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Rising Rate FMU not working properly at high rpm

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Brad Bell

Proven Member
192
5
Aug 15, 2014
Levant, Maine
I'm running a turbo 420a setup with 440cc injectors, 250lph walbro pump, BEGI/Cartech FMU, no FCD/missinglink, running at 10psi currently. I'm asking this question to those who are familiar with or use rising rate fmu's like vortech or BEGI etc.

With my current setup, I have the stock fuel regulator removed with 440cc injectors, I turned the fuel pressure down using the FMU to around 18-20psi to compensate for the larger volume injectors. Which idles around 12-13 AFR on wideband. The FMU works great from low rpm and speed, i.g. around town, streets, etc. Rising at a rate of around 5:1, stopping at around 70psi fuel pressure on full 10psi boost as it should. However, whenever I drive on the highway where my rpm's are already very high (3k-4k), and speeds 60mph+, Whenever I go to boost or give it light throttle to pass someone for example the fuel pressure rises incredibly fast, more of like a 12:1 rise instead of 5:1. Going sometimes 100psi and above of fuel pressure even before I've hit boost. Making the car pretty much fall on its face. Currently I'm using the compressor housing nipple on the turbo for a source on the FMU. What is causing it do this whenever I drive on the highway? Do I need to restrict the signal line more? Use a one way check-valve?

Thanks guys.
 
You probably should be referencing from your intake manifold for vacuum and pressure.
When I used a FMU with a Vortech setup it actually still retained the stock fpr, not sure about the one you are running.
The FMU was connected inline after the stock fpr with the FMU only restricting return fuel to increase fuel pressure when there was positive manifold pressure.
The Vortech FMU runs on the same intake manifold line that the factor fpr was using.
 
You probably should be referencing from your intake manifold for vacuum and pressure.
When I used a FMU with a Vortech setup it actually still retained the stock fpr, not sure about the one you are running.
The FMU was connected inline after the stock fpr with the FMU only restricting return fuel to increase fuel pressure when there was positive manifold pressure.
The Vortech FMU runs on the same intake manifold line that the factor fpr was using.

I've tried running the signal line off the manifold before but it causes the fuel pressure to increase/decrease under vacuum AND boost. Whereas I need the fuel pressure to change under boost only. Unless I installed a one way valve in the signal line. I'll try it again though. I only removed the stock regulator because the hahn racecraft turbo kit manual said it was the only way to get the fmu to function properly. It also allows me to drop the fuel pressure lower than the stock fuel pressure, which is needed because of larger injectors.
 
But how exactly would putting the signal line off the intake manifold than the turbo be any different? I mean they're both receiving the same PSI? Thanks for the responses guys.
The intake sees vacuum AND boost. Putting it on the I/M should prove your idle/cruise AFRs also, you may have to adjust base pressure slightly.

There may be a pressure differential at part throttle where the turbo might be spooled to a slightly higher psi than the I take manifold would see at the onset of boost. Therefore the signal happens too soon and causes you to go pig rich. What ratio disc is in the fmu currently? And what turbo are you running?
 
The intake sees vacuum AND boost. Putting it on the I/M should prove your idle/cruise AFRs also, you may have to adjust base pressure slightly.

There may be a pressure differential at part throttle where the turbo might be spooled to a slightly higher psi than the I take manifold would see at the onset of boost. Therefore the signal happens too soon and causes you to go pig rich. What ratio disc is in the fmu currently? And what turbo are you running?
The ratio is around 5:1 currently, it's not necessarily a set "disc" because the BEGI fmu's use a needle valve to adjust rate of gain and by adding or removing restrictions in the signal line. But like I said earlier the fmu works great at crusing speeds like around town or whatever. But at high speeds and high rpm like on a highway, the fmu runs like a 12:1 and increases all the way up past 100psi of fuel pressure. I just can't figure out why it would do this.
 
I recall that now, it makes me think the ratio is way off as you've set it with no vacuum signal (I can't recall if they have you set the initials for the FMU with engine running or off). setting it to a 5:1 ratio with the vacuum signal should offset the on-boost ratio to a more acceptable level. You should still net the byproduct of better idle/cruise AFRs, even though you say it drives fine: it's still rich at cruise.

Id recommend changing the signal to the I/M and recalibrating the FMU.
 
I recall that now, it makes me think the ratio is way off as you've set it with no vacuum signal (I can't recall if they have you set the initials for the FMU with engine running or off). setting it to a 5:1 ratio with the vacuum signal should offset the on-boost ratio to a more acceptable level. You should still net the byproduct of better idle/cruise AFRs, even though you say it drives fine: it's still rich at cruise.

Id recommend changing the signal to the I/M and recalibrating the FMU.
I'll try re configuring my setup tomorrow and get back to you. Yes the car for the most part is running rich 75% of the time. I believe you're right in that with a vacuum source the fmu will be more stable. Oh and to answer your question earlier its a Hahn super 16g turbo 50mm trim, similar to 4g63 turbo.

Thanks!
 
I'll try re configuring my setup tomorrow and get back to you. Yes the car for the most part is running rich 75% of the time. I believe you're right in that with a vacuum source the fmu will be more stable. Oh and to answer your question earlier its a Hahn super 16g turbo 50mm trim, similar to 4g63 turbo.

Thanks!

I have a feeling it will at least help the situation. You're basically grading the entire FMU curve down by setting it at vacuum, just like a AFPR.

Good luck and let us know when you get a chance.
 
I've never used the BEGI FMU, but back when Vortech FMU's were the standard for low-boost 420A's, we never removed the stock FPR. Some people installed bigger injectors, like you, but replaced the stock FPR with an AFPR to lower the base fuel pressure. I'm not convinced the BEGI FMU is equipped to control fuel pressure completely by itself. I found these two lines in the unit's instruction manual that might help:

The manifold pressure “signal” must come from the intake manifold, after the throttle plate. The signal is transmitted by a sensing line that connects the manifold to the sensing port on the regulator. The regulator will not work properly if the signal comes from anywhere else.
Do not remove the stock regulator, as it still controls the fuel flow under most manifold vacuum conditions.
 
Good points @VelocitàPaola
I would assume (something I shouldn't do) that the BEGI can be run standalone as I've seen it done, but I know it's common to run the stock FPR in series before the FMU, probably for stability!
 
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