Actually, the hemi is designed to run on both 87 and 89 as stated by the manufacturer, otherwise, running 87 would be prohibited.

Another thing is that when one puts 87 in the tank, the hemi does not automatically know this and selects a different timing pattern. The timing for 87 and 89 starts out the same. What happens is that timing is retarded at the first sign of pre-detonation, regardless of octane in the tank. Because pre-detonations can be caused by many factors (air density, cylinder head temperatures, engine load, air temperature), under benign conditions, the timing for both octanes will be the same. Since most of the time we do not drive in conditions that merit ignition retardation (towing on a hot day or pulling a trailer up an incline), we will not notice a difference.

I run 89 when I think I'm going to need all of the hemi power.

 

Very good. Thank you for the technical clarification. Do you happen to know if there is enough latitude in the system that would provide enough increase in ignition timing to take benefit of octanes greater than 89?

Best regards,
Dusty
2010 Ram Big Horn 1500 Quad Cab 2WD, 5.7 Hemi, 545RFE, 3.92 LSD, factory dual exhaust, 20” wheels. Now at: 101,000 miles.

 

A tuner can open up the timing advance to take advantage of the higher octane than 89. The stock tuning will limit the timing advance of the engine to what Chrysler engineers consider safe per their recommended 89.

The only time I would think that running higher than 89 would provide any benefit, is when the hemi is so hot and under load that it pre-detonates even with 89, then the 91 would help some. This would be a very rare occasion and probably not worth the extra $$$.