What is High Octane Fuel?

Everyone note this: Total compression is what matters, not ratio.

@anon508: Compression ratio and displacement are relative to each other. A 10:1 engine with only 120Cu in is no where the same as 10:1 ratio on a 455 cu in engine. Do the math.

@anon481: All of the engines you listed are all small displacement engines, so even a 13:1 engine with a displacement of 120 cu inches is not even close to the total compression of a 455 cu in with a ratio of 9:1.

All small displacement engines (import cars) can use 87 or even lower octane rating because of a relatively low total compression.

Camshaft profiles have an effect on compression ratio too, but that is included in the manufactures' data and will only affect the car's compression ratio if factory camshafts are changed out.

It makes me wonder how often detonation would be an issue. I know compression ratio plays an important part, but what about volumetric efficiency? It would depend on how often you're operating the throttle at or near it's fully open position. After all, most driving is done only at a partially open position. I know that if it were recommended, I would definitely use higher octane fuel.

I also want to comment on a previous posters indication on how manufacturers are quite ambiguous about new cars octane requirement.

Like the example of the Maxima, I've noticed this with other vehicles as well. Back when throttles were mechanically operated, the only thing the computer could do was retard timing, which was quite bad for the engine. With computer operated throttles though, all the computer has to do is limit the opening of the throttle, which isn't bad for the engine in any way, but just makes it act like a weaker engine. Is my observation accurate?

What I'd like to know is, if a roots-supercharged car, without a knock sensor, has been programmed for a minimum of 92 octane, will the higher altitudes of Colorado make up for the dearth of any fuel better than 91 octane?

@Anon88331: no cars are tuned to fire at TDC; it is always at least a few degrees early so that the impulse of the expanding gases is coordinated with the retreat of the piston.

Were the spark to be timed right at TDC or later, the piston would already be moving away by the time combustion can add any pressure to the chamber, resulting in a waste of energy and thus torque, power, and mileage.

If you doubt me, just look up the timing values for any engine with a mechanical distributor. Or watch Marisa Tomei on the witness stand in "My Cousin Vinny"...

Oh, it is also true that any distributorless engine with a knock sensor will benefit from higher octane fuel, at least to a point. On that note, the manual to my wife's Volvo does state that, while regular fuel is acceptable, higher octane will improve power and mileage; so the word is out in that case, even if buried within the pages of the manual.

The higher octane will not give you more mileage. It just doesn't happen. It makes the fuel more stable. This last post, give me a break. What profound information! you mean when i compress air, the temp goes up? who knew? everyone who passed grade 10 science class did.

Area, pressure and temperature are all directly related. So if the area is made smaller (compressed) the pressure and temp goes up every time. As for your theory of cars that fire before top dead center, thereby exerting the force of the engine against itself and basically throwing the simplest theory that engines work on out the window, neat theory.

I think as a former automotive tech I can attest that some improvement can be obtained in performance and MPG on certain higher compression engines even if they are not classified as such. As many have pointed out, most of us know little on what headway the manufacture has built into the timing of its engine. They may have accepted the lost performance in order to keep the 87 octane rating. For many people just the thought of having to buy higher octane fuel would turn them off of the vehicle. It would be nice if every engine maker would specify if higher octane fuel would provide better performance and/or MPG if higher octane was used. I have known engineers who state they burn higher octane not for the extra performance or MPG but because they know regular unleaded is very poor in detergent additives and has more contaminants in it. Obviously Ford, Honda and a couple others created the tier 2 gasoline recommendations for this reason. Ford tested many random gas stations where they had many warranty claims that were fuel related. They were surprised to learn many fuels did not meet even the federal requirements. So it is not surprising that people still have a interest in higher octane fuels.

tests have shown that high octane fuel can give you 6 more horse power in normal cars, depending on the car.

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