Race Fuel: Leaded vs Unleaded and the Importance of Octane

There’s tons of different race fuels out there. There’s leaded and unleaded, ethanol and non-ethanol, and octanes that can reach over 116. So what does all this mean and what fuel should go in your car?


Octane is a rating assigned to different fuels and serves as a measure of it’s performance. The higher the octane, the more compression the fuel can withstand  before it ignites. Race fuel has a much higher octane than pump gas because those engines have higher compression ratios. Most new cars today can run on regular 87 octane because those engines are designed for that kind of fuel. Race cars, however, tend to have higher compression ratios so they thrive on the higher octane fuels.


Long gone are the days of leaded pump gas. The reason lead is used in race fuels is because it is a great octane booster. As mentioned before, the higher the octane, the higher the compression ratio needs to be for that engine. Most new cars today have a low compression ratio and do not need the boosted octane. However, race cars need high octane. The important thing for racers to keep in mind when deciding between leaded or unleaded is that leaded fuel and oxygen sensors do not mix! We still see some race cars with oxygen sensors that run leaded fuel but the sensors end up failing over time. We always recommend our Unleaded 104 Sunoco Fuel to customers that come in with oxygen sensors. The Sunoco 104 is great because it has a high enough octane to be worth your while and won’t blow out your oxygen sensors.

For most carbureted engines we recommend one of our leaded fuels. The octane depends on what engine you have but we offer 110, 112, and 116 leaded Sunoco fuels. Most customers choose the 110 and 112 fuels unless they have a pretty stout engine that requires a much higher compression. In that case they’d go with Sunoco 116.

Another reason lead is a good idea for older engines is because it can extend the life of the exhaust valve seats. Back in the days of leaded pump gas, engines were built without hardened valve seats because the lead in the fuel not only boosted the octane, but also acted as a lubricant. Now that nearly all pump gas is unleaded we no longer have that lubricant so today’s engines are built very differently.

So when it comes to leaded vs unleaded, the rule of thumb is unleaded if you have oxygen sensors and leaded if you don't. In the case of older engines, any leaded fuel with an octane of 110 or close to it works great.


E85 is becoming increasingly popular for it's clean burn and added power. To run E85 you'd need to put in a new fuel system but the pay off can be worth it if you're planning on hitting the track. Read this post to learn more about E85, it's benefits, and the difference between E85 and E85R.

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