There are also mechanical items that can influence an engine's octane requirement. One aspect is combustion-chamber design. Design reflects on how the A/F mixture behaves in the combustion chamber. Early '60s chamber designs often were a closed style. Today's aftermarket heads, depending on the application, often have more open configurations that promote efficient combustion help, contributing to reduced octane sensitivity.
In addition to mechanical considerations, octane requirements in modern Corvettes are influenced by the sophisticated engine control electronics. Today, an LS6 engine with a 10.5:1 compression ratio, and the former ZR-1 with 11:1 compression, will operate on 92-octane unleaded premium. "How that is able to happen is through the engine management computer," Brown says. "It simply takes more timing out of the engine. What we have found is, if you used GT Unleaded (100-octane), that would allow the computer to put more timing back into the engine and you would see an increase in performance, everything else being equal."
To sum up, if you have a new or nearly new Corvette, use the octane grade recommended in your owner's manual. If your older street-driven Corvette has an original high-compression engine, or you've rebuilt it that way, start with Sunoco's 100-octane GT Unleaded premium to make sure there is no detonation. If it's OK there, blend in a 50/50 mix of 94-octane Ultra, and see if knock occurs. When there is none, that's the lowest octane your engine is happy with. Remember, using a higher octane fuel than what your engine needs only costs you more money.
Here's a selection of compression ratios used in Corvette engines over the decades, with the ZL-1 being the ultimate. This was a thinly disguised race engine and required 100-octane fuel, minimum. Note that compression has slowly crept up over time. The reason compression can increase today, yet still use unleaded 92-octane, lies in the refined combustion-chamber configurations and sophisticated engine electronic controls. Also note that the increased compression contributes to increased power, but it is not the sole reason for the power increase.
If you don't know an engine's compression, you need to measure the elements that determine the compression ratio in order to calculate it. There are mathematical formulas to do it manually, but we have discovered an online compression-ratio calculator.
At the Ross Racing Pistons Web site (www.rosspistons.com), there is a c.r. calculator, where you need only to type in the numbers, and, with a click of the mouse, get your answer. Here's an explanation of the factors, and where to find your specifications.
Bore: In inches
Stroke: In inches
Head volume: This is the combustion-chamber volume in cc's. Head manufacturers have these figures listed in their specs. If you're working with a factory head, or one that's been milled, you will have to measure the volume.
Gasket thickness: In thousandths of an inch. Gasket manufacturers will have this specification.
Deck clearance: In inches. Typically, at top dead center, the piston top is located slightly below the block-deck surface. Your machine shop can give you the dimension.
Piston dish or dome volume: In cc's. A dish increases volume, while a dome decreases it. These specifications are available from the piston manufacturer.