Will An In-Line Fuel Pump Help?

[Jeff Chambers]

Originally Posted by 1989GT

I don't think the 30# injectors are maxed out. 350 rwhp with a 15% drivetrain loss would be about 411 bhp. 30(8)=240#'s of fuel/hr. Should be more than enough to support that. Also the 255lph should support over 500 rwhp. Now maybe the lines aren't up to the task for the nitrous shot, I could see that, but there's no reason for you to be running lean when on motor. Obviously your tuner pulled fuel, when tuning the car, and I think by bumping up the pressure you're temporarily tricking the computer to add more fuel. But as soon as you drive the car for a little bit the computer will "learn" and pull the fuel back out and you'll be back to square one. I say go get it retuned, and look to upgrade your fuel lines if you want to be safe about the nitrous shot.

Let's go with your 411bhp at a BSFC of 0.5. That's 205 #/hr fuel requirement. That means the injectors are running at 85%+ duty cycle right now, with all our assumptions. Too close for comfort. In the real world, depending on the A/F commanded/delivered, with losses, inaccuracies of the dyno, etc that 15% overhead may be gone. I've had 331 cars on my dyno max out 36# injectors at 365 RWHP. Even worse is the fact that I see quite often fuel systems that aren't totally maxed on the dyno easily get maxed out on the street. The dyno pulls are short and rarely do we have full electrical load on the vehicle while on the dyno. I've datalogged many vehicles on the dyno that are at that 85% duty cycle (on pump or injectors) that once we make the road test and datalog are maxed out and in danger.

Then let's not forget that fuel pump output is dependent a few parameters that take that 255lph rating and throw it in the trash. The pumps are normally rated at 14-volts; don't get that full 14 volts and the output drops dramatically. Pump output also varies dramatically with the line pressure. The pump may be rated at 255lph at 0-psi load, but put the line pressure at 50 psi and that 255lph becomes 180lph. Take for example the gool ole tried and true Holley Blue fuel pump (I know, its a carburetor pump but bear with me for the sake of demonstration). Its rated at 100gph, fantastic thats what, 375lph? Whoah, but that's at 0psi. Set your regulator to 9psi and now that same pump is rated for only 50gph or round about 189lph. Do some mis-figuring and cause a boatload of problems. Alot of people forget that when they read fuel pressure at the rail, that the fuel had to travel through some pretty restrictive lines to get to that point. 40psi at the rail/gauge may actually be 55 psi at the pump. I've stalled pumps rated for 90psi with 70psi at the gauge/rail. I attached a fuel pump performance curve for a Walbro GSS341. Also don't forget that as these pumps age, their performance degrades. There's some good information out there on the internet on studies done to really quantify fuel system performance.

Do you really think the tuner would have pulled fuel if he was seeing the car run lean? What exactly do you think the computer is learning at this point? I think you need to learn/understand the roles of adaptive learning and how the fuel & MAF curves work in the logic. Sure, tuners can and do make mistakes, but no amount of tuning can solve underlying mechanical faults/weaknesses. Bring the car to me and I'll datalog both fuel pressure and injector duty cycle during the run and show definitely that's there's a problem.