What would happen if you fill your gas tank with 87 octane and the manual calls for 90 what this do to engine?

Using 87 octane gasoline when your car's manual recommends 90 octane will likely lead to pre-ignition or knocking. This is because lower-octane fuel is less resistant to self-ignition under compression. When the fuel ignites prematurely, before the spark plug fires, it creates a sharp, pinging sound and can cause several problems:

* Reduced Engine Performance: The engine might lose power and feel sluggish.

* Increased Engine Wear: The repeated explosions from pre-ignition put stress on the engine's components, leading to faster wear and tear. This can damage pistons, connecting rods, and the crankshaft.

* Catalyst Damage: Pre-ignition can generate higher temperatures, potentially damaging your catalytic converter.

* Increased Emissions: Incomplete combustion due to pre-ignition can increase emissions of harmful pollutants.

* Engine Damage (Severe Cases): In extreme cases, persistent pre-ignition can cause significant engine damage, requiring costly repairs.

It's not a catastrophic immediate failure, but it's certainly not ideal. How quickly problems arise depends on several factors, including:

* How much lower the octane is: The difference between 87 and 90 is relatively small, so the effects might be subtle or slow to appear.

* Driving Style: Aggressive driving and high loads exacerbate the issue.

* Engine Design: Some engines are more sensitive to lower octane than others.

Recommendation: It's always best to use the fuel octane recommended by your car's manufacturer. While a single tank of 87 might not cause immediate harm, consistently using lower-octane fuel than recommended can lead to long-term damage and reduced engine lifespan. Fill up with 90 octane at your next opportunity.