Had a friend who lived in Mass not far from the northrn border and worked in southern New Hampshire. One of his son's had a science project he ran with his dad as the guinea pig. George drove a 1965 Volvo 122S 2-door to work daily, he is a Volvo dealer Parts Manager.
Well, they ran controlled tests one spring after the weather turned nice. They filled the tank up 2 or 3 times with each grade of gasoline, 87, 89, and 91 and they used the same station for all fillups? The car was equipped with a 2.0L gas motor with dual SU HS6 carbs in good condition and properly tuned and jetted. The best gas mileage was obtained by 87 octane. The driving habits were attempted to be consistent as well as the type of trips and usage from tank to tank. The best performance was obtained over a timed measured course for performance as being from the 91 octane.
No adjustments or component changes were made to the carburetors or engine during the test period and air pressures were maintained at a steady setting. Car was tuned up with fresh spark plugs, oil/oil filter, and air filters at the beginning of the test.
So, there findings were valid for normally aspirated carbureted cars
Could be, I suppose. But, I maintain skepticism. It is still a story with no data to support or scrutinize for test replication. A test case of one may be encouraging, but it must be corroborated to be excepted as any sort of proof. Further, it doesn't seem like it was a double blind type test. Like, was the operator aware of the octane used during each test, or put another way, was the human factor eliminated from the physical data collection. I note the absence of temperature, humidity, or barometric pressure, accountability, all of which can change the engine's power output on any given day, and certainly over the span of a "spring".
"Best gas mileage" is not a number or even a range of numbers best to worst. Similarly, why are there no numbers for best performance ,or what the range of performance was?
For example was the best performance .00001% better than worst performance? Was best gas mileage .00001% better than worst gas mileage? Were statistical anomalies account for?
Further, station gas can be variable sample to sample even from the same station, and I see no "controls" for gas selection quality or adherence to standards. Did the station not refill its tanks during the entire spring? In some areas of the country, winter gas is blended differently than summer gas. Did the "test" span this transition? Was the blend even monitored during the span of testing?
Lastly, there are no explanations or rationale presented for why the results obtained don't fit the model for combustion efficiency or power. The conclusion that octane increased power has no supportive explanation other than "faith" that it did. Unfortunately, faith is not a physical measurable factor.
Just how did the engine make more power with the same energy content in each fuel used?
The ignition speed varies with the octane rating. This is why engines that are prone to ping are aided by retarding the spark. So, ignition timing is optimally selected for the fuel being used. A test that varies octane but keeps the ignition timing constant is not a valid test because it is unknown which fuel the test engine's timing was optimized to use.
Please note that adding ethanol to gasoline increases it octane rating, but lowers it's total energy content.
From Wiki:
http://en.wikipedia.org/wiki/Octane_ratingEffects of octane ratingHigher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.
The compression ratio is directly related to power and to thermodynamic efficiency of an internal combustion engine (see Otto-cycle). Engines with higher compression ratios develop more area under the Otto-Cycle curve, thus they extract more energy from a given quantity of fuel.
During the compression stroke of an internal combustion engine, as the air / fuels mix is compressed its temperature rises (PV=nRT).
A fuel with a higher octane rating is less prone to auto-ignition and can withstand a greater rise in temperature during the compression stroke of an internal combustion engine without auto-igniting, thus allowing more power to be extracted from the Otto-Cycle.
If during the compression stroke the air / fuel mix reaches a temperature greater than the auto-ignition temperature of the fuel, the fuel self or auto-ignites. When auto-ignition occurs (before the piston reaches the top of its travel) the up-rising piston is then attempting to squeeze the rapidly expanding (exploding) fuel charge. This will usually destroy an engine quickly if allowed to continue.
There are two types of induction systems on internal combustion engines. Normally aspirated engine (air is sucked in using the engines pistons. Or, forced induction engines (See supercharger|supercharged or turbocharger|turbocharged engines).
In the case of the normally aspirated engine, at the start of the compression stroke the cylinder air / fuel volume is very low, this translates into a low starting pressure. As the piston travels upward, a compression ratio of 10:1 in a normally aspirated engine will most likely not start auto-ignition. But 11:1 may. In a forced induction engine where at the start of the compression stroke the cylinder pressure is already raised (having a greater volume of air / fuel) Exp. 2 Bar (14.7Psi), the starting pressure or air / fuel volume would be 2 times that of the normally aspirated engine. This would translate into an effective compression ratio of 20:1 vs. 10:1 for the normally aspirated. This is why many forced induction engines have compression ratios in the 8:1 range.
Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane.
A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.
However, burning fuel with a lower octane rating than that for which the engine is designed often results in a reduction of power output and efficiency. Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit, which in turn retards the ignition timing when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels are less prone to detonation. Some modern high performance engines are actually optimized for higher than pump premium (93 AKI in the US). The 2001 - 2007 BMW M3 with the S54 engine is one such car. Car and Driver magazine tested a car using a dynamometer, and found that the power output increased as the AKI was increased up to approximately 96 AKI.
Most fuel filling stations have two storage tanks (even those offering 3 or 4 octane levels): those motorists who purchase intermediate grade fuels are given a mixture of higher and lower octane fuels. "Premium" grade is fuel of higher octane, and the minimum grade sold is fuel of lower octane. Purchasing 91 octane fuel (where offered) simply means that more fuel of higher octane is blended with commensurately less fuel of lower octane, than when purchasing a lower grade. The detergents and other additives in the fuel are often, but not always, identical.
The octane rating was developed by chemist Russell Marker at the Ethyl Corporation in 1926. The selection of n-heptane as the zero point of the scale was due to its availability in high purity. Other isomers of heptane produced from crude oil have greatly different ratings.
