LT4_CE

I'm in California, which for the most part, has 91 octane as the highest octane fuel available, outside of using racing fuel. In looking at the 2017 Corvette owner's manual it advises using 93 octane, and cautions against using fuel with a posted octane that is lower.


Directly following that caution is a section called California Fuel Requirements. It says that if the vehicle is certified to meet California Emissions Standards, it is designed to operate on fuels that meet California specifications. It goes on to mention a couple other things to keep Chevy from having to deal with fuel related repair issues.


I did a keyword search on octane in this C7 Tech/Performance sub-forum, and didn't find an answer to this question, which is, in California, with my vehicle that I assume is certified to meet California Emissions Standards, would there be a problem for my new Grand Sport if I use 93 octane fuel?


Which leads to my second question. I can't buy 93 octane, but I can get 100 or 101 in a couple locations, and mix the two. A 10 to 3 or 3 to 1 ratio with 100 octane and 93 octane works out (mathematically) to just over 93 octane. So, is it okay to mix fuels with different octanes to get to the desired 93 octane?


Thanks in advance for any info that's provided!

X25

You really don't need to. The engine would work even with 87 octane, but with reduced power. 91 octane is perfectly fine.

I live in PNW and use 92 octane fuel. Never felt the need to mess with race fuel, etc.

LT4_CE

Thanks. That was fast! It would follow that power would be increased when using 93 compared to 91. I guess I want more power... no particular reason.


Also, I guess I'm getting at a more general quesstion about whether there are any negatives to using a mixture of 91 and 100 octane in order to get to roughly 93 octane? Any chance of causing harm to the motor?

JoesC5

No stock Corvette has ever been required to run 93 octane. Some Corvettes, such as my C6 Z06 REQUIRES a minimum of 91 octane, but GM RECOMMENDS using 93 octane in the C6 Z06.

I can tell a slight difference between 93 and 91(both are available where I live) in my LS7, but I doubt that the LT1 would see much difference in performance running 93(but sure wouldn't hurt performance). The LT4 would probably notice a difference running 93 octane.

X25

One issue with mixtures will be the consistency. The engine controller learns the capabilities of the fuel in time, and each time you fill the tank, unless you're careful at the precise/consistent mixture, it will have to relearn. Obviously, learning happens all the time, but it'd be better for the engine to keep having similar fuel be it 91 or 98.

only9balls

I ran an 11.81 in the quarter in my stock C7 Z51 at Sacramento Raceway on 91 octane, it is all I use, I drive the crap out of it, and it works just fine

It runs extremely well on 91.

tblu92

Weekend racers often mix race fuel with standard calif 91 oct fuel to raise the oct level
No problem there at all----as long as it is all unleaded fuel
As mentioned these new DI engines because of their superior designed combustion chambers can run very well even with poor fuel or low octane fuel---
The ECM will recognize the poor fuel and remove timing accordingly as well as the VVT system will alter valve timing as well----
When going to better fuel like 91 it will add some more timing--nothing huge maybe 1-2 degrees----almost un noticeable
I don't think you would see any gains however from going from 91 to 93 at all---It's an overkill for a DI engine----With a STOCK tune and a stockish bolt on engine
Now on a heavily modded engine things may change somewhat but the only way to know for sure is to have your tuner data log your engine on the dyno and see what timing you engine like the most
PS: as an example on my 2014 car converted to E85 My ECM will add a massive 6* of timing at WOT with 91 Oct fuel--with no hint or detonation ---Even tried it with 87 regular fuel and it still did not detonate and still added 6* of timing at WOT

Mike Jesse

Mixing the higher octane fuel is just throwing money away.
Totally unnecessary.
The only thing I would do is stay with one name brand if possible.
I've been using Shell 92 octane in mine from day one without issue.
Plus my local station still has the separate hoses for the different octane's.

JoesC5

A retired oil company engineer said it's best to rotate gas from different Top Tier brands as each uses a slightly different additive package and rotating the brands will do a better job of keeping the engine clean.

Kracka

https://find93.com/

angryBits

I also live in the SF Bay Area. There's a gas station in Redwood City that sells race gas, I used to use in my ZL1 which ran like crap on CA 91 gas.

However, your Grand Sport will run just fine on 91 I promise, its really only supercharged or turbocharged cars that have a problem with 91 octane (my ZL1 would knock and I had to get it tuned and void my warranty).

So far I've run through 3 tanks of 91 and zero issues. I'm mostly done with break-in and have begun some wide-open-throttle and there have been no signs of knocking. I would even go as far to say your car will probably temporarily de-tune itself on 89 and 87 just fine, you'll just get temporary reduced power until you go back to 91.

Having said that -- being in CA with 91, you're likely not getting the same performance figures everyone else is -- expect a drop of 10hp or so due to 91 vs 93.

angryBits

Useful website, unfortunately it revealed no 93 octane in the entire SF Bay Area, however there are 2 gas stations with 108 race gas but if I recall you'll pay like $5-6 per gallon. 91 around here is typically $3.50 per gallon these days.

only9balls

91 is $2.89 at Chevron up by Folsom... That is amazing there is that much price swing from here to the North Bay like that.

only9balls

Knock can virtually be eliminated with the use of direct injection, that is one of the benefits of having DI on these cars. The fuel gets atomized as it is sprayed in, and is not injected into the cylinder until the piston is already on the way up on the compression stroke, so pre-ignition is no longer an issue.

angryBits

WHAT?!?! I did not know this! That's awesome, I have to do some research

X25

Keep in mind, the manufacturers take advantage of this by increasing the compression, which is why LT1 has as much torque as LS7 in most of its powerband. In other words, in the end of the day, the safety margin is already exploited to some extent, but GM always takes into account that someone will put in 87 octane into its NA cars, so it should be OK : )

only9balls

I also decided to read more about it, and it would appear (according to wikipedia), that there are mixed stages of fuel delivery listed under the theory of operation. I am not sure if this is true with the LT1/LT4 or not, but in conversation, I have been told that the reason these cars are capable of running on octanes as low as 87, is because DI affords some advantages in the combustion process, that was not previously achievable in TPI. More precisely, the timing and amount of fuel can be very accurately applied now. So my previous statement may be a bit too general.

PULLED FROM WIKIPEDIA:

- Ultra lean burn or stratified charge mode is used for light-load running conditions, at constant or reducing road speeds, where no acceleration is required. The fuel is not injected at the intake stroke but rather at the latter stages of the compression stroke. The combustion takes place in a cavity on the piston's surface which has a toroidal or an ovoidal shape, and is placed either in the center (for central injector), or displaced to one side of the piston that is closer to the injector. The cavity creates the swirl effect so that the small amount of air-fuel mixture is optimally placed near the spark plug. This stratified charge is surrounded mostly by air and residual gases, which keeps the fuel and the flame away from the cylinder walls. Decreased combustion temperature allows for lowest emissions and heat losses and increases air quantity by reducing dilation, which delivers additional power. This technique enables the use of ultra-lean mixtures that would be impossible with carburetors or conventional fuel injection.

- Stoichiometric mode is used for moderate load conditions. Fuel is injected during the intake stroke, creating a homogeneous fuel-air mixture in the cylinder. From the stoichiometric ratio, an optimum burn results in a clean exhaust emission, further cleaned by the catalytic converter.

- Full power mode is used for rapid acceleration and heavy loads (as when climbing a hill). The air-fuel mixture is homogeneous and the ratio is slightly richer than stoichiometric, which helps prevent pinging. The fuel is injected during the intake stroke.