The octane requirements of a vehicle vary according to the compression ratio of the engine. The Chrysler Hemi is a relatively high-compression engine and requires fuels that adhere to the manufacturer's recommended octane rating. Higher-compression engines create greater cylinder pressures and heat and therefore require higher-octane fuels to prevent pre-ignition, which results in pinging, knocking and possible damage to the engine.
Octane and performance
Each version of the Chrysler Hemi engine has a recommended octane rating for optimum performance. The minimum acceptable octane represents the lowest octane that these engines can be operated with without significantly degrading performance or risking damage from pre-ignition.The use of fuels with octane ratings above the highest manufacturer's recommended level will not result in any increases in performance. The manufacturer-recommended octane levels therefore reflect the fuels the manufacturer specifically designed the engine to work best with, and deviation from these recommendations will not result in any added benefits and may result in lowered performance and reduced fuel economy.
The SRT8 high-performance Hemi requires 91 octane fuel only. The high-performance SRT8 version of the Chrysler Hemi engine is designed with a slightly higher compression ratio, which results in higher cylinder pressures than the base model Hemi. This higher compression causes the engine to require a higher octane fuel in order to produce its maximum power and avoid the problems with knocking and pinging that are associated with high-compression engines if fueled with low octane gasoline.
Base Hemi-Manual Transmission
Base Hemi engines mated to a manual transmission experience slightly higher cylinder pressures periodically. Because the driver controls the shifting, the loads placed on the engine can be inconsistent and occasionally higher than with an automatic as the engine is forced to work harder. To reduce the chance of pinging and knocking because of higher cylinder pressures, Chrysler recommends 91 octane fuel be used with Hemi engines that are matched to a manual transmission. 87 octane fuel is considered the acceptable minimum for Hemis with manual transmissions.
Base Hemi- Automatic Tansmission
The standard or "base" Hemi engine has a slightly lower compression than the SRT8 performance version. When the Hemi is matched with an automatic transmission, Chrysler recommends 89 octane fuel for peak performance. 87 octane is the minimum octane rating listed for the base Hemi regardless of transmission, but for the best mileage and performance, the higher-octane recommendations should be followed.
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