How does engine/ECM determine octane?
Thread Starter
Joined: Jul 2005
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From: Texas, Europe, Iraq & Afghanistan
How does engine/ECM determine octane? Does anyone know the correct answer to this?
How does the engine/Engine Control Module determine the octane of the fuel?
The General Motors computers have two different fuel tables: Low Octane and High Octane. These tables have different engine timing numbers.
I've heard two different answers.
1.) The engine "tastes" the fuel. Perhaps some kind of specific gravity measurement is taken and this result is used to determine the proper table to use.
2.) Knock sensors determine the table used. If no engine knock is sensed, then the high octane/timing table is used. If knock is detected, then the low octane/timing table is used.
E85 engines add another setting, but knowing how the engine determines what table to use is important to tuning. Moreover, if knock is the determining factor, at what knock level does the computer change tables?
Does anyone know for sure?
Thanks!
How does the engine/Engine Control Module determine the octane of the fuel?
The General Motors computers have two different fuel tables: Low Octane and High Octane. These tables have different engine timing numbers.
I've heard two different answers.
1.) The engine "tastes" the fuel. Perhaps some kind of specific gravity measurement is taken and this result is used to determine the proper table to use.
2.) Knock sensors determine the table used. If no engine knock is sensed, then the high octane/timing table is used. If knock is detected, then the low octane/timing table is used.
E85 engines add another setting, but knowing how the engine determines what table to use is important to tuning. Moreover, if knock is the determining factor, at what knock level does the computer change tables?
Does anyone know for sure?
Thanks!
On The Tree
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The knock sensors determine the signal to the ecu pre programmed retard thresholds.
Therefore limiting the amount of knock retard is the key to improved fuel efficiency regardless of fuel used,each in their merits in conjunction with spark plug heat range,fuel pressures,and density of that fuel.
Therefore limiting the amount of knock retard is the key to improved fuel efficiency regardless of fuel used,each in their merits in conjunction with spark plug heat range,fuel pressures,and density of that fuel.
Last edited by oz304; Nov 11, 2007 at 04:53 PM.
Thread Starter
Joined: Jul 2005
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From: Texas, Europe, Iraq & Afghanistan
Well, both my High and low octane tables are the same. So, no resister is necessary. However, I'm not sure the details of a speed density tune and what tables are tuned.
In any case, without anyone actually saying it, it sounds like the knock sensors determine the engine advance, and not the fuel itself.
Interestingly enough, I've been able to add three degrees advance to both tables on my E85 Avalanche without issue. I only get one degree of knock retard when the engine shifts from V8 to V4 mode. Gas mileage and pep seem to be reliably improved.
Thanks for the feedback!
In any case, without anyone actually saying it, it sounds like the knock sensors determine the engine advance, and not the fuel itself.
Interestingly enough, I've been able to add three degrees advance to both tables on my E85 Avalanche without issue. I only get one degree of knock retard when the engine shifts from V8 to V4 mode. Gas mileage and pep seem to be reliably improved.
Thanks for the feedback!
TECH Senior Member
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From: Texas
so far my basic understanding of tuning is exactly what your saying gregory.
It will run whatever is in the VE / spark advance tables until it gets a signal back, that exceeds the threshold determined by the pcm, from the knock sensors which in turn causes the pcm to pull timing. Essentially you are setting the octane when you dial in your VE tables. Its a fairly simple setup as far as fueling is concerned, determining the type of air can get a little more complicated but I am still learning
It will run whatever is in the VE / spark advance tables until it gets a signal back, that exceeds the threshold determined by the pcm, from the knock sensors which in turn causes the pcm to pull timing. Essentially you are setting the octane when you dial in your VE tables. Its a fairly simple setup as far as fueling is concerned, determining the type of air can get a little more complicated but I am still learning
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On The Tree
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Keep up the good works.
Adding advance or reducing knock retard amount on any fuel,yields better results,as would optimising typical factory 12-14 degrees knock retards to 1 or so degrees on most factory calibrated ecus.
Its amazing what can be done with carefull tuning of the knock sensor retard amounts along with the cruise modes that are carefully leaned as well,even better results become apparent.There is one bloke who claims he uses less than a gallon + a half of fuel per 60 miles on a trip with his lsx tuned engine, with the normal ulp!
Adding advance or reducing knock retard amount on any fuel,yields better results,as would optimising typical factory 12-14 degrees knock retards to 1 or so degrees on most factory calibrated ecus.
Its amazing what can be done with carefull tuning of the knock sensor retard amounts along with the cruise modes that are carefully leaned as well,even better results become apparent.There is one bloke who claims he uses less than a gallon + a half of fuel per 60 miles on a trip with his lsx tuned engine, with the normal ulp!
Last edited by oz304; Nov 12, 2007 at 10:44 AM.
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Thread Starter
Joined: Jul 2005
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From: Texas, Europe, Iraq & Afghanistan
Beats me, but I'll bet the frequency of the LS1 is different from the LS2 as the sensors are located in different areas of the engine.
Any more hints?
Oh, I picked up a reliable 1/2 mile per gallon with the three degrees of advance on both regular and E85.
Any more hints?
Oh, I picked up a reliable 1/2 mile per gallon with the three degrees of advance on both regular and E85.
Last edited by Gregory; Dec 8, 2008 at 03:55 PM.
The Knock Sensor operation is explained here http://delphi.com/manufacturers/auto/sensors/et/flat/. I'll look up the frequency for the LS series in my maintenance manual. I doubt that the frequency is much different for any V8, although I'm sure that the ECM is optomized for the engine series. If knock retard is sensed, the PCM/ECM migrates away from the High Octane advance value toward the Low Octane Table Table value. It's proportional to the amount of KR. If both timing tables are set the same, the PCM can't do anything to reduce knock. In tuning, you want to set the low octane table to be equal to the high octane table to discover where the knock is occurring (by logging the data) and then once, you've gotten KR under control, I like to set the low octane table up for about 7 degrees lower than the high octane table for a daily driver.
Steve
Steve
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I've wondered about this too. I can understand how if the ECU detects ping via the knock sensors, it will pull timing by using a weighted average between the high and low octane tables. But how does it know to advance the timing when e.g. higher octane fuel is used?
It doesn't know when higher octane is used.
When the PCM senses knock, it'll go back to the low octane table, proportional to the amount of knock. It's called the "Low octane table" because it's assumed nothing is wrong, other than you put low octane in the car, which would cause knock.
Get what I'm saying?
When the PCM senses knock, it'll go back to the low octane table, proportional to the amount of knock. It's called the "Low octane table" because it's assumed nothing is wrong, other than you put low octane in the car, which would cause knock.
Get what I'm saying?
Even the people that know what a thorn is can't pronounce it. Don't try.
http://en.wikipedia.org/wiki/%C3%9E
One major reason for this is that Y existed in the printer's type fonts that were imported from Germany or Italy, and Thorn did not.
Yes, I knew that before referencing it on the net.
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From: Western WA
It doesn't know when higher octane is used.
When the PCM senses knock, it'll go back to the low octane table, proportional to the amount of knock. It's called the "Low octane table" because it's assumed nothing is wrong, other than you put low octane in the car, which would cause knock.
Get what I'm saying?
When the PCM senses knock, it'll go back to the low octane table, proportional to the amount of knock. It's called the "Low octane table" because it's assumed nothing is wrong, other than you put low octane in the car, which would cause knock.
Get what I'm saying?
Sorry but I don't believe it.
There must be some method whereby the ECU eventually moves the spark timing up the scale until the knock sensors again detect knock and the ECU backs off. I am wondering what that method is.