DI vs Octane

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I have seen several posts where it has been mentioned that the use of 91+ octane instead of the required 87 will help with reducing intake valve contamination. I would like to see some data or even a reason why this is possible. Ed
 
I don't see how higher octane would change this considering the DI sprays straight down into the cylinder where as he port injection sprays on/past the intake valves to keep'em clean
 
I don’t see the effect on valve deposits either. You might be confusing this with some reports that higher octane reduces fuel dilution of the oil.
 
I can't see it having any real effect on valve deposits. I've read on here that it leads to less fuel dilution in the oil, which I would expect the theory behind to be less enrichment for anti-knock required with higher octane fuel.
 
Originally Posted By: PimTac
I don’t see the effect on valve deposits either. You might be confusing this with some reports that higher octane reduces fuel dilution of the oil.
which may reduce pcv vapors depositing on the valves.. dunno though.
 
Originally Posted By: Dylan1303
I thought premium had the most cleaning juice vs 87 at some stations. Shell comes to mind.


It doesn't really matter. The requirements for Top Tier, which apply to all grades if the brand is wearing the banner, are quite high to begin with.
 
Originally Posted By: Dylan1303
I thought premium had the most cleaning juice vs 87 at some stations. Shell comes to mind.


At one time Shell V-Power had five times more detergent than Shell's regular.
 
Originally Posted By: OVERKILL
I can't see it having any real effect on valve deposits. I've read on here that it leads to less fuel dilution in the oil, which I would expect the theory behind to be less enrichment for anti-knock required with higher octane fuel.


I've mentioned this (less fuel dilution) after reading about it on another forum and after observing my own engine. Though I have heard people say about premium gasoline possibly reducing valve deposits, I agree I don't see how or why it could. I haven't seen any proof, just people saying what they thought. On a forum for my Impala, many believe running synthetic oil and changing it at 5K (or less) will help with the deposits and in the case of the 3.6 engine protect against timing chain problems. Not sure I agree with either but I have no proof either way!

In my area, premium fuel is usually 20 - 30 cents a gallon more, so it's not a big deal to fill up with premium (17 gallon tank). One guy on the Impala forum who uses the tuning software states there are different tables for premium fuel and I have seen a difference monitoring the fuel trims and the knock retard via Torque Pro.
 
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Originally Posted By: oldhp
Originally Posted By: Dylan1303
I thought premium had the most cleaning juice vs 87 at some stations. Shell comes to mind.


At one time Shell V-Power had five times more detergent than Shell's regular.


On their current web page, they state that: "Shell V-Power NiTRO+ contains the highest concentration of the Shell Nitrogen Enriched Cleaning System and cleans up faster than Shell regular."

https://www.shell.us/motorist/shell-fuel...oline-faqs.html
 
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87 octane gas may be less refined and have more impurities than 91.

https://www.sunocoracefuels.com/tech-article/octane-stability-high-octane-vs-low-octane-fuels

"87 octane fuels tend to be less refined and contain more unstable hydrocarbons.
As the months pass during storage these unstable components react to form gums, varnishes and lower octane hydrocarbons.
...
Even in proper storage 87 octane gas can start to degrade in 3 months, 93 octane fuel should last closer to 9 months before degradation is noticeable.
Keep in mind that 93 octane fuels are still susceptible to octane loss and vapor pressure decreases due to butane evaporation."
 
I think most of this is marketing, No one really polices TOP TIER FUEL.
 
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Originally Posted By: Superflop
Originally Posted By: PimTac
I don’t see the effect on valve deposits either. You might be confusing this with some reports that higher octane reduces fuel dilution of the oil.
which may reduce pcv vapors depositing on the valves.. dunno though.


This seems to make sense. The petrol residues in the oil will tend to be heavy end, which will tend to promote coking.

In addition, I've seen it suggested (or stated as a fact, can't remember which now) that boiling off of petrol vapours promotes carry-over of the lighter fractions of the lubricating oil. Think it was by someone who generally seems to know whereof they speak, maybe Sonofjoe.

If this is true, higher octane fuel would indirectly tend to reduce intake tract deposits via its reduction in fuel dilution.
 
Originally Posted By: Ducked
In addition, I've seen it suggested (or stated as a fact, can't remember which now) that boiling off of petrol vapours promotes carry-over of the lighter fractions of the lubricating oil. Think it was by someone who generally seems to know whereof they speak, maybe Sonofjoe.


I seem to recall reading the same thing.

All in all - - anything to reduce fuel dilution in DI engines seems to help everything.
 
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