What happens if I put 87 instead of 91?
Using 87 octane in a vehicle that specifies 91 can reduce performance and efficiency, and may trigger engine-knock protection. In many modern cars, the ECU can adjust timing to prevent damage, but sustained use of lower-octane fuel is not ideal and can risk longer-term issues.
Octane ratings indicate a fuel’s resistance to premature ignition (knock) in high-compression engines. Most everyday cars run fine on regular 87, but engines with higher compression, turbocharging, or performance tuning often require higher octane to maintain full power and efficiency. The impact of using 87 instead of 91 depends on the engine design, boost levels, temperatures, and how aggressively the vehicle is driven.
Understanding how engines respond to fuel octane
Below is a breakdown of the typical effects you might notice when 87 octane is used in a vehicle that recommends 91 octane.
How engines respond to lower octane
The following are common outcomes observed in many modern engines when a lower-octane fuel is used against higher-octane requirements.
- Power and acceleration may feel reduced, especially during hard acceleration or climbing hills.
- Engine pinging or knocking can occur under load, hot conditions, or when the engine is boosted by a turbocharger.
- Fuel economy can decline because the engine may run less efficiently as the ECU retimes ignition to prevent knock.
- Throttle response might seem less immediate due to timing adjustments made by the engine control unit (ECU).
- Repeated knocking, if persistent, can contribute to wear over time; however, most modern engines will limit this risk by retarding timing to protect the engine.
Keep in mind that the exact effect varies by car. Some engines tolerate lower octane with minimal noticeable impact, while others show clear reductions in performance under load.
In summary, while many vehicles will run on 87 without immediate damage, you may lose some power, efficiency, and responsiveness. If your manual specifies 91 or higher, using 87 regularly is not recommended.
What to do if you accidentally fuel with 87
If you’ve inadvertently filled up with 87 in a car that calls for 91, here are practical steps to protect performance and longevity.
Before listing actionable steps, consider these guidelines for handling the situation effectively:
- Drive normally but avoid aggressive acceleration or high-load driving until you’ve filled with the recommended octane again.
- Check your owner’s manual to confirm the minimum octane and any notes about knock or timing adjustments for your specific engine.
- Plan to refuel with the higher-octane fuel at your next fill to restore the engine’s intended operating conditions.
- If you hear persistent knocking or pinging, or if the vehicle seems to lose power under load, stop driving aggressively and seek service guidance.
- Turbocharged or high-performance engines are particularly sensitive to octane; in these cases, using the recommended octane is more important for reliability and warranty coverage.
By following these steps, you minimize potential performance losses and reduce the risk of knock-related damage until you can refuel with the correct octane.
Summary
Octane matters most for engines designed to operate at higher compression or with forced induction. If your car requires 91 octane and you accidentally use 87, you’ll likely notice some drop in power and efficiency and possibly occasional knocking under certain conditions. Modern vehicles often compensate by retarding timing, but this protection comes at the cost of performance. Always refer to your owner's manual for the recommended octane, and if you must use a lower-octane fuel temporarily, plan to refuel with the correct octane as soon as possible. Regularly using the lower-octane fuel can increase wear over time and may affect warranty coverage in some cases.
