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Combustion Basics: How Does Fuel Make a Difference?

Making the Grade

Richard Fong
Oct 9, 2019

The fact that you're reading this now means there's a pretty damn good chance you pump gas into your car. And because of that, you've got a combustion engine under your car's hood. Of course, some of you might "gas up" by plugging into a charger and, if that's the case, this doesn't apply to you, but nevertheless, it doesn't hurt to learn a little more. For the majority of cars currently on the road, the combustion of air and fuel within an engine releases energy used to spin a crankshaft that gives a vehicle motility. In the case of performance builds, how much you're able to raise the output of your engine (of course) requires the right combination of hardware to go with engine management calibration, which monitors your engine vitals while managing spark timing and fuel delivery. Keep in mind, your engine needs to be designed, built, and calibrated for higher-performing fuels in order to realize any sort of gains. Simply filling the tank with race fuel will not miraculously increase the output of the engine. On that same note, using a lower quality fuel in an engine calibrated for high performance can quickly lead to engine damage.

Fueling Basics: Explaining Octane Ratings

Octane ratings are plastered all over every gas pump, and, contrary to marketing terminology, "premium" does not necessarily indicate a fuel is "better" for your car as much as whether it is "right" for your car. Fuel with an 87 octane rating burns more quickly while higher-octane fuels burn more slowly. In engines designed for standard unleaded fuel, efficiency and performance is optimized for 87 octane and could actually perform worse with higher-octane fuel since the burn rate is slower. By contrast, a higher-performing engine, which includes engines with higher compression ratios and/or forced induction, requires the slower burn rate of higher-octane fuels to defend against engine knock. Knocking or preignition occurs when the air/fuel mixture ignites before intended. This happens because higher compression ratios and cylinder pressures cause the air/fuel mixture to heat up, and, in some cases, ignite prematurely. Knocking that goes unchecked inevitably leads to engine damage. Higher-octane fuels help to mitigate preignition events while ensuring proper combustion and higher output for performance engines.

How Does Fuel Make A Difference Premium Fuel Recommended Sticker Photo 5/5   |   Combustion Basics: How Does Fuel Make a Difference?

Recommended Fuel 1 Final - In some vehicles, calibration is optimized for premium unleaded, but the engine management system can make adjustments to the ignition timing and injector duty cycle to safely run on regular unleaded.

How Does Fuel Make A Difference Premium Fuel Only Sticker Photo 5/5   |   Combustion Basics: How Does Fuel Make a Difference?

Recommended Fuel 2 Final - For this vehicle, the manufacturer calls for premium unleaded fuel exclusively, indicating the engine management system does not compensate for fuel quality.

Pricey Performance Options

Grades of performance fuel solutions available at the pump begin with typical 91 octane premium unleaded gasoline and can (in some regions) go up to 110 octane unleaded race fuel. Premium unleaded seems reasonably priced by comparison to the top-tier, at-the-pump race fuels (like 100 octane). For example, in a heavily regulated state like California, where premium 91 octane goes for around $3.89 per gallon, 100-octane race fuel costs more than $10 per gallon. Racing fuels rated in excess of 110 octane are also available; however, these off-road-only fuels rely on lead to help increase the octane and do not play well with oxygen sensors, quickly causing them to foul, malfunction, and have an abbreviated life span.

Ideal Ratios

How Does Fuel Make A Difference Innovate Motorsports Air Fuel Gauge Photo 5/5   |   Combustion Basics: How Does Fuel Make a Difference?
How Does Fuel Make A Difference AEM Air Fuel Gauge Photo 5/5   |   Combustion Basics: How Does Fuel Make a Difference?

Combustion events are most efficient when all the fuel is consumed and there is no additional air remaining. This condition is often referred to as "stoic." If too much fuel is being delivered (referred to as a rich air/fuel ratio), the excessive fuel could rinse the cylinder walls of oil (a condition known as wall wetting), which, in turn, could lead to accelerated ring and cylinder wear. Soon after, a loss of compression, blow-by, and excessive oil consumption will occur. By contrast, if this ratio is higher (known as a lean condition), then there is inadequate fuel being delivered, which results in hotter combustion events that could lead to engine-damaging knock events. In a naturally aspirated engine, a stoic air/fuel ratio is typically 14.7:1. In the case of forced-induction applications (turbocharged or supercharged), a richer ratio of approximately 11.5:1 is considered acceptable.

Knock Count - If you've got a Cobb Accessport or similar device monitoring your engine vitals, it's good to note the knock count, as it is an indication of the calibration of the ignition timing and fuel delivery.

Richer Isn't Always Better

There are numerous ways to keep engine knock at bay when calibrating an engine for optimum performance. One is to simply run a richer air/fuel ratio. The added fuel does not necessarily translate into more output, however. Rather, it serves to cool the cylinders and prevent knocking. While this strategy can typically solve the problem of preignition, it produces two negative byproducts: poorer fuel efficiency and accelerated engine wear.

When too much fuel is delivered to the combustion chamber, the fuel acts as a solvent, washing away the engine oil that lubricates the cylinder walls and piston rings. As a result, the cylinder walls and piston rings wear out prematurely, followed by reduced compression due to blow-by, and, finally, excessive oil consumption. Ultimately, rebuild intervals become much shorter, incurring frequent and expensive engine work and downtime.

Which Grade Should You Use?

The old slogan "You are what you eat" contains a reasonable amount of truth. The same goes for your performance build, as the quality of fuel delivered to your engine greatly influences how it ultimately performs. Of course, it's not feasible to fill up the tank with race fuel just for commuting, yet it's really nice to have your engine running at optimum output anytime-whether you're on the street or at the track. Fortunately, there are options available that enable you to have optimum power under your right foot without breaking the bank.

E85 Final - Flex Fuel vehicles come equipped to work with regular unleaded fuel, E85, or any mixture of the two.

Affordable Strategies for Chemical Intercooling

Fortunately, affordable alternatives to defend against knocking without using race fuels do exist. One solution is a bolt-on upgrade, the other is an alternative fuel, but both chemically intercool the engine. Water-methanol injection supplements the existing fuel delivery by chemically intercooling the air charge as it enters the intake manifold. The alternative fuel is the corn-derived ethanol 85 or E85. Both solutions defend against knock and help to increase output by permitting more aggressive ignition timing and increased boost pressure.

By Richard Fong
19 Articles

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