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that was my take on the question .. if my car gets 30 MPG on 87 will it get 33 on 93 ?
Everything that I have read is no.
On another note - if the manufacturers thought that there would be more MPG then they would advocate the premium fuel. Anything for an edge over the competition. CAFE standards are for the entire line of cars and are not car specific.
But hypothetically IF you got 3 MPG more - with the higher cost of premium fuel - would you actually save any money?
My car will run on anything, but they recommend premium. It is supercharged, with a high compression ratio. In normal driving, if lower octane gas is used, the car will deal with it by dumping extra fuel to reduce or eliminate detonation. So, due to the extra fuel, you'll lose gas mileage as well as power. People checked and there was no benefit eco Impala to using lower octane gas.
Now, if your car doesn't need it, it shouldn't make more power or give you better mileage.
that was my take on the question .. if my car gets 30 MPG on 87 will it get 33 on 93 ?
It might, if the car is actually set up for premium, and on 87 you are running a retarded spark.
Old school cars with simple timing adjustments on the distributor might be able to get better MPG - enough to pay for the difference in fuel cost - by running more spark advance and using premium fuel.
Some cars build up carbon on the piston crowns such that when new they ran fine on 87 but with some miles on them, particularly in summer, they will ping on 87 and need at least mid-grade to run "right".
That said, if your car is designed for 87 and it runs without pinging on 87 - higher octane is not going to help you.
This thread is confusing given the subject mentions diesel but the post compares regular to high octane rated unleaded.
In any case, regarding unleaded gasoline specifically, the relation with octane rating is very commonly misunderstood to mean "slower burning" or "more power". Neither is true. Any common non-diesel road going car burns its air/fuel mixture at about the same rate, the only thing the addition of chemicals to raise the octane rating does is raise the compression the mixture can withstand before compression ignition would occur. This is the reason high compression engines, turbo'd engines, etc. often require higher octane-rated fuel; that ensures the mixture does not self-ignite before the spark plug does it and you don't have knocking which could damage the engine. It has nothing to do with slowing the burn.
What does affect the speed at which a fuel mixture burns is the specific gravity of the fuel. Some fuel vendors will make this information available to you, but it's not very common outside of racing gas suppliers. For example, the unleaded you often find at race tracks can be 98 octane or higher, but may also have a much different specific gravity than regular 87 or 93 octane pump gas. The lighter the SG, the faster it will burn, and race gas often has a lighter SG because it's built for engines running at much higher rpm than road going cars. This gets people into trouble a lot; they take their regular car on the track and think I'm gonna load up with 98 octane "race gas" to get more power, and all they get in return is a car that overheats before the end of the session because the engine wasn't meant for that type of gas with its faster burn.
Getting back to the original posted subject, rather than the post itself, comparing high octane gasoline to diesel; diesel burns slower than any gasoline regardless of octane, because it has a higher specific gravity, it is more dense, therefore it has more energy by volume. The octane plays no role in the comparison, diesel will always burn slower than gasoline.
Simple, most modern cars run a specific compression ratio which is a measure of how much room there is available for fuel when the piston is at the bottom and top of the cylinder. Indeed, your average gasl and diesel powered car, thanks to a fairly common compression ratio of, say, eight to one, is designed to tolerate lower octane fuels. So the regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money”.
Last edited by Vannort54; 09-30-2015 at 02:08 PM..
This thread is confusing given the subject mentions diesel but the post compares regular to high octane rated unleaded.
In any case, regarding unleaded gasoline specifically, the relation with octane rating is very commonly misunderstood to mean "slower burning" or "more power". Neither is true. Any common non-diesel road going car burns its air/fuel mixture at about the same rate, the only thing the addition of chemicals to raise the octane rating does is raise the compression the mixture can withstand before compression ignition would occur. This is the reason high compression engines, turbo'd engines, etc. often require higher octane-rated fuel; that ensures the mixture does not self-ignite before the spark plug does it and you don't have knocking which could damage the engine. It has nothing to do with slowing the burn.
What does affect the speed at which a fuel mixture burns is the specific gravity of the fuel. Some fuel vendors will make this information available to you, but it's not very common outside of racing gas suppliers. For example, the unleaded you often find at race tracks can be 98 octane or higher, but may also have a much different specific gravity than regular 87 or 93 octane pump gas. The lighter the SG, the faster it will burn, and race gas often has a lighter SG because it's built for engines running at much higher rpm than road going cars. This gets people into trouble a lot; they take their regular car on the track and think I'm gonna load up with 98 octane "race gas" to get more power, and all they get in return is a car that overheats before the end of the session because the engine wasn't meant for that type of gas with its faster burn.
Getting back to the original posted subject, rather than the post itself, comparing high octane gasoline to diesel; diesel burns slower than any gasoline regardless of octane, because it has a higher specific gravity, it is more dense, therefore it has more energy by volume. The octane plays no role in the comparison, diesel will always burn slower than gasoline.
Go back to the original question. Maybe it can be interpreted differently.
The way I observed it was:
"I have a car the runs on regular gasoline. If I put in premium gasoline, will it run better and will I get better gas mileage."
That answer is no.
Don't think anyone is advocating putting low octane gasoline in a car that requires high octane.
Modern cars detect ping and retard the timing to prevent engine damage. This markedly reduces engine power and efficiency. Low octane will sometimes result in lower mileage. The great mileage out of turbocharged engines is partly the result of higher compression ratios, which allows more of the energy to be extracted from the burned fuel. So the answer is not "no," the answer is, "it depends."
Diesel engines are more efficient for several reasons. Higher compression ratios are one. Higher carbon (more energy) fuel is one. Less restrictions in intake and exhaust also contribute.
It is supercharged, with a high compression ratio.
No, turbo and supercharged engines are actually designed with LOWER compression ratio pistons, then the compression ratio is brought back up with the turbo/super charger.
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