There are two major elements that bring the Corvette mystique to life. First is power. Power animates the drivetrain and gets your Corvette moving so you can enjoy the thrust and handling enhancements you've made.
The other important engine-related element is fuel. No fuel means no go. Engine power is influenced by engine parameters such as induction-system design, combustion-chamber configuration, and exhaust. As a result of the Corvette engine's high-performance demands, not just any gasoline will do. The fuel you use should be tailored to match the power demands, so the engine can deliver maximum power throughout the rpm range. The most obvious topic of discussion concerning fuel demands is octane rating.
To learn more about octane basics, we interviewed Art Brown, technical operations manager at Sunoco Racing Fuels.
Simply stated, octane is a measure of a gasoline's antiknock quality or a fuel's resistance to pre-ignition or detonation. Pre-ignition occurs when the Air/Fuel (A/F) mixture is ignited before the spark occurs. Detonation is, by definition, A/F ignition at some location in the combustion chamber away from the plug. The higher the octane number, the higher a fuel's resistance to unintended combustion. Contrary to urban legend, octane isn't a measure of how fast fuel burns.
An engine's octane requirement is directly related to the engine's compression ratio, which is the ratio between the cylinder volume when the piston is at Bottom Dead Center (BDC) and when it's at Top Dead Center (TDC). If the volume at BDC is 1,000 cc, and at TDC it's 100 cc, the ratio would be 10:1. Mechanical items that can influence compression ratio are piston-to-deck clearance, piston dish, dome shape, head-gasket thickness, and combustion-chamber volume.
Compression is important for increasing power in naturally aspirated engines. When A/F is squeezed more tightly together, increased cylinder pressure results after ignition to create additional push on the piston during the power stroke. As the piston rises, the A/F mixture is compressed, which adds heat. If there is a hot spot in the combustion chamber, the mixture could be ignited prematurely before the spark plug fires. So, higher octane is the way pre-ignition or detonation can be controlled during the compression cycle.
An important concept to understand is that combustion is not an explosion. Ideally, it is a flame wave, initiated by the spark, which burns across the combustion chamber. This smooth burning generates the rapid rise in cylinder pressure during the power stroke.
Knock is the negative result when combustion occurs somewhere else in the cylinder, in addition to the plug. Like the spark-plug-initiated flame wave, the second wave expands so a collision between the wave fronts occurs. This collision produces a radical spike in cylinder pressure. Instead of a firm, even push down on the piston, the pressure spike hammers it down. This is strong enough that it can ultimately burn a piston, crack a piston ring or ring land, or a spark-plug electrode. These excessive pressure spikes can damage even engine bearings. When you hear a metallic rattle while driving, that is the sound of detonation and, in most cases, the cure is fuel with the correct octane.
No doubt, you've seen the "RON plus MON divided by 2" formula posted on fuel pumps. As Brown explains, there is the Research Octane Number (RON) and Motor Octane Number (MON). Together, this is the Anti-Knock Index (AKI).
RON utilizes a single-cylinder, laboratory test engine running at 600 rpm with an 83-degree F. intake temperature. MON, on the other hand, utilizes a 900-rpm limit and a 300-degree intake temperature. Test data from these engines allows octane to be determined. Because the AKI is an average, you can have very different fuels produce the same AKI. A fuel with a 120 RON octane and a fuel with a MON 102 octane will deliver the same octane number as one with a 113 RON octane and a MON of 109. By blending different fuels, Sunoco can create a cost-effective gasoline with the desired octane.
Knock control was originally accomplished by adding tetraethyl lead. According to Brown, lead was the standard octane booster. He explains, "It does not bind with gasoline, but rather mixes with it." Using lead was a cheap and easy way to boost octane since all you had to do was pour and mix it in. Until the phase-out of lead starting in 1974 (due to toxic lead emissions), it was easy to get enough octane for your Corvette's engine.
Over the past 25 years, compression ratios have climbed back up, thanks to a combination of refined combustion-chamber design characteristics and engine control electronics. Today you can increase compression and power, but remain detonation free with unleaded 92-octane fuel.
Today, Sunoco blends selected fuel stocks in order to achieve a desired octane rating, in addition to meeting EPA-mandated requirements for oxygenates and emissions-reducing formulation. "We formulate fuels with gasoline components as well as oxygenated components and blend them to 100 octane," Brown explains. "You're limited in the amount of octane you can acquire at a reasonable price through the use of hydrocarbon components and oxygenates. This limit is about 104 octane."
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.