Fuel Octane Test - Project Pure Poncho

Octane creep defines the increase in octane that is required as the engine builds carbon deposits. For example, a fresh or new engine in any car may only require 87-octane to support normal combustion, but as mileage accumulates along with carbon deposits, the octane needs to be ramped up or abnormal combustion will occur. There are all different theories on the rate of creep. In the author’s opinion, there are too many variables to define an actual mileage when creep will increase the octane demand. Some are the engine design, fuel used, driving style, service intervals, and operating conditions. That brings me back to proper maintenance and care. A proper tune and use of fuel, along with additives that not only limit carbon buildup but remove it, will keep creep at bay over the life of the vehicle. A driving style that minimizes engine idling and short-cycle trips goes a long way to hinder creep.

Spark knock zone is a phrase that has nothing to do with the fuel, but will impact the octane. This has more to do with skewing and variations in the ignition timing caused by sloppy components that alter the ionization event on each bore and are not linear in the amount of lead in relation to engine rpm. A poorly made or worn distributor bushing, or excessive timing chain slop or wear, are the key contributors to altering the spark knock zone. Often an engine is tuned to eliminate audible ping or detonation on only one or two cylinders. This is due to sloppy and inconsistent ignition events that scatter the timing a few degrees in each direction. For this reason, it’s best to confirm the operation of the distributor at higher rpm and not just at idle or off idle.

Another caveat is that we are only testing under full throttle and maximum power. The octane of modern gasoline is the average of the Motor Octane Number (MON) and Research Octane Number (RON) values. This average is identified as the AKI for anti-knock index. The RON is more critical for part-throttle operation under lower inlet temperature, while the MON better represents full-throttle octane tolerance and hotter charge air. The AKI was established since a vehicle can and will be operated under many different conditions and thus, needs to perform well in all of them.

The Results

We left the engine on the dyno from our baseline testing covered in the Aug. ’12 issue. HPP came back to RaceKrafters the next morning to perform the octane test with the Rockett Brand fuel. This meant that nothing was disturbed and the weather conditions were almost exactly the same or as close as earthly possible. The baseline was with the local sourced unknown brand 92-octane fuel, and the results were thoroughly covered in the last issue. Next we would test the regular grade 88-octane Rockett Brand Racing Fuel and the company’s 94 and 100 octane blends back-to-back on the same morning.

The weight (specific gravity) of each fuel was different:

Variation in the chemical composition required minor jet changes to optimize each fuel and are noted in the dyno charts.

Ignition advance was not a function of abnormal combustion in our test engine since none was present. It was altered to optimize the power and did not vary as much as you would have thought.

This is total timing in at 2,900 rpm for each fuel:

Local-sourced: 92-octane, 32 degrees

Rockett Brand: 88-octane, 32 degrees

Rockett Brand: 94-octane, 32 degrees

Rockett Brand: 100-octane, 34 degrees

As you can see, the Pontiac did not need any significant alteration to the ignition curve even when the octane varied by 12 points.

What was even more impressive was the average power and torque for the entire test run from 3,300 rpm to 5,300 rpm.

The difference in average power and torque from the 88-octane fuel to the 100-octane was only 3.03 and 3.39, respectively.