I did it.
Here were my reasons:
First of all, when supplying the heads through two 1/8" pipe ports and returning through two more 1/8" pipe ports you can plan to see as much as 10 or more psi
of pressure differential between the supply and the line between the heads and the regulator because of restriction.
So if you hold 60psi at the reg in a situation where you might have a 10psi differential, then your poor pump is actually running 70psi constantly. No dice say the pump manufacturers.
Secondly, I was interested in the fact that there was a pressure drop, and what effect that might have. If you have a drop across an 1/8" port from the supply line to the much larger rail inside the head, are you giving an opportunity for the fuel to vaporize slightly across this fitting and actually put fuel vapor into the system right then and there? After wondering this I went outside and lowered my fuel pressure from 68 down to 55 and the truck immediately dropped tailpipe smoke and "felt" crisper. I don't pay to much homage to "feelings" when it comes to these things, but the tailpipe smoke wasn't subjective, it was real. Even if my feelings weren't.
Thirdly, temperature. Although people always think the flow-through setup most commonly used keeps the fuel cooler because it keeps flowing it through instead of just sitting, the opposite is actually true.
When idling or cruising at very light pedal, the fuel will be hotter with the Quad Feed setup. This is obviously because the fuel is sitting in the rails and very little is being used by the injectors. So you're probably dealing with fuel at around 190 or so degrees. But idle and light pedal are not where I'm concerned with fuel temperature. In these conditions there will also be almost no flow of this hot fuel, and definitely no chance of pressure drops, meaning no chance for vaporization across these non-existent pressure drops. And because the fuel in the circulatory section of the system (from tank to pump to reg back to tank) is running at near ambient all the time, the rest of the system is as cool as can be and happy. Now when you nail the pedal to WOT
, you had better believe that those rails are going to empty in a heart beat. Now you've got some serious flow through all four of those -6 lines feeding those heads. And here's the important takeaway from a temperature standpoint, you are now feeding the engine with circulatory fuel from the tank at near ambient temperature. So if it's 70 degrees out, you're probably running fuel at maybe 75 to 80 or so degrees, if even that hot. And the pump is pumping fuel at this low temperature. The pump suction side is by far the
biggest opportunity for fuel vapor to form because of fuel temperature and inherent filter restriction. And with the Quad feed, the fuel is at nearly ambient temperature the entire time the engine is running. No matter the fuel level, or the distance driven.
So how does this differ from the flow-through method?
The flow-through method is actually creating higher fuel temperatures in the instances and places where it matters. If you think about it, the fuel rail is just a passage through the head. There are many others in the head as well. We call them the water jacket. They just have coolant flowing through them instead of fuel. And when you run the fuel through the head and back out with good circulation, you have just turned your fuel system into an auxiliary cooler. You can expect the fuel temperature EVERYWHERE in a flow-through system to run around 140 to 150 or so degrees, all the time after the truck is up to temp. Worse if the tank is about empty.
So in the situation of idling or very light pedal cruising the fuel temperature in the heads will be around 50 degrees cooler with the flow-through method vs the Quad Feed, being that it will be right at or slightly above coolant temperature vs the 140 to 150 or so with the flow-through. Problem is, the fuel temp in the head is of little concern. It would be nice to have that fuel cooler, and it might pay dividends for mpg
, but I haven't checked. But the main problem is ahead of the pump. With the flow-through setup the temperature of the fuel pre-pump is easily 70 or more degrees higher than with the Quad Feed. You have heated the fuel and then run a serious risk of entraining fuel vapor pre-pump because of it. That will drop power and hurt milage without a shadow of a doubt. Not to mention really piss your pump off.
And here's the clincher for me. When you go WOT
....... the flow-through setup keeps on pumping the 140 to 150 or so degree fuel the whole time, and the Quad feed quickly drops down to damn near ambient temperature as soon as the injectors start to empty that rail. Considering how much fuel it takes to maintain fuel pressure on a hot 7.3 I think we can all imagine how fast that rail is emptied of the hot fuel when the pedal hits the floor, or anywhere close. At this point the Quad Feed setup is flowing fuel at near ambient in both places that count, at the heads and before the pump.
That's why I did it.
And just FWIW
, I didn't change anything on the fuel system except to switch over to the Quad Feed and the pump instantly sang a higher note when I keyed the truck on. Just purred along instead of the laboring sounds I was accustomed to hearing out of it.