Octane Required

Riley P

Riley P

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This isn't specifically a C7 thing but more a 6.2 Engine thing, including trucks when they went to the direct injection.

Because the computer now times when fuel is injected into the cylinder, why does it matter what octane is used? Octane does not have more energy than regular gas (not including ethanol). Octane is only a resistance to detonation and was used in performance engines because of the higher compression ratio therefore the fuel being under higher pressure needed to resist detonation until the spark made it fire.
Now that the 6.2's have direct injection and there is no fuel to preignite, why does the octane rating matter?
 
6.2's still have spark plugs, right ? ignition is not caused by direct injection. Diesels ignite by compression but not gas engines. Thus same rule still applies no matter how the fuel is delivered to the cyl in a gas motor. The computer commands a better and more precise timed burn than a carb system. But the better the fuel the better the bang. Someone correct this ol school hotrodder if I'm wrong.
 
It makes sense what you are saying but I'm not sure the exact timing of the injected fuel. Is it at some point during the compression stroke or is it fired in the cylinder at the exact time the plug fires?
 
I honestly don't know when the fuel is injected during the compression stroke . I have never even seen or heard one of these things run. My guess the air needs to be mixed with the fuel spray at some point so it must be very close to the end of the compression stroke ??? I know they are using a high compression ratio and getting away with low grade fuel so there is a trick there in todays technology. To me it still makes sense the better the fuel the better the explosion.
 
I skimmed through it Eric but I didn't see where they say at what point the fuel is injected exactly. They do say it is aimed at the fuel bowl in the piston so it is very close to the end of the stroke as I guessed. I know I will never own one so not something I will be rebuilding one day. If it stops on the road make sure one has a CAA card. Like many things 2day they build stuff so complicated and expensive to accomplish the near same result as old technology. I got a car mag someplace doing a test on the track of a 66 Pontiac 2+2 421 tripower 4 speed. 0-60 was 3.9 seconds, and 1/2 second slower than some Ferrarri on the lap. Hey, what would you guys do to me if I put a diesel into my Corvette ? Nice rattle sound and a trail of black smoke !!!!
 
Riley P said:
This isn't specifically a C7 thing but more a 6.2 Engine thing, including trucks when they went to the direct injection.

Because the computer now times when fuel is injected into the cylinder, why does it matter what octane is used? Octane does not have more energy than regular gas (not including ethanol). Octane is only a resistance to detonation and was used in performance engines because of the higher compression ratio therefore the fuel being under higher pressure needed to resist detonation until the spark made it fire.
Now that the 6.2's have direct injection and there is no fuel to preignite, why does the octane rating matter?
Click to expand...
You can use regular still but there will be less output than with premium. The owners manual even says so. This engine could easily have had more stock horsepower but Corvette decided that there was still the odd guy out there that would want to stick with regular. Maybe premium isn't always available for someone in backwater Utah, or Alberta for that matter. My other car has an 07 twin-turbo I6 that uses DI so I have some familiarity with this technology. Use of a high quality, low volatility synthetic oil is imperative. The EGR system still dumps waste into intake system which can result in a carbon buildup just before the intake valves. Unintended consequences? Anyways, I've got 170K on the BMW I6 using premium only and the oils recommended and have yet to see any loss of power or fuel economy degrade. The power and efficiency of these new engines is amazing. More horsepower and better fuel economy.
 
Michael. I would give you a big thumbs up on the road for your diesel rattler Corvette... Just make sure you tune the hell out of it.... nothing I like better than punching my Stroker and seeing that burst of unburned fuel smoke out the back....lol.... and mine is just a baby tune... some of the boys out there are running scary HP and torque....
 
Not a technical guy: I use 94 octane from Chevron in my C7 - maybe $5.00 more per fill up - the computer system makes adjustments, I would think, to ensure engine efficiency - I hope?
 
Back a few years ago when premium hit $1.50 here in Ontario, I decided to experiment with regular gas on my Acura (premium is recommended). The use of regular results in false savings because I clearly saw a decrease in fuel economy and power. I am not technically savvy enough to explain why, but I assume the timing difference in combustion that results from the octane difference results in a degrading of efficiency and power. I don't think it does any harm to the engine (at least in the short to medium term). While I may be willing to accept that on my DD, no way I will do that with my C7.
 
There is a misconception about octane and why we use higher octane fuel in some instances. I think all performance engine guys will tell you that you want to run the lowest octane fuel you can run without detonation. A very broad statement but basically true.
The lower the octane the easier it will ignite and burn the problem is that in a high performance engine the burn will be slow and hot. So much heat that the engine will run poorly and not produce the power it was designed to.
If the engine is equipped with electronics to monitor the combustion the system will detect the detonation and retard the ignition timing and adjust the fuel delivery to try and eliminate the detonation and reduce the heat being produced by the poor burn.
Higher octane makes the fuel harder to ignite allowing the engine to create a hot fast burn at the top of the compression stroke creating the most power possible. Our engines control valve timing, ignition timing and fuel delivery, making the proper fuel very important. The manual says minimum 91.
We have done hundreds of fuel tests here in BC and the best fuels we have found are Chevron 94 and Shell Gold I believe it is 92.
Our tests showed these fuels are among the best anywhere on the continent. We have tested fuel from all over North America as part of technical inspection at races.
In my car I run 94 if I can get it or 91 If I cant get it and when in the USA I carry octane boost to add if I can't get good 91 without ethanol.
Ethanol is not ideal for our engines. It is basically an octane substitute.

Later....
 
EZERUSS said:
There is a misconception about octane and why we use higher octane fuel in some instances. I think all performance engine guys will tell you that you want to run the lowest octane fuel you can run without detonation. A very broad statement but basically true.
The lower the octane the easier it will ignite and burn the problem is that in a high performance engine the burn will be slow and hot. So much heat that the engine will run poorly and not produce the power it was designed to.
If the engine is equipped with electronics to monitor the combustion the system will detect the detonation and retard the ignition timing and adjust the fuel delivery to try and eliminate the detonation and reduce the heat being produced by the poor burn.
Higher octane makes the fuel harder to ignite allowing the engine to create a hot fast burn at the top of the compression stroke creating the most power possible. Our engines control valve timing, ignition timing and fuel delivery, making the proper fuel very important. The manual says minimum 91.
We have done hundreds of fuel tests here in BC and the best fuels we have found are Chevron 94 and Shell Gold I believe it is 92.
Our tests showed these fuels are among the best anywhere on the continent. We have tested fuel from all over North America as part of technical inspection at races.
In my car I run 94 if I can get it or 91 If I cant get it and when in the USA I carry octane boost to add if I can't get good 91 without ethanol.
Ethanol is not ideal for our engines. It is basically an octane substitute.

Later....
Click to expand...
The C7 engine is designed for 91 octane. Unless your engine is modded, running a higher octane will do nothing for you and is the same as running 91 in a car designed for 87. The higher octane gas burns slower and that is why a higher compression engine needs it. It's not cause of "poor burn" as you stated. The detonation sounds or pinging is from the fuel burning a little too early and causing pressure on the piston before tdc. Everyone will do what they think is better for the engine but the owner's manual says 91 for best performance but regular can be used as well with reduced HP.
 
I too use Chevron 94 octane gas in my C7 and did so in my 07 Shelby GT 500 and all my other corvettes before - a few extra dollars - so why not?
 
Tony Gull said:
The C7 engine is designed for 91 octane. Unless your engine is modded, running a higher octane will do nothing for you and is the same as running 91 in a car designed for 87. The higher octane gas burns slower and that is why a higher compression engine needs it. It's not cause of "poor burn" as you stated. The detonation sounds or pinging is from the fuel burning a little too early and causing pressure on the piston before tdc. Everyone will do what they think is better for the engine but the owner's manual says 91 for best performance but regular can be used as well with reduced HP.
Click to expand...

Actually the High octane does not burn slower it ignites at a higher temperature that is created by higher compression. A slow burn creates heat and a slow burn is inefficient and produces poor power numbers. If you read what I posted I stated that the low octane fuel causes a poor burn in a performance engine not high octane fuel.
But like you state 91 octane is the fuel listed in the manual as a minimum. Section 9-53 states 91 octane or higher.
 
Don't agree with the temperature statement as I've read enough and have owned many performance cars all my life. The octane can be higher but there is no horsepower gain above the 465. So feel free to run above 91, pay more money and probably have reduced HP. Facts are in my replies if you took the time to read them as I did yours.
 
I would equate the octane of gas to the alcohol content of a drink. The higher the content the less one has to drink to get buzzed, making the drink more efficient. The higher the octane, the easier it is for the gasoline to ignite, making it more efficient. FYI, I actually don't drink.
 
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tyho said:
I would equate the octane of gas to the alcohol content of a drink. The higher the content the less one has to drink to get buzzed, making the drink more efficient. The higher the octane, the easier it is for the gasoline to ignite, making it more efficient. FYI, I actually don't drink.
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That's pretty much a perfect analogy tyho. I have a question though...... Given the higher alcohol...errrr... octane content and getting the engine buzzing more efficiently, are the results also similar to the more efficient drink? Will my car refuse to get up the next day and insist on laying around the garage complaining of 'head' aches ..... :Vomit: ;)
 
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Rruuff Day said:
That's pretty much a perfect analogy tyho. I have a question though...... Given the higher alcohol...errrr... octane content and getting the engine buzzing more efficiently, are the results also similar to the more efficient drink? Will my car refuse to get up the next day and insist on laying around the garage complaining of 'head' aches ..... :Vomit: ;)
Click to expand...

Absolutely. Keep it in a dark and quiet room. It will feel better quicker. Enjoy your octane.:Biggrin:
 
I could write a few jolly paragraphs on this but I'll leave you with a couple of useful links. There's a LOT of misinformation and marketing speak on this topic.

Regular or Premium? - Feature

Octane is not a value or measurement of power.
All things being equal, 87 and 91 gas should have the same thermal and chemical energy properties.

And yes there are corner cases where running above your rated octane level can be beneficial.

Resume your boozing... :)
 
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