MysticBowtie

I was having the tune done yesterday by the great Mr. Ed Wright. The car keeps going lean under WOT. When the pedal is pegged, it drops to rich for about millisecond and shoots up to lean and the motor starts to "break up" or miss.

I'm running 62psi of fuel pressure with 1 255 walbro pump in-line, and 1 new 255 in-tank pump. I have the Procharger FMU installed, and the fuel pressure guage is connected to the fuel rail and is showing good fuel pressure. I have 85lb injectors that according to the data logger are doing good . Ed told me that he has, "run out of numbers" and that there is somthing wrong with the car mechanically. My boost guage stops at 15psi, and the D1 is pushing right past that.

I understand that the factory lines are good enough for high horsepower motors, but is the computer? Are you guys that are running a lot of boost using an aftermarket fuel system or computer? Here is a pic of the A/F, ignore the file name, the dyno guy didn't change it from the last guy in.

Kevin Blown 95 TA

Are you saying the pressure never drops at the rail? Is 62# what you see in boost or your base setting? If you had 62# at the rail in boost with 85# injectors, no way you should run lean unless your injectors are all plugged up. FP normally needs to go up with boost or else the fuel delivery is fighting against boost pressure, so unless you are bringing in that much extra fuel in the enrichment tables, it has to be done at the regulator (or FMU). What I'm getting at is that if your fuel pressure at 0# of boost is 45#, then your pressure at 15# should be 45+15# just to maintain the same relative pressure or you could go lean if it isn't adjusted for in the software somehow and there might not be enough room to do that. The FMU should be jacking up the pressure for you. Unless I misunderstand you and the FMU is jacking the pressure up to 62 off some other base setting. It should vary, though and maybe it can't because there isn't enough volume. If your inline pump is in series with your in tank pump there is a possibility that it is becoming a restriction at WOT, but that would show up as a drop in pressure. When my car had that problem, it would just kind of fall on its face when it got into good boost. There could also be another problem if you're making enough power in that your fuel rails are in series and its possible that the drivers side is not getting enough fuel, but again, I would think that would show up on your gauge. With 85# injectors, I don't see any reason for you to use an FMU. Those can be hard to tune because they are inconsistent. You just need to make sure you have enough volume, enough injector (which you have), and a boost-referenced regulator, then the rest is in the tune. There is a certain point at which the stock lines are no longer viable, but there are guys that have made 700 hp on them, so I can't make that conclusion on your setup. I will say that many of us have done dual in-tank pumps, some with parallel fuel rails, and that works pretty well, much better for high hp apps than an in-tank and in-line in series.

Jason Short

The first thing I would do is ditch the "in-line" 255lph pump. Using (2) 255lph pump in line with each other (one in the tank, one "in-line") is only going to cause pressure issues when being pushed. They were never designed to work in series with one another. The FMU on top of that is just complicating things further. In theory, you could have (3) different fuel pressures at different points in your current fuel system.

Depending on where your power level target/goal is, you need to do one of two things. If you are planning sub-600ish rwhp (not an exact number, rather a reference), keep the 255lph intank pump, ditch the "in-line" pump, ditch the FMU, and add a Boost-a-pump. If you plan on more than 600ish rwhp, you'll need to go with a dual 255lph intank setup if you want to retain the stock lines.

Your fuel system (in my opinion) is actually over-complicated at this point. With your setup (D1/383) potential, I'd advise to just go to a dual 255lph intank pump setup, and run that right to your regulator/fuel rails. Get rid of everything else.

RealQuick

Agreed.

MysticBowtie

I really appreciate the advice! I remember watching for the fuel pressure to drop, but not to increase. I should have known to look for that, embarrassing! Going to 2 pumps in the tank makes sense, but why remove the FMU? If all it does is shut off the return line under boost does it lead to excess pressure?

Jason Short

The FMU is really nothing more than a band aid. From a marketing standpoint, ATI (Procharger) and Vortech offered the FMU so that the base kits could be used on nearly a nearly stock motor. Essentially it just spikes the fuel pressure under boost (overloading/driving the injector). It makes your injectors "act" larger than they really are.

The proper way to fuel a FI (or any) motor is through consistant fuel delivery volume (flow), consistant pressure, and the proper size injector. You already have an excellent injector for your combo. The dual intank pumps provide the volume, the single regulator (without FMU) will provide a constant pressure behind the injector.

mzgp5x

I run a 383ci MM6 97ss with a D1 (7.6 --> 3.4 " pulley). I ran lean intill I installed (2) two 255 lph walbro pumps and larger 3/8" -6 an twin fuel lines. They "Y" to a single -8an line to the fuel rails. I had a starve fuel flow problem before the change @ WOT mode. Now, I can tune into rich area, but, that too can be a problem.

You probably have an OBD1 PCM witch will not trim the injector open time @ WOT when the MAF reaches high frequency limit. I had this problem with OBD2 PCM, and due to LT1-edit version 2.2, I recalibred my MAF sensor, and had a feature to set BLM to 1 @ WOT mode.

I think you will be OK with twin 255 LPH in tank pumps. I reccomend gettting a WBo2 sensor to dial in the WOT and low speed AFR. The WBO2 sensor helped me quite alot in the tune.
Hope this helps. B.

MikeGyver

A walbro only supports 530hp forced induction. Fuel pumps flow more at lower psi, and the most at zero psi (open flow). An inline pump is only going to help out by simulating zero psi for the first pump at best. Since fuel pumps are positive displacement pumps, the second pump can't suck fuel through the first pump, so the first pump is still what limits your hp. A completely unrestricted walbro will support around 700hp at zero psi. I think you need to do 2 parallel walbros for your boost level.

MysticBowtie

I got the new secondary fuel pump in the mail yesterday. Hopefully I get a chance to get to it this weekend! I can't wait to see what the car will do. This has been the longest build, and now the wife is starting to talk kids....