I'm in the process of modifying my oem hanger for a Walbro 525 pump and running a -8AN feed and -6AN return line. I planned all this out and have all the fittings including a 90* bulkhead -8AN fitting for the feed to come out from the hanger. Seen many pics of this and did a lot of research, watched vids, etc. Now that I have it all apart, I'm having a little trouble understanding the theory of this. The outlet of the pump is only 5/16 inside diameter. So how is it that the entire feed line couldn't be 5/16? As far as I know, the rule of flow/restrictions means you can have an indefinite length of any kind of line, be it for fuel, an exhaust system, intake, etc. and the flow will be determined by the smallest restriction or biggest choke point right? So what makes the fuel system flow so well out of such a tiny pump orifice?

Mechanical engineering is my day job so I can answer this for you and there are a few factors at play. First off, the outlet size of the pump in this case should be considered irrelevant. It was probably designed to optimize pump performance with regards to flow and pressure, and based on the data sheet you should know what flow and pressure the pump is capable of providing. To your question about line size vs outlet pressure. To start with, the area of a circle increases with the square of the radius so as the radius of a circle increases, the area of the circle gets exponentially larger. The second aspect is that the flow velocity through a pipe is directly related to the volume of flow and pressure in the pipe. All of these factors are also directly related to the length of a pipe and as the length gets longer, pressure loss (from friction) gets larger. For an example, lets assume the volume and length of pipe is constant. If the pipe size increases, the velocity of flow decreases and the pressure in the pipe also decreases but more importantly pressure loss in the of pipe decreases. If the pipe size decreases, the flow velocity increases along with the pressure thus the pressure loss in the pipe is greater. Applying this to the length of a pipe, as the length of pipe increases in order to maintain a constant volume, the pressure at the beginning of the pipe must increase to make up for the increased loss in pressure from the increased length. If the pipe extends to infinity, eventually the additional pressure at the beginning of the pipe required to maintain flow at the outlet will increase the velocity and thus pressure loss at the to a point that no flow can be achieved in the outlet.

Hopefully this made sense but the basic gist of it is that you want the most fuel to reach the engine with minimal pressure loss from the pump. To accomplish this, you want to use a larger piping size from the pump to the engine. If the pump was right next to the injectors with a very short fuel line then a smaller size could be used since there would be less pressure loss thus a higher velocity would be acceptable in the line. Theres a lot of fluid dynamics that also come into play but on a basic application like this that can be ignored.

Interesting question and I appreciate the insight by Muffinizer. A bit off topic but I too am trying to sort out a fuel solution. I haven't landed on the solution just yet. Kind of thinking about modifying my existing hanger for a larger pump (currently 255lph) until I make a decision about the transmission, as a single E85 pumping solution can more than provide for all the power the A340E can handle. I hope that as you're doing these modifications that you will document and take pictures along the way. I'm not sure of your power goal (I want to say 600-700?) but are you planning for enough pump for E85 or are you stay with conventional petroleum?

One more thing to add is that you can have too large a fuel pump. If your fuel regulator can't handle the flow your mileage will be very poor from the injectors spraying too much fuel. Additionally if your pump is capable of huge flow rates and you're using basically nothing sitting in traffic, the energy that the pump is expending ends up heating your fuel and on a non return style pump in addition to heating fuel, this can burn out the pump from churn when it is running and pumping nothing. I don't have any experience with this personally but they make fuel pump controllers for this reason.

I suppose that's true but in his case, as long as he's running a return line (which he stated he is) I don't see a concern. With an aftermarket fuel pressure regulator & gauge you should be able to identify if your fuel pressure is correct at idle and adjust. I know the N/A engines didn't have a return line but I assume anyone going turbo puts in a return line, yes? Over-pressurizing the fuel rail is unlikely unless you don't have or have an undersized return line. In such a scenario, yes you're right, your injectors will over-inject fuel at low engine loads and lead to a headache full of tuning and driveability issues for the poor subject.

Thanks for that great info. It’s obviously the right thing to do since everyone does it. Just wanted to understand why exactly. And yes, I have a return system in the making with aftermarket FPR. Getting rid of the OEM regulator and filter for an in-line external one in the feed line and FPR in the return after the rail.

I will be staying on pump gas even though I am building my entire system to be e85 ready. Might as well since the cost of the parts is relatively the same. Will be ready for anything the future holds. Yes my power goals are along those lines. Things have come a long way and now there is no longer any need for multiple pumps in the under 800whp zone. The single 525lph I got will be good for anything I'll ever do. I'll do my best to document and explain the setup.

You will have a lot to change out if you want to run e85. Basically the whole system. If you go that route you might as well make a return line and go larger on the pump.. You know modest power goals don't last long