Rep Power: 807

A big question that has been asked is how much HP can a returnless fuel system support. The answer is probably as much as you want as long as you have enough money. But, at some point, it’s probably cheaper to convert the car to a return system than try to keep it returnless.

I also know that there are people out there saying I’m making this much power with a returnless system. While that’s all fine and dandy, they have no data, other than a dyno graph. Is their system holding pressure? They don’t know.

BELOW is a graph (figure 1) of pressure drop across the fuel injector, as measured by the sensor that’s mounted on the fuel rail. I know the graph is kind of busy, but I thought I’d stick them all on then offer the explanation of each one.

Let’s start with the dark blue line. This is an ’03 Cobra with a KenneBell running 21# of boost with 50# injectors and a Boost-A-Pump on the highest setting. It made 605 RWHP. As you can see it holds good fuel pressure, asking for 40 psi, until about 6400 RPM and then it starts to drop fast. It’s down to 32 PSI by 6800. Now, his A/F never started drifting lean because the injectors were big enough that they increased their pulsewidth, automatically and kept A/F flat. Is he making the power, yes. Would I run WOT across Nevada, I don’t think so.

Next, the pink line. 2001 Cobra with a D1SC, all forged parts, stock ’03 Cobra pump and tank. Making about 17# of boost and has 50# injectors. This car on this run made 520ish RWHP. We stopped the run because A/F was drifting lean. You can see fuel pressure starts dropping at about 5700 RPM. This car had aftermarket rails on it, but I think it was stock line from the tank. This car has less than 30 psi, so it’s down over 10 psi of fuel pressure.

Next line to look at is the yellow one. This one is the same car as above, but with a KenneBell Boost-A-Pump installed. As you can see this does help. Fuel pressure does start to drop about the same point but it stops falling around 5700 rpm and flat lines at about 35 PSI. This time the car made around 530 RWHP. Is this fuel system adequate? In my opinion no. I feel any pressure loss is bad.

The next line is pretty interesting. It’s a stock ’03 Cobra running about 12# of boost with Nitrous. It made 575 RWHP (this car was fast) and like 700 ft-lbs. I am pretty sure this had no boost a pump in it at all. Notice how the pressure is not falling at power levels higher than the ’01 Cobra mentioned above. Why? Well, if you go back the explanation on fuel pump flow and how the flow drops with boost, it’s simple to figure out. This car is running 5 psi less boost, so the fuel pumps have a higher flow rate since there is less boost in the system. The nitrous doesn’t add boost (kind of but not really) so it doesn’t impact the flow rate of the fuel pump like boost does.

The next line, I guess you would call it purple, is an ’01 Cobra with an ’03 motor in it running a KenneBell with 15# of boost and no Boost-A-Pump. It made about 530 RWHP when the runs were stopped. This car you can see starts loosing pressure at 5000 RPM and the run is stopped by 6000 RPM when the fuel pressure was 30-psi pressure drop. The A/F ratio on this car also did not drift lean at all since it has 55# injectors, the EEC compensated just like it should. This person has added a Boost-A-Pump and from what he says it’s OK. I will see this car again in 10 days and we’ll see how the fuel pressure is holding up.

The next line is the brown line. This is a ’99 Cobra, Novi 2000, 20+# of boost, stock ’03 pumps and tank. Car would probably make 600+ RWHP if it had a fuel system, these runs were stopped and it was making 550 when the runs were stopped. You can see this one starts to lose fuel pressure at around 4900 RPM and is less than 30-psi pressure drop across the injectors by 6000 RPM. At 6000 RPM, the A/F did start to drift lean, but up to that point the EEC compensated due to the 55# injectors. I think he has now installed a Boost-A-Pump but I have not yet seen the car again. It has no cooler on it and with all that boost it was recommended to put a bigger pulley on it (this car will be in a future article about blowers, heat, coolers, etc).

The next line of the graph, the teal colored line, is actually an ’01 Bullitt making 415 RWHP, S-Trim, 10# of boost, and the Focus pump that many people like to use. This one holds really good pressure and then at 6000 RPM the bottom falls out and it drops fast. Due to the rate at which it dropped, I would suspect that this one sucked the bucket dry. Most of the time when you run out of pump, the fuel pressure drops slowly, this car dropped much faster than the others.

Now, the question is, how much HP can you support with a returnless system. My answer is still the same, as much as you want if you don’t care how much money you spend. However, looking at the data, it’s kind of obvious that the Cobra pump and tanks assembly, with Boost-A-Pumps, is limited in what it can support. Very few of these cars actually had A/F problems due to the loss of pressure, since they had big enough injectors. How much power it will support varies if you are going to use nitrous or a larger amount of boost. I think this proves that around 600 RWHP, all made with blower, is near the limit of what you can support.