95 or 98 octane after a tune: how much power the fuel is worthBuild Ladder blog

Why fuel changes the power

Higher octane resists knock, so the engine computer can run more ignition advance and more boost before it has to pull timing. A file written for 98 asks for more than a file written for 95. On a stock car the difference is small; on a tuned turbo car it shows in the published figures.

A note on the numbers: Europe and Israel quote RON (95, 98). American tuners quote pump octane, a different scale: US 91 is roughly 95 RON and US 93 is roughly 98 RON.

Same tuner, both fuels, ten cars

Each car links to its guide, where every figure links to the tuner's own page. Only tuners that publish both fuels for the same file family are included. The Supra figures are for US 91 and 93.

What the table shows

• 98 is worth 3–9% on Stage 1. On a 300 hp hot hatch that's 10–20 hp; on the RS3 8Y and RS6 C8 it's around 50 hp.
• Big turbo engines gain the most. The five-cylinder RS3 and the V8 RS6 run high boost, where knock resistance matters more.
• The headline figure is often the top fuel. APR, for example, publishes one Stage 1 range from its lowest supported fuel up to race fuel or E85. The bottom of the range is the pump-fuel figure.

Choose the file for the fuel you actually buy

If a file is written for 98 and you fill up with 95 on a trip, the engine computer pulls timing to protect the engine. With a mild file that only costs power; with an aggressive one it's a real risk. So:

• 98 is easy to find on your routes: a 98 file gets the extra 3–9%.
• You sometimes can't find 98: run the 95 file, or a tune with switchable maps (REVO and bootmod3 offer them on some cars).
• E85 figures don't apply in Israel: it isn't sold at filling stations. See tuning a car in Israel.

Keep reading

• APR or Unitronic? Stage 1 compared on seven cars
• Tuning a car in Israel: fuel, heat and the law
• Best first mods for a Golf GTI Mk7