PE vs. RPM Trouble - Help Needed!
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 1,209
Likes: 0
From: Toronto Ontario
St. Jude Donor '13
PE vs. RPM Trouble - Help Needed!
Need to clarify PE understanding. The PE vs. RPM table has negetive and positive values depending on the tune and what not.
Percent Change verses RPM
Example --> 2800 0.8 or another Example --> 3200 -0.8
What does the positive or negative number mean? Is it modifying the AFR by that percent? Or is adding or subtracting fuel by that percent?
A negative fuel percentage would numerically increase the AFR (making the mixture leaner) and a negative AFR would numerically lower the AFR (richer) number. So you can see the predicament and importance of understanding this modifier.
Need some clarification on this,
Thanks,
Victor
Percent Change verses RPM
Example --> 2800 0.8 or another Example --> 3200 -0.8
What does the positive or negative number mean? Is it modifying the AFR by that percent? Or is adding or subtracting fuel by that percent?
A negative fuel percentage would numerically increase the AFR (making the mixture leaner) and a negative AFR would numerically lower the AFR (richer) number. So you can see the predicament and importance of understanding this modifier.
Need some clarification on this,
Thanks,
Victor
Last edited by 89'Bowtie; Sep 12, 2005 at 08:37 PM.
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
Originally Posted by 89'Bowtie
Need to clarify PE understanding. The PE vs. RPM table has negetive and positive values depending on the tune and what not.
Percent Change verses RPM
Example --> 2800 0.8 or another Example --> 3200 -0.8
What does the positive or negative number mean? Is it modifying the AFR by that percent? Or is adding or subtracting fuel by that percent?
A negative fuel percentage would numerically increase the AFR (making the mixture leaner) and a negative AFR would numerically lower the AFR (richer) number. So you can see the predicament and importance of understanding this modifier.
Need some clarification on this,
Thanks,
Victor
Percent Change verses RPM
Example --> 2800 0.8 or another Example --> 3200 -0.8
What does the positive or negative number mean? Is it modifying the AFR by that percent? Or is adding or subtracting fuel by that percent?
A negative fuel percentage would numerically increase the AFR (making the mixture leaner) and a negative AFR would numerically lower the AFR (richer) number. So you can see the predicament and importance of understanding this modifier.
Need some clarification on this,
Thanks,
Victor
Victor, the smaller the number the less fuel the ecm/pcm will fuel the engine during PE. Also PE engages only after the PCM sees that the TPS threshold for PE has been exceeded.
WOT (PE) fuel is calculate in an EE PCM (94/95) by this equation:
WOT AFR = 14.7 / (1 + %Change vs Cool/100 + %Change vs RPM/100)
For the selected RPM, suppose the percent change to Fuel/Air Ratio from the RPM table is = - 5.1 and the percent change to Fuel/Air Ratio from the COOLANT table at the selected coolant temperature is = 22.7, then the approximate WOT AFR would be:
14.7 / (1 -0.051+.227) = 12.5:1 AFR
Negative numbers are used, and it just means less fuel. It is just a constant that the PCM uses to calculate PE fuel. I made an excel spreadsheet to help calculate PE Fueling which makes it real easy to estimate fueling during PE.
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 1,209
Likes: 0
From: Toronto Ontario
St. Jude Donor '13
Pe
Thanks TJ for the great explanation, that helps clear it right up. From your knowledge would it be safe to assume the 1996 ECM utlilizes that same formula?
Thanks,
Victor
Thanks,
Victor
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 1,209
Likes: 0
From: Toronto Ontario
St. Jude Donor '13
Afr Table
Just as an after thought, if the ECM is using the above formula to calculate WOT FUELING, then what purpose does the WOT AFR table serve and how does it work with the PE's (temp/rpm, etc)?
It would appear to me that the PE's have a greater effect on AFR than the actual WOT AFR table. I recall some people measuring the factory AFR to be overly rich (10.5-11.00 at WOT) in a stock LT1 tune even though the factory has 12.9-13.00’ish in the WOT table at operating temp. I am assuming the PE's would be the culprit in driving the mixture that rich.
Thanks
Victor
It would appear to me that the PE's have a greater effect on AFR than the actual WOT AFR table. I recall some people measuring the factory AFR to be overly rich (10.5-11.00 at WOT) in a stock LT1 tune even though the factory has 12.9-13.00’ish in the WOT table at operating temp. I am assuming the PE's would be the culprit in driving the mixture that rich.
Thanks
Victor
