Saving Old Seagulls

Moving from Petrol to 80% Ethanol: Does it Increase Power Output?

A fair question. Alcohol fuels have long been used in motorsport, so yes — they are inherently capable of producing more horsepower for a given displacement.

The extra power comes from two main factors:

Cooler intake temperatures: Ethanol evaporates more readily than petrol, which cools the intake charge. Cooler air is denser, meaning more oxygen enters the engine. More air allows you to add more fuel — and more fuel means more power.

Higher octane rating: Ethanol has an octane rating of around 110, compared to 95–98 for standard pump petrol. This allows for a higher compression ratio without knocking. In practice, this means you can fit a high-compression cylinder head (John might have one available!) to extract more power safely.

I didn’t specifically set out to increase the power output of my Seagull, but since I needed to retune the main jet for ethanol, it gave me the opportunity to dial it in perfectly.

And because my fuel mix is 100% defined and consistent, I was able to tune at least the wide-open throttle (WOT) range very accurately. The combination of cooler intake air and precise jetting results in a noticeably stronger output.

I don’t have a way to measure the exact power increase — but at WOT, she empties the test bin faster than she ever did on petrol.

If you'd tried your original motor on your boat you could have measured the revs & speed, if you repeated the same test with the worked on motor, carrying the same load on the same hull, if the revs & speed have increased then logically power output also has.

I have now tested it on the water, there is no change in top speed... same speed that I achieved with this engine when running on petrol pre mix. Ah well.