6.2 engine fuel

M1ck3y · December 22, 2021

19 hours ago, Supreme Pizza said:

All I can say about octane is that you must use at least the minimum octane rating recommended by the manufacturer.

So why does it always say "minimum octane rating". Well, because more is OK, less is death.

Higher octane doesn't make a car faster, but higher octane is required by faster cars.

More octane = less volatile fuel.

You can raise your octane rating by diluting your fuel with a less volatile fuel, like kerosene.

High octane fuel should be cheaper than low octane fuel. But its not. Because people that need high octane fuel have more money. And because people think paying more for fuel will get you better fuel.

If the fuel is "too combustible", then it will ignite before the spark happens. That is called pre-ignition, spark knock, or detonation. High compression engines have issues with pre-ignition, so they use higher octane fuel, so that the gas wont "blow up" before its supposed to during the compression cycle. Its counter-intuitive, but lower grade (deluted) fuel works better for higher compression engines. I see people put "octane booster" or "premium gas" in their crappy cars, hoping it will make them faster or more reliable. Doesn't work that way. You want to use the minimum octane fuel that is acceptable for your engine. ~~Low octane fuel is more powerful than high octane fuel.~~

RCF71 · December 23, 2021

On 12/21/2021 at 2:07 AM, Supreme Pizza said:

All I can say about octane is that you must use at least the minimum octane rating recommended by the manufacturer.

So why does it always say "minimum octane rating". Well, because more is OK, less is death.

Higher octane doesn't make a car faster, but higher octane is required by faster cars.

More octane = less volatile fuel.

You can raise your octane rating by diluting your fuel with a less volatile fuel, like kerosene.

High octane fuel should be cheaper than low octane fuel. But its not. Because people that need high octane fuel have more money. And because people think paying more for fuel will get you better fuel.

If the fuel is "too combustible", then it will ignite before the spark happens. That is called pre-ignition, spark knock, or detonation. High compression engines have issues with pre-ignition, so they use higher octane fuel, so that the gas wont "blow up" before its supposed to during the compression cycle. Its counter-intuitive, but lower grade (deluted) fuel works better for higher compression engines. I see people put "octane booster" or "premium gas" in their crappy cars, hoping it will make them faster or more reliable. Doesn't work that way. You want to use the minimum octane fuel that is acceptable for your engine. Low octane fuel is more powerful than high octane fuel.

All of that bolded info is incorrect.

Edited December 23, 2021 by RCF71
Accuracy

Supreme Pizza · December 23, 2021

4 minutes ago, RCF71 said:

All of that bolded info is incorrect.

"I see people put "octane booster" or "premium gas" in their crappy cars, hoping it will make them faster or more reliable."

How is this statement incorrect in any way? And how would you be able to determine if I have seen people pour octane booster into their Geo Metro, while they were bragging about how much faster it makes their car?

Anyway,

I would suggest that people do their own research and choose the option that they feel is best. Especially when its something as important as using the correct octane, diet, medical treatments, vehicle to purchase, the best brand of refrigerator, etc.

Read the forums, talk to the experts, research all the available information, and draw your own conclusions. Facts and opinions can be a grey area. Interpretation can be a factor as well.

Even if one direction is the "correct direction" for most, its not going to be ideal for all.

Don't follow anyone's advice, but rather use that advise in combination with your other sources of information in order to make the decision that you feel will suit your needs the best.

Transient · December 23, 2021

RCF71 · December 23, 2021

9 hours ago, Supreme Pizza said:

"I see people put "octane booster" or "premium gas" in their crappy cars, hoping it will make them faster or more reliable."

How is this statement incorrect in any way? And how would you be able to determine if I have seen people pour octane booster into their Geo Metro, while they were bragging about how much faster it makes their car?

Ok, fixed it.

A few notes:

- octane and volatility are not correlated, octane has more to do with the chemical structure of each molecule, not its size (which is what most affects volatility)

- higher octane fuel is definitely not cheaper to produce, refineries have processing units that cost 100’s of millions of dollars to convert low octane molecules into higher octane molecules… why should higher octane fuel be cheaper?

- lower octane is not “more combustible”, it simply has a lower activation energy - combustion is a chemical reaction, and all reactions need a certain amount of energy to start/continue

- “diluted” fuel is not better for high compression engines, unless you are “diluting” the gasoline portion with an additive that has much higher octane (ethanol, methanol, mtbe, etc)

- lower octane fuel may or may not have more energy than higher octane fuel, again, not directly correlated. In the case of ethanol addition to raise octane, it is true that the energy content is lowered (ethanol has approx. 30% lower energy content than typical gasoline)

- do not put kerosene in your gasoline, it will not burn completely and will leave you with carbon deposits (at best, because it will not vaporize and mix thoroughly), but kerosene can go into diesel to lower power pour point/prevent gelling

Edited December 23, 2021 by RCF71

TJay74 · December 23, 2021

To reinforce what RCF71 just said:

Lower octane fuel burns faster, but doesnt produce more BTU's (energy) than higher octane fuel. The speed of the burn from the octane is what can cause the issue when used in a engine that has a high compression ration.

The sound of detonation or knock is caused when pre-ignition happens. As the fuel-air mix is pushed into the combustion chamber and is compressed the leading edges of the valve or the valve reliefs on the pistons can be hot enough they ignite the fuel-air mix before the top of the compression cycle and before the spark plug actually fires to ignite the fuel-air mix. This causes an issue due to when the hot spot in the chamber ignites the fuel-air mix it starts a flame front (propagation) to race across the combustion chamber. As the flame front is racing across the chamber the spark plug then ignites the mix as well, now you have 2 different flame fronts traveling across the chamber towards each other at the speed of sound. When they collide the cause the audible "knock" or pinging that we hear. This also can cause damage of the knock is severe enough, when inspected the tops of the pistons will have marks on them similar to if you hit the piston top with a ball peen hammer.

The way you control this is with a higher octane fuel that burns slower and resists pre-ignition, this allows the spark plug to do its job and to ignite the fuel-air mix with a single spark source.

M1ck3y · December 23, 2021

57 minutes ago, TJay74 said:

To reinforce what RCF71 just said:

Lower octane fuel burns faster, but doesnt produce more BTU's (energy) than higher octane fuel. The speed of the burn from the octane is what can cause the issue when used in a engine that has a high compression ration.

The sound of detonation or knock is caused when pre-ignition happens. As the fuel-air mix is pushed into the combustion chamber and is compressed the leading edges of the valve or the valve reliefs on the pistons can be hot enough they ignite the fuel-air mix before the top of the compression cycle and before the spark plug actually fires to ignite the fuel-air mix. This causes an issue due to when the hot spot in the chamber ignites the fuel-air mix it starts a flame front (propagation) to race across the combustion chamber. As the flame front is racing across the chamber the spark plug then ignites the mix as well, now you have 2 different flame fronts traveling across the chamber towards each other at the speed of sound. When they collide the cause the audible "knock" or pinging that we hear. This also can cause damage of the knock is severe enough, when inspected the tops of the pistons will have marks on them similar to if you hit the piston top with a ball peen hammer.

The way you control this is with a higher octane fuel that burns slower and resists pre-ignition, this allows the spark plug to do its job and to ignite the fuel-air mix with a single spark source.

You shouldn't state that low octane fuel burns faster. It's entirely dependent on the composition of the fuel. Some of the highest octane fuels are also some of the fastest burning. Resistance to pre-ignition is the purpose of high octane. Burn speed really only becomes a factor when you have a high reving, high strung engine and you require a faster burn speed at high rpm's. Tuning is also a big factor here. Adjustments in timing can be made to accommodate the fuel's burn speed; making as much, or even more power then the slower burning fuel. Or vice versa for that matter..

Edited December 23, 2021 by M1ck3y

swathdiver · December 23, 2021

I wonder how many folks know that ethanol is used to reduce spark knock these days?

TJay74 · December 23, 2021

1 hour ago, M1ck3y said:

You shouldn't state that low octane fuel burns faster. It's entirely dependent on the composition of the fuel. Some of the highest octane fuels are also some of the fastest burning. Resistance to pre-ignition is the purpose of high octane. Burn speed really only becomes a factor when you have a high reving, high strung engine and you require a faster burn speed at high rpm's. Tuning is also a big factor here. Adjustments in timing can be made to accommodate the fuel's burn speed; making as much, or even more power then the slower burning fuel. Or vice versa for that matter..

This was a very basic run down, once you get into oxygenated fuels you get into a whole new type of conversation.

TJay74 · December 23, 2021

27 minutes ago, swathdiver said:

I wonder how many folks know that ethanol is used to reduce spark knock these days?

Ethanol is a cheap and amazing way to get rid of knock. My 2016 6.2/A8 has been on E85 for 4 years now, it runs great. Even still if a person only has 91 they can add up to E10 with no harm. If the trucks tank is 26 gallons that means you can easily add 2.5-3 gallons of E85 to the 91, it will help with detonation and make the truck run better as well.

JohnH4260 · December 27, 2021

I’ve been using Costco 91 octane gas since new in March 2021. No problems so far…

GullLakeMi · December 31, 2021

I had a '16 with 6.2L and 8 speed. Damn 8 speed was so clunky I often ran 87 octane. Why the hell not. Shitty transmission didn't deserve any better. It's only $3k for the 6.2L. Sold it after 5 years to CarMax for nearly what I paid for it excluding sales tax.

Don't see the reason to pamper a full-size truck when they consistently are at bottom of any reliability rating anyway.