89 octane vs 93 octane
03-15-2016, 05:14 AM
#16
The heat content of gasoline is identical between 87 and 93 octane. Octane rating pertains to the ability to resist combustion due to compression (compressing things makes them heat up). Our engines run with a compression ratio of 11.0 to 1. That is very high.
Higher compression ratio improves the efficiency of the engine and is why a diesel engine is more fuel efficient. It also increases the likelihood of self-ignition of the air-fuel mixture. If the fuel starts burning before TDC (top dead center) you will get knock or pinging as the pressure is trying to force the piston backwards from it's upward motion. Get that too severe and it will blow the engine.
Engines use computers and knock sensors to track when this is occurring and will adjust timing and fuel delivery to avoid it. We can run 87 Octane in our cars but we will get lower hp. Fuel economy might be the same running at part throttle but you will definitely not have the same hp or immediate throttle response. Most importantly, this does not mean there isn't some low level of pinging going on in the engine that you might not detect. It might not do any permanent damage but I'd rather run with the 91/93 octane.
It takes 1.5 gal of ethanol to get the same heat content as 1 gal of gasoline. That's why your hp and fuel economy go down when you run a mixture, as I'm often forced to do here in AZ. Ethanol does have a higher octane rating so theoretically you can get some of that back by running a higher compression ratio.
Theoretically you can get the same hp from either but to get there engines will start burning the fuel at or even before top dead center.
Higher compression ratio improves the efficiency of the engine and is why a diesel engine is more fuel efficient. It also increases the likelihood of self-ignition of the air-fuel mixture. If the fuel starts burning before TDC (top dead center) you will get knock or pinging as the pressure is trying to force the piston backwards from it's upward motion. Get that too severe and it will blow the engine.
Engines use computers and knock sensors to track when this is occurring and will adjust timing and fuel delivery to avoid it. We can run 87 Octane in our cars but we will get lower hp. Fuel economy might be the same running at part throttle but you will definitely not have the same hp or immediate throttle response. Most importantly, this does not mean there isn't some low level of pinging going on in the engine that you might not detect. It might not do any permanent damage but I'd rather run with the 91/93 octane.
It takes 1.5 gal of ethanol to get the same heat content as 1 gal of gasoline. That's why your hp and fuel economy go down when you run a mixture, as I'm often forced to do here in AZ. Ethanol does have a higher octane rating so theoretically you can get some of that back by running a higher compression ratio.
Theoretically you can get the same hp from either but to get there engines will start burning the fuel at or even before top dead center.
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2008G-Man (03-16-2016)
03-16-2016, 01:55 PM
#17
Why are you looking to use a lower octane? To save some cash?
So our tanks are 20 gallons.. Lets say that you are an extreme scenario and you are literally running on fumes twice a week and you go to fill up with 89 octane instead of 93 octane. Right now I am seeing 89 at $2.00 and 93 at $2.30 per gallon (local station nearby)
So in view of the foregoing, you have saved a maximum of $6.00 per fill up/ $12.00 per week. And that is an extreme scenario.
Most people only fill up once a week. So most will save $6.00 a week/ $24.00 a month
Frankly, and I don't know your financial situation, I can think of must less risky ways to cut back. Like getting a car that is actually designed to run optimally on 87 octane. The car itself will probably be less expensive to both purchase and maintain. So much so that it may save you even more than $6.00 a week.
So our tanks are 20 gallons.. Lets say that you are an extreme scenario and you are literally running on fumes twice a week and you go to fill up with 89 octane instead of 93 octane. Right now I am seeing 89 at $2.00 and 93 at $2.30 per gallon (local station nearby)
So in view of the foregoing, you have saved a maximum of $6.00 per fill up/ $12.00 per week. And that is an extreme scenario.
Most people only fill up once a week. So most will save $6.00 a week/ $24.00 a month
Frankly, and I don't know your financial situation, I can think of must less risky ways to cut back. Like getting a car that is actually designed to run optimally on 87 octane. The car itself will probably be less expensive to both purchase and maintain. So much so that it may save you even more than $6.00 a week.
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