March 30, 2010, 7:30 am

Can Using the Wrong Octane Fuel Lead to Disaster?

By PAUL STENQUIST

Last week, Gunnar Heinrich, who runs the site Automobiles De Luxe, wrote that a 2010 Cadillac SRX Turbo he had borrowed from General Motors for a review sustained serious internal damage during a test drive and that he had to be plucked from the side of a mountain road.

According to Mr. Heinrich’s article, using the wrong fuel was partly to blame:

The V-6 in the Cadillac requires a minimum 91 octane. Lab tests had shown that the SRX was running on regular, which meant that yours had inadvertently fed the car 88 octane gas and not the factory mandated premium.

At some point during travel, between 2,000-2,500 r.p.m. — or normal highway cruising speed — the engine’s management system had adjusted the air fuel mixture to work too lean causing a retarded spark – but crucially – it allowed for a simultaneous turbo boost, which led to a catastrophic pressure build-up in the cylinder chambers.

Could a difference of only three octane cause a vehicle to break down? Asked about the incident, a Cadillac spokesman, David Caldwell, said in an interview that the low-octane fuel that Mr. Heinrich bought was a contributing factor but that it wasn’t the principal cause. So the quick and simple answer is no. Using regular fuel in a turbocharged engine or any engine designed for premium generally won’t lead to immediate and serious damage. The long answer is more complicated.

The demon that led to the demise of the 2.8-liter turbocharged V-6 in this Cadillac was detonation, Mr. Caldwell said. It’s a condition caused by the fuel igniting prematurely and in more than one location within the combustion chamber. The multiple flame fronts cause extremely high cylinder pressure that can pound engine parts to death. High-octane fuels don’t ignite as readily as lower-octane blends, so detonation is less likely to occur when they’re used.


But nearly every engine produced today is equipped with a knock sensor. On engines designed for premium fuel, this device compensates when regular is used. Mounted on the engine, the sensor detects the rattle of detonation and sends a signal to the engine computer, which changes the calibration to eliminate the cause. In most cases, that means retarding ignition timing. With the spark coming later, less heat is generated, so unburned fuel isn’t ignited prematurely and detonation doesn’t occur.

In Mr. Heinrich’s case, a failure might have rendered the computer incapable of responding to conditions. Mr. Caldwell said G.M. was still testing and could not say conclusively what caused the problem. G.M. engineers are checking to make sure the calibration can cope with even the most extreme circumstances.

Extreme circumstances were certainly part of the equation in Mr. Heinrich’s incident. According to his article, he was driving up a mountain near Ventura, Calif., with the engine at about 2,000 r.p.m. That’s a high-load situation, which in itself causes high cylinder pressure and combustion chamber heat. Coupled with the detonation-prone nature of regular fuel, those conditions put the knock sensor into overdrive. As the spark was retarded, the engine calibration might have allowed for more boost to avoid a significant loss of power – either by design or because of component failure. That diminished the effectiveness of late spark in regard to eliminating detonation. Consequently, the hammering in the combustion chambers became so violent that a connecting rod was damaged.

The takeaway is straightforward. First, you can usually get away with putting regular fuel in an engine designed for premium, but it’s best to follow the manufacturer’s recommendation.

Second, power-plant engineering is a tricky business. In high-performance engines, power versus reliability is a balancing act, and a slight hiccup in an engine calibration can tip the scales. But no manufacturer wants to be saddled with failures, and you can be sure that it doesn’t take more than one very major and public incident to send the engineers racing back to their calibration computers

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Our low-tech Ram managed to eke out a few extra dyno ponies on premium fuel, but at the track its performance was virtually identical. The Mustang's knock sensors and EEC-V computer found 2 hp more on the dyno and shaved a more impressive 0.3 second off its quarter-mile time at the track. The Accord took a tiny step backward in power (minus 2.6 percent) and performance (minus 1.5 percent) on premium fuel, a phenomenon for which none of the experts we consulted could offer an explanation except to posit that the results may fall within normal test-to-test variability. This, of course, may also be the case for the gains of similar magnitude realized by the Ram and Mustang.

The results were more dramatic with the test cars that require premium fuel. The turbocharged Saab's sophisticated Trionic engine-control system dialed the power back 9.8 percent on regular gas, and performance dropped 10.1 percent at the track. Burning regular in our BMW M3 diminished track performance by 6.6 percent, but neither the BMW nor the Saab suffered any drivability problems while burning regular unleaded fuel. Unfortunately, the M3's sophisticated electronics made it impossible to test the car on the dyno (see caption at top).

Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends, as any performance gain realized will surely be far less than the percentage hike in price. Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes. And finally, if a car calibrated for regular fuel begins to knock on anything less than premium or midgrade, owners should invest in a tuneup, emissions-control-system repair, or detergent additives to solve, rather than bandage, the root problem. Class dismissed.