HEMIMANN
Senior Member
- Joined
- Dec 7, 2020
- Posts
- 9,615
- Reaction score
- 25,248
- Location
- Minneapolis, MN
- Ram Year
- 2017 2500 Laramie Crew Cab
- Engine
- 6.4L HEMI
So - there's a bunch 'o discussion on this in other, off-topic forums.
There are only two octane gasolines in N.A. for vehicle fuels, 87 & 91 (Av gas is higher). (BP is carrying 93, but it has a lot of ethanol added). 89 is a blend of 87 & 91. I use 91 because I am unsure how that blend really behaves during combustion. Optimal octane is important based on your cylinder pressure and spark timing.
You can get more power out of an engine with higher compression ratio, turbos, blowers, etc. These require higher octane gas so it won't pre-ignite before the spark. Except for the custom engine builders, what we get from the factory is what we have to deal with. Hemi recommends 89 because that's where the spark timing programming is set. If you use 87, it will start pre-igniting multiple times before the controller pulls back the spark timing. Not a good strategy IMO. And again, I don't like blended 89, so I use 91.
The optimal gasoline is octane rating set for the compression and spark timing designed into your engine. Any higher may not burn completely, any lower will pre-ignite.
Alcohol blended gasoline does not store long - maybe 1 month. It adsorbs moisture from the atmosphere and drops out to the bottom, causing corrosion. There are several states (and CA?) that mandate alcohol blends from bribes by the agrilobby to legislatures. I live in one such state, Minnesota. I use the minimum ethanol I can find, 10%. They are pushing 15%. Don't buy it. And certainly never 85%. And "Nonoxy" (no alcohol) for two cycle engines and small engines like Briggs.
Octane alone has nothing to do with storage. Alcohol does.
Race engines are a different story.
There are only two octane gasolines in N.A. for vehicle fuels, 87 & 91 (Av gas is higher). (BP is carrying 93, but it has a lot of ethanol added). 89 is a blend of 87 & 91. I use 91 because I am unsure how that blend really behaves during combustion. Optimal octane is important based on your cylinder pressure and spark timing.
You can get more power out of an engine with higher compression ratio, turbos, blowers, etc. These require higher octane gas so it won't pre-ignite before the spark. Except for the custom engine builders, what we get from the factory is what we have to deal with. Hemi recommends 89 because that's where the spark timing programming is set. If you use 87, it will start pre-igniting multiple times before the controller pulls back the spark timing. Not a good strategy IMO. And again, I don't like blended 89, so I use 91.
The optimal gasoline is octane rating set for the compression and spark timing designed into your engine. Any higher may not burn completely, any lower will pre-ignite.
Alcohol blended gasoline does not store long - maybe 1 month. It adsorbs moisture from the atmosphere and drops out to the bottom, causing corrosion. There are several states (and CA?) that mandate alcohol blends from bribes by the agrilobby to legislatures. I live in one such state, Minnesota. I use the minimum ethanol I can find, 10%. They are pushing 15%. Don't buy it. And certainly never 85%. And "Nonoxy" (no alcohol) for two cycle engines and small engines like Briggs.
Octane alone has nothing to do with storage. Alcohol does.
Race engines are a different story.
