U.S. octane requirement question

454cid · Post by **454cid** » 09 Mar 2026, 22:00

Here in MI "Premium" is 93 and that's what I always put in my 850. Unfortunately, most gas in MI is 10% ethanol, including the 93 Octane.

Post by **CIK7** » 09 Mar 2026, 22:06

abscate wrote: 09 Mar 2026, 03:37 Compression ratio is the static ratio of chamber with piston down and piston up. When a turbo boosts, it pushes extra air into the cylinder and the effective pressure in the cylinder is higher? How much? I’m not sure.

I think this answers my question. The average pressure might be lower than the naturally aspirated, but the turbo pressure might be what adds back in the need for premium.

Post by **abscate** » 10 Mar 2026, 03:21

CIK7 wrote: 09 Mar 2026, 22:06
abscate wrote: 09 Mar 2026, 03:37 Compression ratio is the static ratio of chamber with piston down and piston up. When a turbo boosts, it pushes extra air into the cylinder and the effective pressure in the cylinder is higher? How much? I’m not sure.
I think this answers my question. The average pressure might be lower than the naturally aspirated, but the turbo pressure might be what adds back in the need for premium.

It gets complicated quickly. When you boost, the timing gets massively retarded to prevent pinging. The high pressure mixture is much more explosive than the regular compressed mixture so you can ignite later.

You can mess with all this with a tune, run around happy for a few months, then come back to the forum with a cracked block complaining how Volvo engines are crappy