Best Way To Account For Fuel Pressure Drop
05-24-2009, 01:30 PM
#1
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Best Way To Account For Fuel Pressure Drop In my logs I am seeing at PE, a momentary drop in AFR to the
target value (roughly) and then a relaxation to a leaner level
and significant AFR error. Looking at fuel pressure I see it drop
from 58PSI at idle / cruise, to ~52PSI wide open. Seems like
about what other people say. Fuel filter is pretty new.
Short of replumbing the fuel system for at-rail regulation, we
have to live with some flow/pressure drop. A philosophical
question is, how best to address it.
I have been thinking that maybe the best place is the IFR
table, zero / low vacuum cells. If these were tailored to the
actual fuel pressure (sqrt) seen at various vacuum levels
it ought to fix up the fuel delivery accuracy side of things
(presuming the pump pressure drop tracked manifold vac
cleanly, which is probably not entirely true).
I fo not want to fix it with the VE table or other airflow stuff
because it is a fuel delivery issue. Nor PE / OLFA since it is
not a command problem but a delivery one.
Any other suggestions in a fuel-delivery-model vein?
target value (roughly) and then a relaxation to a leaner level
and significant AFR error. Looking at fuel pressure I see it drop
from 58PSI at idle / cruise, to ~52PSI wide open. Seems like
about what other people say. Fuel filter is pretty new.
Short of replumbing the fuel system for at-rail regulation, we
have to live with some flow/pressure drop. A philosophical
question is, how best to address it.
I have been thinking that maybe the best place is the IFR
table, zero / low vacuum cells. If these were tailored to the
actual fuel pressure (sqrt) seen at various vacuum levels
it ought to fix up the fuel delivery accuracy side of things
(presuming the pump pressure drop tracked manifold vac
cleanly, which is probably not entirely true).
I fo not want to fix it with the VE table or other airflow stuff
because it is a fuel delivery issue. Nor PE / OLFA since it is
not a command problem but a delivery one.
Any other suggestions in a fuel-delivery-model vein?
05-24-2009, 07:15 PM
#2
jimmy, you're correct, as per usual.
things to look into: if you calculate the fuel delivery at 52psi and mate it with the estimated airmass at that moment, what AFR would you get? aka does the fuel seem to be the source of the problem.
start logging and graphing fuel pressure on various axis. since injectors work off MAP and battery voltage, why not graph that? does the FP follow MAP or the voltage more? what other variables tend to have a significant correlation to the FP drop? RPM? airflow? airmass?
if the FP drop occurs purely as a function of MAP, then IFR table would be perfect for expressing it. I have a spreadsheet for it if you want. the better way of dealing with it would be of course to simply eliminate the FP drop altogether, usually with fuel pump, plumbing, or the electrical wiring to the pump itself. If the FP drop depends on something else altogether, then it really becomes a more difficult case to which the answer is completely dependent on the details.
great post though, i hope this will show the people how to properly attribute changes in fueling, not just fudging the numbers with one table, whether it be PE, IFR, MAF, or VE.
things to look into: if you calculate the fuel delivery at 52psi and mate it with the estimated airmass at that moment, what AFR would you get? aka does the fuel seem to be the source of the problem.
start logging and graphing fuel pressure on various axis. since injectors work off MAP and battery voltage, why not graph that? does the FP follow MAP or the voltage more? what other variables tend to have a significant correlation to the FP drop? RPM? airflow? airmass?
if the FP drop occurs purely as a function of MAP, then IFR table would be perfect for expressing it. I have a spreadsheet for it if you want. the better way of dealing with it would be of course to simply eliminate the FP drop altogether, usually with fuel pump, plumbing, or the electrical wiring to the pump itself. If the FP drop depends on something else altogether, then it really becomes a more difficult case to which the answer is completely dependent on the details.
great post though, i hope this will show the people how to properly attribute changes in fueling, not just fudging the numbers with one table, whether it be PE, IFR, MAF, or VE.
05-24-2009, 08:20 PM
#3
Jimmy, Have you checked voltage going to your fuel pump? Larger wiring, and or better pump relay and wiring fixes alot of that. The more "reserve" the pump has, the more it can force thru the regulator when needed. Also a kenne bell boost a pump is another quick fix. Voltage is mostly always the cause of what you are seeing on a NA car. I cheat on most my cars and switch the system to stay 14.7-15.5 volts. that helps alot.
05-26-2009, 11:25 AM
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Jimmy, Have you checked voltage going to your fuel pump? Larger wiring, and or better pump relay and wiring fixes alot of that. The more "reserve" the pump has, the more it can force thru the regulator when needed. Also a kenne bell boost a pump is another quick fix. Voltage is mostly always the cause of what you are seeing on a NA car. I cheat on most my cars and switch the system to stay 14.7-15.5 volts. that helps alot.
That sounds like an intriguing technique.
Thanks.
..WeathermanShawn..
