To run neat ethanol in an engine built for gasoline, enlarge the carburettor jets by about 26% in diameter (or lengthen injector pulses by about 50%), advance ignition timing 5-10 degrees, fit ethanol-proof fuel lines and, if you can, raise compression from about 9:1 to 12-14:1. The jet change comes from the fuel-air arithmetic: ethanol needs 1.63 times the fuel mass and 1.53 times the volume for the same air.
Part of Ethanol Fuel Production
Modifications that let a gasoline carburettor or fuel-injection system run on neat or high-percentage ethanol.
The Fuel Must Roughly Increase By Half Because Ethanol Carries Its Own Oxygen
A chemically correct mixture needs 14.7 kg of air per kg of gasoline and 9.0 per kg of ethanol. For the same air flow:
- Mass of fuel: 14.7 ÷ 9.0 = 1.63 times.
- Volume of fuel: ethanol is denser (0.789 kg/L against about 0.74 for gasoline, assumed), so volume rises by 1.63 × 0.74 ÷ 0.789 = 1.53 times.
- Through the same hole at the same pressure, a denser liquid flows slightly less by volume, by a factor √(0.74 ÷ 0.789) = 0.97. So the hole’s area must rise 1.53 ÷ 0.97 = 1.58 times, and its diameter by √1.58 = 1.26 times (26% larger).
A 1.00 mm gasoline jet becomes about 1.26 mm. The parent says “increase fuel flow by 30-40%”, which is below this arithmetic at the chemically correct mixture; the figure above (+53% by volume) is where to start the search, and trim from there. The same working gives these starting points for other blends (jet diameter relative to gasoline):
| Fuel | Fuel volume for same air | Jet diameter |
|---|---|---|
| E10 | 1.06 | 1.03 |
| E20 | 1.12 | 1.06 |
| E85 | 1.46 | 1.23 |
| E100 | 1.53 | 1.26 |
What You Need
- Drills or reamers in small steps, or ready-made larger jets
- Spark plug gauge, a timing light or the engine’s own timing marks
- An inspection mirror and a means of reading plug colour
- Ethanol-proof fuel lines and seals (Fuel Line Material Compatibility)
Method: Carburettor
Step 1: Record the starting jet size and keep the old jet.
Step 2: Enlarge in steps. Go up about 10% in diameter, run, then another 10%, until you approach 1.26 times. Drilling a jet is permanent; stop and test rather than going straight to the number.
Step 3: Read the engine. Lean: popping in the intake, hot exhaust, hesitation, pale plugs. Rich: sluggish, wet plugs. Aim for a smooth, responsive engine whose plugs are not pale.
Step 4: Advance timing 5-10 degrees from the gasoline setting, a little at a time, until the engine runs best under load without pinging.
Step 5: Start on gasoline in the cold and switch when warm; ethanol starts poorly below about 10°C.
Method: Fuel Injection
An injector delivers more fuel in proportion to how long it stays open. At the 1.53 volume factor, an open-loop system needs pulses about 50% longer. A computer with an oxygen sensor will correct a modest shortfall and then run out of range; flex-fuel engines have extra range built in. Without reprogramming, injection engines are best kept to E10-E20.
Compression
Ethanol resists knock, so raising compression from about 9:1 to 12-14:1 (Ethanol in Gasoline Engines for the octane figures) lets the engine burn it more efficiently. This means machining the head or fitting thinner gaskets, and is a rebuild-grade job, not a field tweak.
Common Mistakes
- Drilling the jet to the final size in one go and discovering a rich or lean fault with no way back.
- Running a modified carburettor on gasoline later and overfuelling it.
- Advancing timing past knock. Pinging damages pistons; back off at the first sound.
Lean running burns valves and fuel leaks burn people
A lean engine overheats and can backfire through the carburettor into the engine bay, and ethanol’s flame is pale and hard to see. Fit a fire extinguisher or sand, check the lines for leaks every time, and see Fire Safety.
What’s Next
- Learn next: Fuel Line Material Compatibility, to avoid leaks
- Combine with: Engine Use, for how much fuel you will need