Safest Highest Fuel pressure for overall system

[turbogate]

Relevant Mods:

MKIV TT Fuel pump, "NO" 12v mod
Aeromotive AFPR already set at ~40 "Without vacuum attached"
RC 550s
NO J-tube
S-AFC
RonR FFI long runner
Q45TB
LEXUS AFM (screw as much back out as possible)
T04 60-1 turbo

Here's the scoop, at 12psi, I have no problems with fuel cut, my S-AFC HI settings are in the -15 at their lowest points, and I stay in the ~11.5 range per the wideband readings. Before upgrading the TB and Intake, I was able to tune to the same A/F range at 19psi with no fuel cut.

The other night I was re-tuning the car, now that I have the bigger TB and FFI, and while at 12psi it didn't require to much adjustment, at 19psi it was a whole different story. We found that at around 4500 rpm and up, it would go very lean, which meant we had to raise the S-AFC settings from -15 range to around -5 at some points which told the stock ECU that more air is coming in, hence "FUEL CUT". So I need to get the AFC settings back down to the -15 where it will hopefully get around fuel cut, so I need to raise the Fuel pressure. Being that I am already at ~40psi, how much higher can I go? off course everything else will be monitored with a wideband, EGT etc. I just don't want to go in a pressure level that will be dangerous for the fuel system (Fires, Leaks, etc)

So what the highest fuel pressure someone has run without problems?

Thanks

 

[Turbo. Targa. Life.]

Very good question, free bump

 

[suprarich]

I run 55 psi no problems.

 

[tubbie]

I say its time for a vpc or map ecu or something of that nature. Time to remove the fuel cut (or raise it) without having to adjust your fuel pressure too high. Adjusting a few psi up/down is good, but to go lots up or down is no good.

As the pump has to pump more fuel pressure it begins to pump less fuel. There was a thread somewhere on SF that someone did a fuel psi to flow rate test on the mk3, mk4, and walbro fuel pumps. When pressure goes high, the amount of flow decrease, being more psi does not equal more fuel in a linear flow.

To determine your maximum fuel pressure, you need to know at what pressure the fuel pump stops pumping fuel. Say for instance the pump is only good to 100psi. At 100psi, the pump will not be able to supply any fuel because the pressure is too high. If you set your fuel pressure to 75psi, under boost, your fpr will increase fuel psi 1psi for 1psi of boost. So you run 26 psi of boost. Your 75psi of fuel + 26psi of boost = 101psi of fuel. Your pump is only good to 100psi of fuel. Now your at 101psi of fuel, the pump will not be able to pump any fuel. You go lean and boom.

The way to see the max psi of your pump is if you have a fuel pressure gauge, clamp (plier) the fuel return line and the pump will pump up to it's maximum psi. Anything more than that max psi and you will not be flowing any fuel.

 

[drjonez]

honestly, w/those sort of mods, it's time to go standalone. you've drastically changed the VE of your engine, the ECU's fuel/timing maps are marginally close @ best.

that said, the only problem w/elevated FP is pump flow- it goes DOWN w/press. get a good fuel press gauge in cabin and keep an eye on it....i'm guessing that's your problem. either that or pump voltage- the wiring ages and as RPM increase, pump V goes down....

 

[turbogate]

Thanks guys, I agree that I'm pretty close to standalone necessity, I never thought that this turbo could do what it does at 19psi, and its just starting to wake up. I have the cash, but I am debating between the AEM or Arz big brake kit. My engine is still on stock internals, and I've had a slight rod knock for the past 5 years which is just starting now to get louder (Thanks to real boost now). So I didn't want to go AEM and not enjoy its potential yet. When this engine goes, I have a spare engine that I will fully rebuilt with Internals, the works, which is when I thought it would make sense to get the AEM.

Either ways, my MKIV fuel pump should be in good shape, I will try to find out more info on the Pumps Specs in that comparison mentioned earlier. being that I'm currently at ~40psi, probably another 5-10 psi should put me where I need to be to get around fuel cut. If the engine goes, oh well, time for a rebuilt, its not my daily driver anymore. One thing though, the day I go standalone (AEM) I will have something to give back to the community, because one of the best AEM tuners resides in Miami, so a lot of secrets and tricks will be revealed for the 7M. He is good friends with my cousin.

Can I check with a voltmeter how many volts is being supplied to my fuel pump at WOT? like I said earlier I havent done the 12v mod, maybe I should consider doing that also.

Thanks for the replies

Greg

 

[drjonez]

Either ways, my MKIV fuel pump should be in good shape, I will try to find out more info on the Pumps Specs in that comparison mentioned earlier. being that I'm currently at ~40psi, probably another 5-10 psi should put me where I need to be to get around fuel cut. If the engine goes, oh well, time for a rebuilt, its not my daily driver anymore. One thing though, the day I go standalone (AEM) I will have something to give back to the community, because one of the best AEM tuners resides in Miami, so a lot of secrets and tricks will be revealed for the 7M. He is good friends with my cousin.

SHOULD is the operative word there....you don't know unless you find out. in-cabin FP gauge will tell you that....

Can I check with a voltmeter how many volts is being supplied to my fuel pump at WOT? like I said earlier I havent done the 12v mod, maybe I should consider doing that also.

yup, stick a VM on the FP wires and do a run.

the 12V mod isn't going to change anything @ WOT (unless there's a problem w/the relay...). the ECU switches over to 12V for WOT.