typhoon-crazy
New member
is running over 50psi fuel pressure bad?? and i know i should look on a scanner at my O2 volts, but when trying to get the fuel pressure right about what number should i look for on the O2 volts??
quick fuel pressure question
- Thread starter typhoon-crazy
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Daron
Active member
Re: quick fuel pressure question
It isnt bad per se, but may be the wrong question. Depending on your fuel system mods and the boost you are running, going too high (60 psi + 20 psi additional at 20 pounds of boost) may be an issue with the stock pump and the "low pressure" walbro's. O2 readings arent much of a tuning tool.
typhoon-crazy said:
It isnt bad per se, but may be the wrong question. Depending on your fuel system mods and the boost you are running, going too high (60 psi + 20 psi additional at 20 pounds of boost) may be an issue with the stock pump and the "low pressure" walbro's. O2 readings arent much of a tuning tool.
Use 'Block Learn Memory' (BLM) to 'see' your long term fuel delivery correction. 128 is 0% correction. Numbers less than 128 = computer is adjusting injection of LESS fuel because your FP is too high or other rich problem. Numbers greater than 128 = computer is calculating an injection of more fuel because of low FP or other lean problem.
BLM is stored in 'cells'. Each cell is based on RPM and LOAD. I forget how many cells for the SyTy ECM, but if you look at the mid cells, it should give you a good idea of what's happening.
Also, don't read BLM immediately after adjusting FP, it takes some time to update. The O2 sensor is first watched by the Integrator (INT) when in closed loop. If the O2 sensor is cycling off center (of 0.450V), the Integrator changes above or below 128 (128 is 0% correction) to add or subtract fuel. This is short term fuel correction. The BLM watches these numbers over several minutes before it begins to add or subtract fuel. The BLM stops updating when the INT returns to 128.
I would suggest BLM at 123-125 as a good average setting unless you have a problem with WOT spark knock (not heard, but seen as overactive knock signal and excessive retard). You could go down to 120 but keep in mind that too high a fuel pressure can actually decrease power by flooding out (quenching) the combustion process.
BLM is stored in 'cells'. Each cell is based on RPM and LOAD. I forget how many cells for the SyTy ECM, but if you look at the mid cells, it should give you a good idea of what's happening.
Also, don't read BLM immediately after adjusting FP, it takes some time to update. The O2 sensor is first watched by the Integrator (INT) when in closed loop. If the O2 sensor is cycling off center (of 0.450V), the Integrator changes above or below 128 (128 is 0% correction) to add or subtract fuel. This is short term fuel correction. The BLM watches these numbers over several minutes before it begins to add or subtract fuel. The BLM stops updating when the INT returns to 128.
I would suggest BLM at 123-125 as a good average setting unless you have a problem with WOT spark knock (not heard, but seen as overactive knock signal and excessive retard). You could go down to 120 but keep in mind that too high a fuel pressure can actually decrease power by flooding out (quenching) the combustion process.
SyTy's have two cells- open and closed throttle.
The BLM is only updated during cruise. If the ECM is adding fuel (BLM above 128) then it will take that into account during WOT. If it is below 128 it will not subtract any fuel out.
Use the O2 sensor for trend information. 900-1000mv is Usually safe with a good 02 sensor. Look at the other parameters in conjuntion with the 02 reading.
A local Ty needed 50psi after an injector change. My truck runs very rich on the stock calibration at 50 psi.
The BLM is only updated during cruise. If the ECM is adding fuel (BLM above 128) then it will take that into account during WOT. If it is below 128 it will not subtract any fuel out.
Use the O2 sensor for trend information. 900-1000mv is Usually safe with a good 02 sensor. Look at the other parameters in conjuntion with the 02 reading.
A local Ty needed 50psi after an injector change. My truck runs very rich on the stock calibration at 50 psi.
Thanks, had to refresh old GM Training memory. This means 'power enrichment' mode would be more rich if BLM were at 123-125 (during cruise). Did I get that right? Would that be too much with a stock chip and no mods?Sy769 said:
Are you looking at the O2 trends for this or at BLM? Did BLM eventually correct for the higher pressure setting?Sy769 said:
typhoon-crazy
New member
unfortunatly i dont have dm. im using auto exray :-? . but im tyring to run 18psi on the stock turbo with 33# injectors and found that when i run about 55 psi or so i dont get any kr and the truck seems to pull alot better. thanks for all the info 
Yes, boost pressure is applied to the vacuum line of the fuel pressure regulator to assist the spring that controls base fuel pressure. This gives you a one psi FP 'increase' for each one psi of boost pressure.
When at idle and part-throttle, vacuum is applied to the fuel pressure regulator to work against the spring that controls base fuel pressure. This gives you one psi FP 'decrease' for every two inches Hg of vacuum.
The objective of this 'automatic' pressure adjustment is to keep the pressure drop at the tips of the injectors constant under all operating conditions. If not adjusted, intake vacuum would 'pull' extra fuel from the injectors causing the engine to run rich and boost pressure in the intake would 'push' back on the injectors, causing the engine to run lean. This is, of course, opposite of how you want it to work.
When at idle and part-throttle, vacuum is applied to the fuel pressure regulator to work against the spring that controls base fuel pressure. This gives you one psi FP 'decrease' for every two inches Hg of vacuum.
The objective of this 'automatic' pressure adjustment is to keep the pressure drop at the tips of the injectors constant under all operating conditions. If not adjusted, intake vacuum would 'pull' extra fuel from the injectors causing the engine to run rich and boost pressure in the intake would 'push' back on the injectors, causing the engine to run lean. This is, of course, opposite of how you want it to work.
TyMeUp said:
Yeah sort-of. More simply stated it maintains the pressure differential equal to your base fuel pressure. If you have 50 psi base then at 10 psi boost your fuel pressure will be 60psi hence the 50psi differential is maintained.The opposite is true for operation in vac.
You have to look at the whole picture with the stock O2. The Ty I mentioned had a lot of knock and low 02's after the injector change. Also the histogram is a nice tool to look at in datamaster. The Int and BLM both indicated it was lean. We kept raising the fuel pressure a few lb.s at a time until most of the knock was gone and the BLM stayed around 128. When you get the pressure to high the truck will run like crap at idle and off idle not to mention killing spool and track times.
Like I said you have to be careful trusting the O2 readouts. Another truck was reading high 700's to low 800's until we replaced the sensor. After that with no other changes it read low 900's.
There are some crafty syty tuners out there that can shed a lot more light on this subject than me.
I have a WB box almost ready to go. A very handy tool compared to the stock sensor.
Like I said you have to be careful trusting the O2 readouts. Another truck was reading high 700's to low 800's until we replaced the sensor. After that with no other changes it read low 900's.
There are some crafty syty tuners out there that can shed a lot more light on this subject than me.
I have a WB box almost ready to go. A very handy tool compared to the stock sensor.