Mix race gas with pump gas - find the octane of a mix, or exactly how much race fuel to add to hit a target rating for a track day or aggressive tune.
Blending a few gallons of race fuel into pump gas is the classic way to buy knock headroom for a track day or a spicy tune file without running a dedicated race-fuel setup.
Gasoline-to-gasoline blends track the volume-weighted average within about a point, which is plenty for planning. The notable exception is ethanol: its effective blending octane in gasoline is higher than its rated octane, so E85 mixes outperform the linear estimate - use the E85 Blend Calculator for those, which works in ethanol percentage instead.
Octane is measured two ways in a standardized test engine. RON (Research Octane Number) is run under mild, low-load conditions. MON (Motor Octane Number) is run under higher load and intake temperature, so it reads several points lower than RON for the same fuel; the gap between them is called sensitivity. US pumps post AKI, the anti-knock index, which is just the average of the two - that is what the (R+M)/2 on the pump sticker means. Europe posts RON alone, which reads about 4 to 5 points higher than the US AKI for an identical fuel (93 AKI is roughly 98 RON). Race fuels are labeled both ways depending on brand, so before you blend, make sure both fuels in the calculator are quoted on the same scale - mixing a RON number with an AKI number will throw your result off by several points.
Octane rating is a measure of how well a fuel resists auto-ignition under heat and pressure. Knock, or detonation, happens when the unburned end gas in the cylinder ignites on its own ahead of the spark-initiated flame front; the two pressure waves collide and the sharp spike can crack ring lands and pistons. A higher-octane fuel simply waits for the spark, which is what lets an engine run more compression, more boost, and more spark advance safely. That is the key point about whether premium "adds power": it only helps if the engine was actually limited by knock. A naturally aspirated engine designed and timed for 87 octane is already optimized, and pouring 93 into it changes nothing measurable. An engine that was pulling timing to stay out of knock - common on turbocharged or high-compression builds - can recover or add timing on higher octane and make genuine power, especially once a tune is calibrated for that fuel. Forced induction and high static compression both raise cylinder pressure and temperature, which is exactly what pushes the end gas toward auto-ignition, so the more boost or compression you run, the more octane you need to hold the same knock margin.
Suppose you have 10 gallons of 93 AKI pump gas in the tank and want to reach 100 AKI for a track day using 110-octane unleaded race fuel. Use the target formula:
Adding 7 gallons of 110 to the existing 10 gallons of 93 gives a 17-gallon tank. Checking it with the mix formula: (10 × 93 + 7 × 110) ÷ 17 = (930 + 770) ÷ 17 ≈ 100.0 AKI. Note how much race fuel a seemingly small 7-point jump requires - over 40% of the tank - because the race fuel is only 10 points above the target. The closer your target creeps to the race-gas octane, the more race fuel each additional point costs, which is why chasing the last point or two gets expensive fast.
Many high-octane race fuels (110+) are leaded - even small amounts permanently damage O2 sensors and catalytic converters. For street-driven cars with cats, stick to unleaded race fuels (e.g. 100-104 unleaded) or ethanol blending.
Volume-weighted average: (volume1 × octane1 + volume2 × octane2) ÷ total volume. Mixing 5 gallons of 110 race gas with 10 gallons of 93 pump gas yields roughly (5×110 + 10×93) ÷ 15 ≈ 98.7 octane. Linear blending is a good approximation for gasoline-to-gasoline mixes.
For gasoline mixed with gasoline, linear-by-volume is accurate within about a point. Ethanol blends behave non-linearly - ethanol's blending octane in gasoline is effectively higher than its rated octane - so for E85 mixing use a dedicated ethanol blend calculator instead.
RON (Research Octane Number) and MON (Motor Octane Number) are two lab tests run under different conditions; MON uses higher engine load and reads several points lower than RON for the same fuel. AKI, the (R+M)/2 number on US pumps, is simply the average of the two. Europe posts RON, which reads about 4 to 5 points higher than the AKI for the same fuel, so 93 AKI is roughly 98 RON.
Octane rating measures a fuel's resistance to auto-ignition under heat and pressure. A higher-octane fuel waits for the spark instead of igniting on its own ahead of the flame front, which is what knock is. Resisting that premature ignition lets an engine run more compression, more boost, and more spark advance without detonation damaging the pistons.
Only if the engine can use it. A naturally aspirated engine designed for 87 octane will not make more power on 93 because its timing and compression are already optimized for regular. An engine that was pulling timing on knock - many turbo or high-compression engines - can recover or add timing on higher octane and make real power, especially with a tune calibrated for that fuel.
Yes. Both boost and high static compression raise cylinder pressure and temperature, which makes the end gas more likely to auto-ignite. The more boost or compression you run, the more octane you need to keep the same knock margin, which is why heavily boosted builds turn to race gas or ethanol blends.
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