FUEL: Premium versus Regular
Safety Car
Joined: Jan 2015
Posts: 3,941
Likes: 499
From: Northern Virginia
Gasoline as it comes out of the ground from Mother Nature has an octane of about 60.
Safety Car
Joined: Jan 2015
Posts: 3,941
Likes: 499
From: Northern Virginia
No, SMFCPACFP is correct. The higher the compression ratio, the higher octane fuel that you need. The more sophisticated computerized engine controls in today's cars allow for much higher compression ratios for a given octane level than could be used in yesteryear's cars, but the physics is still the same.
Last edited by meyerweb; Apr 20, 2015 at 08:17 PM.
Safety Car
Joined: Jan 2015
Posts: 3,941
Likes: 499
From: Northern Virginia
Regular unleaded gasoline rated at
87 octane or higher can be used,
but acceleration and fuel economy
will be reduced, and an audible
knocking noise may be heard. If this
occurs, use a gasoline rated at
91 octane or higher as soon as
possible. Otherwise, the engine
could be damaged.
87 octane or higher can be used,
but acceleration and fuel economy
will be reduced, and an audible
knocking noise may be heard. If this
occurs, use a gasoline rated at
91 octane or higher as soon as
possible. Otherwise, the engine
could be damaged.
Advanced
Joined: Jan 2004
Posts: 73
Likes: 2
From: Los Angeles CA
Wow! I can hardly believe all the misinformation and ignorance in this thread. IC gasoline engines are not simple and complex. The simple is higher compression demands higher octane(slower burning, harder to ignite). Other factors are heat, spark advance and air/fuel ratio.
In the good old days of the late 1960s engines were made entirely of cast iron: block, I take and heads. Cast iron is a slow heat transmitter. So a cast iron head with a 10:1 compression ratio required damn near 100 octane to not knock and ping. Then aluminum heads were invented which transferred heat very quickly and suddenly you could run 10.5:1 compression on 93 octane.
The EFI and knock sensors allowed a 9.5:1 engine to run on 87 octane, sometimes higher depending on the size of the engine. Smaller displacement cylinders could run higher compression with the low octane fuel. My BMW K1300S for example has a 13:1 compression ratio, makes 175 HP and can run on 89 octane due to the knock sensors. On hot days with 89 and sometimes 91 it will knock a bit a low rpm until the sensor tells the computer to retard the timing. This "allows" it to run on low octane fuel, but at reduced power and efficiency. To get full power in all conditions it needs 93 octane.
The corvette has another technological trick called direct injection. This squirts the fuel into the cylinder under very high pressure at just the right moment to be ignited. So you can run 11:1 or slightly higher on pump gas, but again anything less than 93 octane and the ecu is pulling timing and richening the mixture to combat knock and ping. So while it will run on 87, it will do so at reduced power and efficiency.
The point of all this is the engine in the Vette is designed to run optimally on 93 octane, anything lower and the sensors and ecu keep the engine from becoming a grenade. Don't believe me? Get one of those Bluetooth OBD dongles and a monitor program for your smartphone and have it log knock sensor and timing activity. You will be shocked how often knock is detected and timing is retarded. If a knock sensor fails on high octane fuel, no big deal, but on 89 or worse 87 on a very hot day, you will get very severe knock.
Just because you can't doesn't mean you should. The low octane capability is there to protect idiots and cheapskates.
In the good old days of the late 1960s engines were made entirely of cast iron: block, I take and heads. Cast iron is a slow heat transmitter. So a cast iron head with a 10:1 compression ratio required damn near 100 octane to not knock and ping. Then aluminum heads were invented which transferred heat very quickly and suddenly you could run 10.5:1 compression on 93 octane.
The EFI and knock sensors allowed a 9.5:1 engine to run on 87 octane, sometimes higher depending on the size of the engine. Smaller displacement cylinders could run higher compression with the low octane fuel. My BMW K1300S for example has a 13:1 compression ratio, makes 175 HP and can run on 89 octane due to the knock sensors. On hot days with 89 and sometimes 91 it will knock a bit a low rpm until the sensor tells the computer to retard the timing. This "allows" it to run on low octane fuel, but at reduced power and efficiency. To get full power in all conditions it needs 93 octane.
The corvette has another technological trick called direct injection. This squirts the fuel into the cylinder under very high pressure at just the right moment to be ignited. So you can run 11:1 or slightly higher on pump gas, but again anything less than 93 octane and the ecu is pulling timing and richening the mixture to combat knock and ping. So while it will run on 87, it will do so at reduced power and efficiency.
The point of all this is the engine in the Vette is designed to run optimally on 93 octane, anything lower and the sensors and ecu keep the engine from becoming a grenade. Don't believe me? Get one of those Bluetooth OBD dongles and a monitor program for your smartphone and have it log knock sensor and timing activity. You will be shocked how often knock is detected and timing is retarded. If a knock sensor fails on high octane fuel, no big deal, but on 89 or worse 87 on a very hot day, you will get very severe knock.
Just because you can't doesn't mean you should. The low octane capability is there to protect idiots and cheapskates.
Safety Car
Joined: Jul 2001
Posts: 4,661
Likes: 67
From: Ocean Springs MS
Yes, it probably is just a little under 90 octane, but doubt the engine can even tell the difference unless tracking the car..
Kind of go by what the knock vibration sensors are telling me.. My other car is a Shelby GT 350 which recommends 91 octane as well, but have never heard the marbles in the engine sounds in it either under hard accel..
Kind of go by what the knock vibration sensors are telling me.. My other car is a Shelby GT 350 which recommends 91 octane as well, but have never heard the marbles in the engine sounds in it either under hard accel..
Safety Car
Joined: Jan 2015
Posts: 3,941
Likes: 499
From: Northern Virginia
Wow! I can hardly believe all the misinformation and ignorance in this thread.
In the good old days of the late 1960s engines were made entirely of cast iron: block, I take and heads. Cast iron is a slow heat transmitter. So a cast iron head with a 10:1 compression ratio required damn near 100 octane to not knock and ping. Then aluminum heads were invented which transferred heat very quickly and suddenly you could run 10.5:1 compression on 93 octane.
In the good old days of the late 1960s engines were made entirely of cast iron: block, I take and heads. Cast iron is a slow heat transmitter. So a cast iron head with a 10:1 compression ratio required damn near 100 octane to not knock and ping. Then aluminum heads were invented which transferred heat very quickly and suddenly you could run 10.5:1 compression on 93 octane.
Today's gasoline is rated by something called the anti-knock index (AKI), which is the average of RON and MON {(R+M)/2}.
So the 100 octane number you refer to and the 93 AKI number are not even remotely comparable, and trying to compare one to the other as a measure of automotive technology is totally meaningless.
There's no formula you can use to map AKI to RON, as different fuels will have somewhat different RON to MON variances. But, in general, RON runs 4 to 6 points higher than AKI, so you can figure 93 octane pump gas is roughly the same, in terms of knock resistance, as 97-99 octane gas from the 60s.
Not quite the massive difference you seem to think.
More info here: http://en.wikipedia.org/wiki/Octane_rating
The point of all this is the engine in the Vette is designed to run optimally on 93 octane, anything lower and the sensors and ecu keep the engine from becoming a grenade.
....Just because you can't doesn't mean you should. The low octane capability is there to protect idiots and cheapskates.
....Just because you can't doesn't mean you should. The low octane capability is there to protect idiots and cheapskates.
Last edited by meyerweb; Apr 21, 2015 at 11:42 AM.
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