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Since I got my 89 Formula with the 5.0 TBI I've been using 89 octane gas. Should I just use regular? I haven't found a good answer when it comes to this. The car runs fine on 89.
 

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Unless the manufacturer states Premium, which is 91-93 octane, is suggested, the engine doesn't require it. Many engines that suggest Premium don't necessarily have to run on it.

However, without vehicle details listed, if the engine were dyno tuned for performance and 93 octane helps the engine achieve its best output, then you're running 91-93 all day. Running it on 87-89 will cause dangerous detonation under hard acceleration.

Bottom line
1. Stock: 87 is fine
2. Dyno'd is 91-93
3. High octane race fuel 101-108 if suggested but anything over 101 will harm the O2 sensors.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Three factors to consider:

1 - As noted above, what octane does the manufacturer recommend? You can find this in the Owner's Manual, if you are fortunate to have one.

2 - The 1989 5.0L engines have a knock sensor. That provides protection from damage when using moderately lower than recommended octane fuel. The engine will appear to “run fine”, but the engine will experience a small loss of power due to the ignition spark timing retard. Go to really low octane fuel and you can still do significant damage. But detonation (knock) should be evident, with a rattling sound often described as ball bearings rattling in a tin can.

3 - What is the elevation (relative to sea level) where you live? The higher the elevation, the lower the air density, and that reduces the effective static compression ratio. At very high elevations (e.g. Denver at 5,000-ft) they likely don’t even sell 90+ octane fuels.
 
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