30A4 10:1 compression is high considering the minimum octane of 86 recommended by Honda.

The Powertrain Control Module contains one or more timing maps. These maps Honda engineered Y axis RPM and X axis load. Load is calculated from the intake air sensor on the V6. Basically the sensor calculates temp drop of a heated wire from airflow and converts to engine load. The intersection X and Y is the timing advance number. Timing can be adjusted to engine knock onset then stopped by the knock sensor.

The combination of timing map, gasoline grade and knock sensor enables a potentially high compression engine to operate on Regular gasoline.

I haven't been able to verify if the J30A4 has a high octane map but my guess is that it does given 10:1.

If this is true, the engine should operate efficiently using higher octane gasoline because the PCM "learns" in closed loop. Don't know if this is done in 20 miles or one tank.

Conventional advice is to use lowest octane that the engine will run on without knocking. Regular unleaded for J30.

Worth a test Mid Grade as cost is 4 percent over Regular.

MPG should be 4 percent higher. For me, that's 1.2 MPG additional, freeway driving, little less than 1.0 around town.

This is my analysis as a longtime DIY mech.

When I posted the comments above, I neglected to provide additional information about the importance of pressure in determining an engine's need for higher octane gasoline:

The presence of engine deposits not only increases the internal engine temperature, but also the engine's compression ratio. These deposits take space that would otherwise be occupied by an air-fuel mixture. This, in effect, increases this engine's compression ratio and it's need for higher octane gasoline.

Ambient air pressure is very important in determining the octane needed to prevent knocking. Normally aspirated gasoline engines (those without a turbo-charger) experience a drop of about one octane for each 1,000 feet increase in elevation above sea level. (Tests indicate that this elevation effect varies between 0.8 and 1.2 octane and averages about 1.0 octane). Engine makers normally design their engines to operate at or near sea level, and some (including Nissan) acknowledge that at higher elevations lower octane gasoline is acceptable. Almost all regular gasoline in the Rocky Mountain West (generally sold at 4,000 feet elevation or above) is produced at 85 octane (rather than 87 octane in the remainder of the USA). I do most of my driving around home at about 6,000 feet elevation or above. But at this elevation, 85 octane gasoline performs about like a 91 octane gasoline at sea level. So, unless your vehicle has a turbo charger, you are OK burning lower octane gasoline in the Mountain West.

SilverMax

Your discussion on gasoline octane is correct as far as it goes. But some of your information is incomplete, particularly the paragraph about what determines an engine's need for higher octane gasoline. You state that "The octane level required by an engine is determined by its compression ratio." -- which is partly correct. In fact an engine's need for octane is determined by these three basic factors:

- Pressure (the pressure inside the engine, which is determined by the ambient outside air pressure, the engine's compression ratio, and -- if installed -- the increased pressure produced by a turbo-charger). Higher pressures require higher octanes.

- Temperature (the temperature inside the engine, which is determined by the ambient outside air temperature, the temperature of the inside of the engine and -- potentially -- the presence of engine deposits which tend to get hotter than the inside of the engine). Higher temperatures require higher octanes.

- Engine Spark Advance (the advance in the firing of the spark in a gasoline engine before top-dead-center). More spark advance requires higher octanes.

All 3 of these factors work together to determine an engine's need for higher octane gasolines. For example, the new Honda Accord engine has a 10.0 to 1 compression ratio and Honda recommends regular gasoline. The new Nissan Maxima engine has a 10.3 to 1 compression ratio (not that much higher than the Honda) and Nissan recommends premium gasoline -- but acknowledges that regular gasoline will work, but will potentially reduce performance -- which may be caused by reducing the spark advance in the Nissan engine if the knock sensor detects knocking with regular gasoline.

SilverMax