E85 vs. Pump Gas: How Much More Injector Do You Actually Need?

What's in this article

 

It is the "liquid gold" of the tuning world. E85 (85% Ethanol) allows for more boost, more timing, and significantly cooler intake temperatures. It's essentially cheap race fuel from a pump.

However, there is no such thing as a free lunch. Ethanol has a lower energy density than standard 98-octane pump gas. To make the same amount of power, you have to spray a lot more fuel into the cylinder. If you are planning to make the switch, the first question you need to answer is: Are my injectors big enough?

The 30% Rule

The most important number to remember when switching to E85 is 30%.

Because ethanol contains less energy per gallon than gasoline, the stoichiometric ratio (the ideal air-to-fuel ratio for complete combustion) drops significantly. While gasoline is roughly 14.7:1, E85 is closer to 9.8:1.

In practical terms, this means your fuel system needs to move approximately 30% to 35% more volume to maintain the same power level. If your current injectors are running at 80% duty cycle on pump gas, they will be physically incapable of providing enough fuel for E85.

Doing the Math: How to Size for Ethanol

When sizing injectors for an E85 build, you shouldn't just aim for "enough." You need to build in a safety margin.

Here is a quick breakdown of how injector requirements scale:

If you are building an LS engine with a turbocharger, this is where many enthusiasts get caught out. An injector that seems "huge" on paper can quickly reach its limit once the ECU detects ethanol content.

Why "Headroom" Matters

At Excess Injectors, we generally recommend aiming for a maximum Duty Cycle of 80%.

Running injectors at 95% or 100% (known as "static") is dangerous. When an injector stays open constantly, it loses its ability to pulse accurately, the coil gets hot, and you lose control over your fueling. By sizing your injectors 30% larger than your pump gas requirements, plus an extra 20% for safety headroom, you ensure your engine stays safe even on the hottest track days.

Fuel Temperature and Density

E85 doesn't just require more volume; it’s also more sensitive to temperature. As fuel gets hotter, it becomes less dense. In a high-performance environment where fuel is constantly being cycled through a hot fuel rail, your injectors have to work even harder to compensate for that loss of density. This is another reason why "barely big enough" is never big enough.

Can Injectors Be "Too Big"?

A common fear is that "going big" for E85 will ruin the car's driveability on normal pump gas.

In the past, this was true. Massive injectors used to have terrible spray patterns at low pulse widths, making for a shaky idle. However, modern injectors (like the Excess 1100cc or 1500cc kits) use advanced internal geometry that allows them to remain incredibly precise at tiny pulse widths.

With our Exact Match Data, your ECU can control a large injector just as smoothly as a factory one, giving you the best of both worlds: a perfect idle on pump gas and enough flow for big boost on E85.

Calculate with Power

If you are making the jump to E85, don't just "hope" your fuel system can keep up. Calculate your power goals, add 30% for the ethanol requirement, and build in a 20% safety margin.