A short discussion about octane
07-28-2005, 05:52 AM
#1
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Join Date: Nov 2004
Posts: 48
A short discussion about octane
Higher octane fuel resists detonation (knock). That is all. It doesn't necessarily have more heat value in BTU's than a lower octane fuel per unit volume or mass, in fact alcohol has an extremely high octane number but much less mass per volume and therefore fewer miles per gallon.
A modern engine has an engine control computer and a knock sensor. If the the knock sensor detects detonation, the computer retards the ignition timing (causing the spark to occurr much closer to top dead center on the compression stroke.) This in turn reduces the amount of power you can extract from the burning and expanding gas in the combustion chamber, because the peak brake mean effective pressure (BMEP) is lower, and the piston doesn't get as much push.
The higher the Octane number the slower the burn. So, if you run a higher octane fuel, the knock sensor doesn't alarm the computer and the ignition can occurr much earlier (more advanced) in the compression stroke as the piston approaches top dead center. The combustion process or burn takes longer, and pushes the pistion down rather than the detonation being an uncontrolled and rapid hammering of the piston. Therefore your BMEP is higher and you get more power with the same amount of fuel and BTU's being expended.
If your octane were to be too high, such as if you were to run 100/130 octane leaded avgas, the combustion process would take so long that at high rpm it would still be occurring as your exhaust valve began to open. This heats the valve and burns the edges of the valve leading to loss of compression and loss of power over time.
So, don't run any higher octane than is necessary but run at leat what the manufacturer says to run, to keep the knock sensor from robbing you of power which leads to lower mileage and less acceleration and more unnecessary downshifts.
When I changed my knock sensor, even though I always run 91 octane, I noticed several improvements, including a measurable increase in efficiency (from 24 mpg to 27+ mpg.)
A modern engine has an engine control computer and a knock sensor. If the the knock sensor detects detonation, the computer retards the ignition timing (causing the spark to occurr much closer to top dead center on the compression stroke.) This in turn reduces the amount of power you can extract from the burning and expanding gas in the combustion chamber, because the peak brake mean effective pressure (BMEP) is lower, and the piston doesn't get as much push.
The higher the Octane number the slower the burn. So, if you run a higher octane fuel, the knock sensor doesn't alarm the computer and the ignition can occurr much earlier (more advanced) in the compression stroke as the piston approaches top dead center. The combustion process or burn takes longer, and pushes the pistion down rather than the detonation being an uncontrolled and rapid hammering of the piston. Therefore your BMEP is higher and you get more power with the same amount of fuel and BTU's being expended.
If your octane were to be too high, such as if you were to run 100/130 octane leaded avgas, the combustion process would take so long that at high rpm it would still be occurring as your exhaust valve began to open. This heats the valve and burns the edges of the valve leading to loss of compression and loss of power over time.
So, don't run any higher octane than is necessary but run at leat what the manufacturer says to run, to keep the knock sensor from robbing you of power which leads to lower mileage and less acceleration and more unnecessary downshifts.
When I changed my knock sensor, even though I always run 91 octane, I noticed several improvements, including a measurable increase in efficiency (from 24 mpg to 27+ mpg.)
07-28-2005, 07:32 AM
#5
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Join Date: Jul 2005
Posts: 24
I have a problem here..I have a 98 max, pearl mist green, the car is mint, the whole things stock (for now), car hasnt been run in a year and 3 months, just bought it offa lady..the last few days my gas has gone below half way and my car has had a little bit of trouble starting..the starter turns, the battery is an interstate, and is charged..im thinkin i have a clogged fuel injector..or maybe my alternator is shot..can someone help?
any input would be appreciated.
any input would be appreciated.
07-28-2005, 08:42 AM
#6
by your gas going below half, do you mean that you have used some gas so it is below the half mark? Or that its leaking? And by trouble starting, does it start, or not? Check your plugs, fuel filter, air filter, TPS, etc. etc. etc, regrease starter it it takes awhile to start. Your fuel injectors and coil packs you can test yourself with a multimeter, howto in the stickies.
As for the starter of this thread, thank you for smacking all the idiots that run 87 octane.
As for the starter of this thread, thank you for smacking all the idiots that run 87 octane.
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