“In regards to blowers, I suppose you could look at it as a percentage of your blower overdrive, but I still go to worse-case-scenario and at what total pressure?” Craig explains.
As you might surmise, a naturally-aspirated and turbocharged engine of roughly equal horsepower would have very similar fuel demands for this reason.
Weldon — and any fuel system manufacturer worth their salt — will generally factor a safety margin, if you will, into the fuel system for a given combination. This is done to account for system losses — the hoses, lines, and fittings — and the actual use of the vehicle. Drag racing applications are going to require more fuel for the potential of where the customer might go with the car, while someone with a pure street car may settle on a given combination and leave it alone.
“I throw about a 30 to 35 percent add-on to cover that,” Craig shares.
When it comes to fuel pumps, Craig says the key thing to look for is low amp draw at system pressure and also the flows at that given pressure. “Using our example, if the base is 40 psi and we’re putting 20 psi of boost to it, we’re at 60 psi. You have to look at what the flow is with that particular pump at 60 psi to make sure that you have enough pump to cover it. So the most critical thing in sizing a pump is flows and amp draws, at pressure. Because you can have a pump that’s advertised enough flow, but it’s at a lower pressure, and doesn’t show amps, so when you get to the required system pressure, there isn’t enough volume.”
In our build, we utilized Weldon’s D2035-A pump, which flows a minimum of 180 gph at 80 psi and draws in the neighborhood of 20 amps. At 60 psi, Craig puts the flow numbers in the vicinity of 200 gph. The less system pressure there is, the more flow you have — a fuel injected engine at 50 psi will support around 2,000 horsepower while at 80 psi that rating falls to 1,800 horsepower.
…let’s say you’re making 1,000 horsepower on the street, you need 140 gph … but that’s at 80 psi. Let’s back up the flow curve, and you’re at base pressure of 40 or 50 psi, now you’re moving 200-some gph through the system as soon as you turn the fuel pump on. You don’t need that on the street. – Jim Craig. Weldon Racing
Jumping over to the bypass regulator, a properly-sized return orifice (not an AN line size, but the internal diaphragm assembly that establishes the system pressure) is one of the keys that Craig notes when piecing together your fuel system. This orifice should adequately match the amount of fuel volume you’re trying to push through the system. “Ideally, with this engine you’d need to have at least a 1/4-inch return hole, or orifice, to accommodate 200 gph. As long as your return orifice is sized to the pump, .250-inch and higher, you’ll be able to accommodate 200 gph or higher through that orifice and still maintain your system pressure under wide-open throttle and no throttle. It essentially allows for fuel pressure response so you don’t have that drop-off in fuel pressure.”
Craig says ideally you don’t want to go under .200-inch return orifice with a regulator, as at that point that regulator may well become the restriction in the system. Commercially available, he says, it would be difficult to over-size a regulator.
Weldon markets one basic size of constant-flow regulator, with some variation in its makeup. Ours is the A2040-281-A-120, which is intended for fuel-injected engines and is adjustable between 28 and 110 psi. As Craig notes, most fuel injected engines are between 40 to 60 psi as a base. The regulator also sports a boost port to compensate for fuel pressure as your boost increases, and a .821-inch bypass return orifice, which Craig says is sized to maintain pressure operationally with any of Weldon’s electric fuel pumps.
Moving to lines and fittings, sizing is dictated by the volume of the pump. Upon determining what is the pump capable of flowing, Craig and his team then look at the size.
“If you’re trying to draw 200 gph through the system, you need at least a -10 line all the way through. If it’s above 200, you’ll want to go to a -12.”
Street Versus Strip
An electric fuel pump, as most know, runs at a constant speed, regardless of the fuel demands of the engine at a given time. It simply circulates back through the regulator to the tank and continues the cycle. In drag racing, where the engine may only run for a couple of minutes at a time, this doesn’t generally become cause for concern. But if you’re driving for miles — or perhaps hours — at a time, in stop-and-go traffic, well that’s a different scenario entirely.
