If you live west of the Rockies, you already know the pump tops out at 91 octane. (If you want the long, smoggy history of why, it's here: Why 91 Octane Is A Thing.) On a boosted EJ or FA, octane is horsepower insurance. Run out of knock margin and the tune has to pull timing and boost, or the engine pays for it.
So sooner or later every Subaru owner chasing power asks: E85 or race gas? Both buy knock resistance. They get there in very different ways, they cost very different amounts, and they bite you in very different places. Let's lay it out.
First, What Octane Is (and Isn't)
Octane is a measure of a fuel's resistance to autoignition, meaning its ability to sit under heat and pressure without lighting off on its own before the flame front gets there. The pump number in the US is (R+M)/2, the average of the Research (RON) and Motor (MON) octane methods. Higher octane doesn't mean more energy. It means more margin before knock, and that margin is what lets a calibrator run more boost and more timing.
Keep that distinction in your head, because the two fuels in this fight are about as different in energy content as two liquids you'd put in a car can be.
What E85 Really Is
"E85" is a marketing name, not a recipe. The ASTM D5798 spec covers flex fuel with 51% to 83% ethanol by volume. The Department of Energy's Alternative Fuels Data Center (AFDC) explains that E85 is adjusted seasonally and geographically, and that winter blends often carry less ethanol to get enough vapor pressure for cold starts. Stations have to "blend down" leftover summer fuel with gasoline when the season changes.
Translation for tuners: the stuff coming out of the "E85" nozzle in January may not be what came out of it in July, or at the station across town.
Here are the fuel property numbers from the U.S. Department of Energy's Handbook for Handling, Storing, and Dispensing E85 and Other Ethanol-Gasoline Blends (February 2016):
| Property | Ethanol | Gasoline | E85 |
|---|---|---|---|
| Octane (R+M)/2 | 113 | 86–94 | 95–97 |
| Lower heating value (Btu/gal) | 76,300 | 116,900 | 83,600–95,450 |
| Stoichiometric air-fuel ratio (by weight) | 9 | 14.7 | 9.6–9.8* |
| Specific gravity | 0.794 | 0.72–0.78 | 0.78 |
*E85 stoich is our figure, 9.6–9.8:1. The DOE handbook lists 10.
Two things should jump out:
- E85's pump octane rating is only 95–97, according to the U.S. Department of Energy. That's good, but on paper it isn't a dramatic jump over premium.
- It has noticeably less energy per gallon and needs a lot more fuel per pound of air. At stoichiometric, gasoline wants 14.7 parts air to 1 part fuel by weight, and E85 wants 9.6–9.8:1. That works out to roughly 50–53% more fuel mass for the same air (simple arithmetic: 14.7 ÷ 9.8 = 1.50 and 14.7 ÷ 9.6 = 1.53). Your injectors, pump, lines and fuel system all have to keep up.
Why E85 Punches Above Its Octane Number
If E85 is "only" 95–97 octane, why do turbo cars wake up on it? Two reasons: chemistry and cooling.
MIT researchers Kasseris and Heywood (SAE 2012-01-1284) separated the two effects on a turbocharged engine with both port and direct injection:
- Chemical benefit: ethanol blends resist autoignition better chemically. That benefit exists with port injection too, but they found diminishing returns above about E40.
- Charge cooling benefit: ethanol's high heat of vaporization cools the charge as it evaporates. In direct injection, that thermal benefit rose from about 5 octane numbers for gasoline to about 18 for E85.
The cooling effect is biggest when the fuel evaporates inside the cylinder (DI). Port injected engines like the EJ257 still get the chemical benefit and some cooling in the port, but you shouldn't assume DI-sized numbers on a port engine.
The DSPORT team's EJ257 compression guidance reflects this: on pump gas, keep compression between the original ratio and about 9.0:1. With E85 or high octane race fuel, about 9.0:1 to 10:1 is likely best, because those fuels have "a much lower propensity to knock."
What Race Fuel Really Is
"Race fuel" covers a huge range of products. Some are unleaded, some leaded, some oxygenated, some not. You have to read the spec sheet for the exact product. Two real examples from VP Racing's published typical values:
VP C16
- MON 116, RON >120, (R+M)/2 >118
- Leaded, 0% oxygen
- Stoichiometric A/F 14.70
- Specific gravity 0.7350
VP Q16
- MON 116, RON >120, (R+M)/2 >118
- Leaded, 9.41% oxygen
- Stoichiometric A/F 13.33
- VP says it requires 4–6% more fuel flow because it's highly oxygenated
Notice that C16 has the same stoich as gasoline. Swap it in on a port-injected EJ and your fueling stays close to a gasoline tune, with a massive jump in knock resistance. Q16 is oxygenated and needs more fuel, so it is not a drop-in on a gasoline map. Every race fuel needs a tune made for that fuel.
The leaded problem
Leaded race fuel isn't legal for street cars. EPA's ban on leaded gasoline for on-road vehicles took effect January 1, 1996, with exceptions for off-road uses including racing cars. Lead also damages catalytic converters, which is why unleaded fuel arrived with catalyst-equipped cars in 1975. If the car is street driven and has a cat, leaded fuel is off the table. Period.
Think in Lambda, Not AFR
One thing that trips people up when they switch fuels: air-fuel ratio numbers don't carry over. "11.5:1" means something completely different on E85 than on gasoline, because stoich is 9.6–9.8:1 on E85 and 14.7:1 on gas (and 13.33:1 on an oxygenated race fuel like Q16).
That's why calibrators think in lambda: the actual air-fuel ratio divided by that fuel's stoichiometric ratio. Lambda 1.0 is stoich on any fuel. Richer than stoich is below 1.0, leaner is above. A wideband reading lambda is telling you the same thing whether the tank is full of 91, E85 or C16. A wideband displaying "gasoline AFR" while you're running E85 is just multiplying lambda by 14.7 and lying to you with a straight face.
This matters even more on flex fuel. The ECU has to blend fueling, timing and cold-start tables as ethanol content changes, and the target that makes sense at E30 doesn't make sense at E80. Get the fuel's stoich right in the calibration, read lambda, and the numbers start making sense again.
The Altitude Myth
Out here in Colorado, Utah, Montana, New Mexico and Wyoming, somebody will eventually tell you that you don't need octane because you're at altitude. On a naturally aspirated economy car, thinner air does reduce cylinder pressure, and the engine doesn't make as much power. On a turbo car, the turbo simply works harder to make the manifold pressure it's asked for, and the cylinder doesn't know what elevation it's at.
As I put it in the 91 octane article, in a high performance engine, a shortage of oxygen molecules in the atmosphere won't hold back your horsepower, but a lack of knock resistance will. Altitude doesn't get you out of buying octane. It just makes the turbo spin faster on the way to the same knock limit.
Colorado density altitude is often above 8,500 ft, even at 4,500 ft of actual elevation. A smaller turbine housing (hotside) can help here, but it'll hinder the same car at sea level.
Head to Head
Knock resistance
- Race fuel: very high and very consistent from drum to drum, because it's made to a spec.
- E85: very strong in practice, thanks to chemistry and cooling, but only as consistent as the blend in your tank.
Fuel system demand
- Race fuel: depends on the product. A non-oxygenated fuel like C16 is close to gasoline. Oxygenated fuels need more flow.
- E85: a lot more fuel. Plan on injectors, a pump and possibly lines. On a DI-only FA, the factory high-pressure system becomes a ceiling fast. (See Direct Injection vs. Port Injection.)
Consistency
- Race fuel: you know what's in the can.
- E85: Ethanol content varies by season and station. Run an ethanol content sensor with a flex-capable tune, or don't run ethanol. It's not worth burning a piston because you guessed.
Availability & cost
- Race fuel: you buy it by the drum or pail. VP's online store adds an EPA-driven hazmat surcharge to the shipping on every 6-gallon case. It's expensive enough that most street cars won't live on it.
- E85: pump price where it's available. Around here, availability can be spotty, and that's a real argument for flex fuel instead of a dedicated E85 tune.
Around here, when you can find E85 at all, it's often E70.
Materials & cold weather
- Race fuel: ordinary gasoline-compatible components.
- E85: the DOE handbook notes ethanol blends have higher electrical conductivity, which can affect material compatibility through corrosion of certain metal junctions. Use E85-rated pumps, lines and seals. Cold starts get harder as ethanol content and temperature go down, which is why winter blends exist.
Legality on the street
- Race fuel: leaded race fuel is off-road and race only.
- E85: legal at the pump. Your tune still has to respect emissions rules. Our dyno page explains our policy on emissions equipment and race-only vehicles.
What About Octane Boosters?
There's a middle ground. We stock two octane enhancements: Maxima Hi-Test, which is a simple pour-in-and-forget-it, and Race Fuel Concentrate, which needs calculated mixing and can take pump gas up to 105 octane. That's not E85 and it's not a drum of C16, but for some setups and some events it's the practical answer. Mixing ratios matter, and so does tuning for what you're actually running.
If you're thinking about ethanol, know this: there's no way to guarantee ethanol quality or consistency without a flex fuel kit. Use an ethanol content sensor, or don't use ethanol.
How to Choose
Daily driven, mostly street, flexible on fuel stops: flex fuel E85 with an ethanol content sensor. You get pump-gas safety when you can't find E85, and the power when you can.
Dedicated E85, track days, available locally: a dedicated E85 tune with a fuel system sized for it, plus an ethanol content sensor.
Race-only car, event days, no cat: race fuel can make sense. Pick a specific product, get a tune for that product and stick with it.
Pump gas with a small bump: a properly mixed octane enhancer and a tune that's honest about it.
Whatever you pick, the tune has to match the fuel that's actually in the tank, not the fuel you hoped was in the tank.
FAQ
Is E85 really 85% ethanol?
Not necessarily. ASTM D5798 allows 51% to 83% ethanol, and the blend changes seasonally and by region. Run an ethanol content sensor.
Why does E85 make more power if it's only 95–97 octane?
Its pump octane (95–97, according to the U.S. Department of Energy) understates its real knock resistance in a turbo engine. Ethanol's chemistry resists autoignition, and its high heat of vaporization cools the charge, especially with direct injection.
Can I run race gas on my street Subaru?
Not leaded race gas. It's illegal for on-road vehicles and it damages catalytic converters. Unleaded race fuels exist, but you still need a tune for that specific fuel.
Do I need new injectors for E85?
Usually. E85 needs roughly 50–53% more fuel mass than gasoline at stoich (simple arithmetic from 9.6–9.8:1 vs. 14.7:1). Many factory fuel systems can't supply that at higher power.
Can I mix E85 and pump gas?
Yes, and the ethanol content changes with the mix. That's exactly why a flex fuel sensor and flex-capable tune are worth it.
What air-fuel ratio should I target on E85?
Think in lambda, not AFR. E85's stoich is 9.6–9.8:1, compared with 14.7:1 for gasoline, so the same AFR number means a different mixture on each fuel. Your tuner sets lambda targets for the fuel and the engine, and a wideband reading lambda keeps everyone honest.
Is race fuel always better than E85?
Not always. Race fuel gives you consistency and very high octane ratings, but it's expensive and you have to haul it. E85 gives you charge cooling and pump availability, if your area has it. "Better" depends on how the car is used.
Let's Get Your Fuel and Tune on the Same Page
We're a Cobb Tuning dealer & Protuner supporting Subaru, GTR & Porsche, and we calibrate on our SuperFlow roller dyno here in Grand Junction, at altitude, on the fuel you'll really be buying. If you're weighing E85, flex fuel, race gas or an octane enhancer, give us a call at 970-242-0919 or book a pre-tune inspection. Bring your fuel questions, we'll bring the data.