Is it normal for rail pressure to drop on consecutive runs?

proboner

Specialist
Sep 13, 2020
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I know rail pressure is a common question around here, but I searched and didn't see this specific issue addressed. When I do consecutive WOT datalogs in 3rd gear, the rail pressure will drop across the board on each consecutive run. So on the first run in may peak at 2300psi and drop as low as 1750psi, on the second run it may peak at 1900psi and drop as low as 1550psi, on the third run it may peak at 1850psi and drop as low as 1475psi. Then if I come back and do 3 more runs a few weeks later, it'll do the exact same thing. Each consecutive run it just gets lower across the board. Is this normal?

The car is FBO with Pure DDs and 450 walbro in the tank running an "E30" mix. Trying to decide if maybe this is a failing HPFP or just to be expected.

This actually leads to a bonus question as well! So when I say "E30", I put it into quotations for 2 reasons:
  • It's not actually E30, the Motiv Flex fuel says it's E30 because it's reading incorrectly
  • I'm not actually tuned for E30 yet, but I do have an the Flex Fuel box checked in MHD
Given that I have the Flex Fuel box checked in MHD, but I'm not running an OTS tune, and my tuner has NOT tuned the car for E30 yet, do you think MHD is automatically adding fuel when it sees E30 in the rail?
 

wheela

Captain
Jun 4, 2021
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Twin Cities, MN
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2015 e84 X1 35i Msport
Do you have any logs of consecutive pulls?

If it's not an e30 map, your car will have to add a ton of fuel trim to keep AFR on target. I'm not super familiar with n54, but upgraded twins on e30 you may be at the limit of the stock hpfp flow capability.

I'd be curious to see how ltft%, stft%, and load% compare on back-to-back pulls where your rail pressure is dropping.

I'm a little confused, do you know your actual ethanol content? And you're running motiv flex fuel? But aren't tuned for it? I'm not familiar enough with motiv flex fuel to know if/ what problems that may cause. My understanding is flex fuel re-writes logic to interpolate between pump and e-maps based on e-content. I think to make room for the pump and e-maps, they may repurpose some existing rom maps. So if you're running flex fuel but not tuned for flex fuel yet, not sure which maps are getting interpolated based on e-content? Somebody please correct me if I'm mistaken here, I don't know flex fuel.
 

proboner

Specialist
Sep 13, 2020
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Logs are here: https://datazap.me/u/kriswwashburn/hpfp-filter-removed?log=0&data=3-22

My actual ethanol content is not E30. The Motiv Flex Fuel has been nothing but a PITA for me and continues to be so, by erroneously reporting inflated ethanol numbers. It says I have an e21 blend when running 91 octane pump gas. It also says I have an E30 blend with 2.5 gallons of E85 and ~12 gallons of 91 octane pump gas. I have confirmed by physically testing, that local 91 octane fuel is ~e8 and e85 is spot on. But I digress...

The only way to view the ethanol content in MHD is to turn on the Flex Fuel option. I did so, because I wanted to start seeing ethanol content and because I was having my tuner make a custom E30 tune for me.

My tuner, who shall remain nameless, misunderstood my request for an E30 tune and just retuned me for 18psi on 91 octane. The excuse on his end being that "he tunes x number of cars a year, so how should he be expected to keep track of who wants what", which to me just sounds like "I'm too busy to give a shit about your car or give you good service". He wants several hundred more dollars to tune it correctly. I don't want to give him any more money when he obviously has too much on his plate. But I digress once again...

The Motiv Flex Fuel is being removed this week and a tie in with my P3 vent gauge to display ethanol content (unfortunately not tied into MHD in any way) will be installed in it's place.

My understanding is the stock HPFP should handle up to ~21psi on Pure DDs on an E30 blend. I'm running 18psi on (what I'm assuming is) a 91 octane blend. This is why I'm curious if maybe MHD is interpolating and adding fuel for the E30 blend, despite the tune not being built for E30. I'm trying to squeeze as much as I can out of this on the stock HPFP, as I don't want to go PI due to the ratio of cost/power.
 

SlowE93

Major
Jul 2, 2017
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E93
Logs are here: https://datazap.me/u/kriswwashburn/hpfp-filter-removed?log=0&data=3-22

My actual ethanol content is not E30. The Motiv Flex Fuel has been nothing but a PITA for me and continues to be so, by erroneously reporting inflated ethanol numbers. It says I have an e21 blend when running 91 octane pump gas. It also says I have an E30 blend with 2.5 gallons of E85 and ~12 gallons of 91 octane pump gas. I have confirmed by physically testing, that local 91 octane fuel is ~e8 and e85 is spot on. But I digress...

The only way to view the ethanol content in MHD is to turn on the Flex Fuel option. I did so, because I wanted to start seeing ethanol content and because I was having my tuner make a custom E30 tune for me.

My tuner, who shall remain nameless, misunderstood my request for an E30 tune and just retuned me for 18psi on 91 octane. The excuse on his end being that "he tunes x number of cars a year, so how should he be expected to keep track of who wants what", which to me just sounds like "I'm too busy to give a shit about your car or give you good service". He wants several hundred more dollars to tune it correctly. I don't want to give him any more money when he obviously has too much on his plate. But I digress once again...

The Motiv Flex Fuel is being removed this week and a tie in with my P3 vent gauge to display ethanol content (unfortunately not tied into MHD in any way) will be installed in it's place.

My understanding is the stock HPFP should handle up to ~21psi on Pure DDs on an E30 blend. I'm running 18psi on (what I'm assuming is) a 91 octane blend. This is why I'm curious if maybe MHD is interpolating and adding fuel for the E30 blend, despite the tune not being built for E30. I'm trying to squeeze as much as I can out of this on the stock HPFP, as I don't want to go PI due to the ratio of cost/power.
Fuck your tuner !
 

wheela

Captain
Jun 4, 2021
1,236
708
0
Twin Cities, MN
Ride
2015 e84 X1 35i Msport
The excuse on his end being that "he tunes x number of cars a year, so how should he be expected to keep track of who wants what", which to me just sounds like "I'm too busy to give a shit about your car or give you good service".
Wow, how can he be expected to keep it straight? Maybe because he's being paid to? How can you be confident you even get the right file back if he can't be expected to keep things straight?
 

wheela

Captain
Jun 4, 2021
1,236
708
0
Twin Cities, MN
Ride
2015 e84 X1 35i Msport
I looked at your logs, I saw the first one started higher than the other 2, but they all seemed to drop to pretty similar levels during the pull? Load actual and afr seem pretty similar between the 3 so I suspect your getting similar fuel delivery in these 3 logs.

I didn't see ltft% in your logs, does your tuner not ask you to log ltft%?
 

proboner

Specialist
Sep 13, 2020
84
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0
I looked at your logs, I saw the first one started higher than the other 2, but they all seemed to drop to pretty similar levels during the pull? Load actual and afr seem pretty similar between the 3 so I suspect your getting similar fuel delivery in these 3 logs.

I didn't see ltft% in your logs, does your tuner not ask you to log ltft%?
These aren't actual tuning logs, they're just logs I'm taking for my own data gathering. I've been doing small fuel delivery modifications to try and gauge if they'll make any difference on rail pressure... drilling the rail, upgrading the line size, adding an inline filter and pulling the filter from the HPFP. None of these runs are being sent to a tuner.

I would assume there wouldn't be any difference in fuel delivery between each run, as nothing in the tune was changed. My goal was to do each modification and do 3 test runs to measure rail pressure after each run and average them out to see what kind of difference it made. What I noticed though is that the first run would usually be pretty significantly higher than each subsequent. When I drilled out the rail in particular, it was like 800psi higher on the top end on the very first run, but then dropped to only a couple hundred better than baseline on subsequent runs.

I've done 3 modifications and performed 3 data logs after each, and every time the first run shows the best results with the next two getting worse and worse.