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To further clarify, the average street car is designed to run on the ethanol which is sold everywhere, but some engine configurations need a higher octane fuel

To further clarify, the average street car is designed to run on the ethanol which is sold everywhere, but some engine configurations need a higher octane fuel in order to combust the fuel completely. I had a buddy who rebuilt an engine with a compression ratio that required 91 octane, or he'd get knocking. This was during the late 80's, and more than one station around these parts had pumps selling 95 octane (it might have even been 100) for the real hot-rods.

p_l 6y ago
This whole comment thread is surprising to me because I haven't seen sub-90 octane rating in ages - I have even just checked, and one of the main petrol companies in Poland sells only 95, 98, and special extra variant of 98.
- ferdek 6y ago
  USA, they have all that weird stuff there...
- saagarjha 6y ago
  It’s complicated, as that number measures different things in different countries: https://en.wikipedia.org/wiki/Octane_rating#Measurement_methods https://en.wikipedia.org/wiki/Octane_rating#Measurement_meth...
- formerly_proven 6y ago
  As per usual, the US uses a different scale (AON) than the rest of the world (RON).
jakogut 6y ago
> some engine configurations need a higher octane fuel in order to combust the fuel completely This is incorrect. Higher octane ratings mean a fuel can withstand more compression before detonating. Pre-detonation, or "knocking", where an engine experiences detonation in the cylinder before top dead center can cause severe damage, so higher octane is used with higher compression engines. Lower compression engines see no benefit.
- TheRealDunkirk 6y ago
  If the fuel pre-ignites, proper flame propagation will be thwarted, and the fuel will not be burned as thoroughly, leaving residue, and exhausting (albeit tiny amounts of) fuel. Sure, I put the cart in front of the horse, but I can always count on HN to point out the pedantic.
  - jakogut 6y ago
    Reading this again, you're right, it was a little pedantic. This is one of those topics I feel is just rife with misinformation, so I tend to jump on explanations that don't seem correct at first glance.

p_l 6y ago

This whole comment thread is surprising to me because I haven't seen sub-90 octane rating in ages - I have even just checked, and one of the main petrol companies in Poland sells only 95, 98, and special extra variant of 98.

ferdek 6y ago
USA, they have all that weird stuff there...
saagarjha 6y ago
It’s complicated, as that number measures different things in different countries: https://en.wikipedia.org/wiki/Octane_rating#Measurement_methods https://en.wikipedia.org/wiki/Octane_rating#Measurement_meth...
formerly_proven 6y ago
As per usual, the US uses a different scale (AON) than the rest of the world (RON).

jakogut 6y ago

> some engine configurations need a higher octane fuel in order to combust the fuel completely This is incorrect. Higher octane ratings mean a fuel can withstand more compression before detonating. Pre-detonation, or "knocking", where an engine experiences detonation in the cylinder before top dead center can cause severe damage, so higher octane is used with higher compression engines. Lower compression engines see no benefit.

TheRealDunkirk 6y ago
If the fuel pre-ignites, proper flame propagation will be thwarted, and the fuel will not be burned as thoroughly, leaving residue, and exhausting (albeit tiny amounts of) fuel. Sure, I put the cart in front of the horse, but I can always count on HN to point out the pedantic.
- jakogut 6y ago
  Reading this again, you're right, it was a little pedantic. This is one of those topics I feel is just rife with misinformation, so I tend to jump on explanations that don't seem correct at first glance.