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# Differential Fuel Pressure compensation table

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Hi,

I´m trying to build a Differential Fuel Pressure compensation table to account for variations across rpm and boost.

My Fuel differential pressure goal is 400Kpa and the FPR holds it most of the time but at high RPM and load it drops sometimes to around 370 kpa.

FPR is 1:1 rising rate, map referenced.

Hi need to know how much fuel % i have to add on each site on the table to compensate for this loss, does somenone has a formula to calculate this ?

Do you have a data log showing your Differential Fuel Pressure, Lambda Target and Measured Lambda?

We could calculate the necessary correction, although I bet it will be close to the % difference in the fuel pressure.

A 10% drop in Differential Fuel Pressure would need a 10% increase in delivered fuel. For example, the 350 kPa would be 350/400 = .875 (a 12.5% drop), so you would make the trim be 112.5 as a starting point. You could then use your data logs to fine tune that.

The relationship between pressure and flow is exponential, not linear.

The multiplier would be √(base pressure/measured pressure). Multiply that by 100 if you want the result in percent.

So an example, if your base pressure is 400kpa and your actual pressure is 350kpa, the correction required would be √400/350 = 1.143. Or in percent 114.3%.

oops you forgot the square root ... √400/350 = 1.069 or 106.9%

If we knew what measured Lambda vs. Target Lambda, then the correction would be "have over want" or Measured / Target. If the compensation table was not 100, then you would multiply the current value by the ratio calculated.

“oops you forgot the square root ... √400/350 = 1.069 or 106.9%”

Lol. Thanks for the save. The intent was there but my burning toast got in the way.

Thanks for the replies !

I attached a picture of a log with RPM,MAP, target and measured lambda, fuel pressure delta and correction.

Tested adding 1% of fuel below 380Kpa but in some areas is not enough. I´m targeting 0,78Lambda.

I´m going to test that formula and capture some logs to see, should look like this, right ?

Target, measured, Correction %

400, 350, 107%

400, 360, 105%

400, 370, 104%

400, 380, 103%

400, 390, 101%

400, 400, 100%

So that trace shows that either the compensation is already happening in the background, you have closed loop fueling enabled and it's compensating, or you "baked in" these changes in your fuel map. I say that because your measured lambda is tracking your target lamba very well even with 0% compensation due to the differential fuel pressure.

yes, i configured 1% correction on the table before logging that run but i will test again with the new values.

i´m using a "real" VE table and the numbers are tuned so it should track lambda target. I´m using closed loop fuel until 30% TPS and 120Kpa, past that is open loop.

The fuel pressure is unstable under load so i´m sure i have an issue with the adjustable regulator or the fuel pump, i have to solve that.

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