(Image/Jeff Smith)

I need help understanding EFI high pressure fuel systems. I have researched all over but the answer eludes me. What determines line size? I’m trying to build a Whipple Supercharged SBC 427 c.i.d. C3 Corvette engine with approximately 8 psi boost and 700 hp FAST XFI System. I just purchased a Walbro 525-liters per hour, (F90000285) in-tank fuel pump which should be overkill and I plan to use the existing 3/8-in. line for a return and install a new supply line. I should have the pressure needed, but what determines the volume and the line size?

I’m thinking larger hose for more fuel and power but that doesn’t seem to be the case with EFI. I’m using the formula 700 hp x .75 = 525 lbs./hr. divided by 6.2 lbs./gallon = 84 gph. If this is correct, how do I determine the line size that’s required? As I understand it, higher pressure means lower volume, correct? If so, how do I get more horsepower from high pressure which would mean lower volume of fuel. Can I get 700 hp from 48 psi with six 72-lb. injectors with 8 lbs of boost from a 3/8-inch supply line? I have an early EFI system that only uses six injectors instead of eight. Any help would be greatly appreciated. — J.C.

Jeff Smith: This is a great question. Your math on the fuel pump size is good. With 700 hp you multiplied it times the brake-specific fuel consumption (BSFC) number (on the high end of the scale) to get 525 lbs. of fuel.

BSFC is defined as the pounds of fuel per horsepower per hour consumed by an engine. We have included a chart from Aeromotive listing the BSFC numbers for gasoline, E85, and methanol in three different configurations of naturally aspirated, nitroused, and supercharged that also includes turbochargers.

As the BSFC numbers get larger, this means the engine uses more fuel to make the same power.

Naturally aspirated engines are the most efficient.

Aeromotive BSFC Data

Gasoline N.A 0.45 – 0.50
Gasoline Nitrous0.50 – 0.60
Gasoline Supercharged0.60 – 0.75
E85 N.A.0.60 – 0.70
E85 Nitrous0.70 – 0.80
E85 Supercharged0.85 – 0.95
Methanol N.A. 0.90 – 1.10
Methanol Nitrous1.20 – 1.30
Methanol Supercharged1.80 – 2.00

We found TI Automotive’s rating of the fuel pump in an online test that shows around 118 gallons per hour of fuel at 50 psi. We can convert this to pounds per hour (lbs./hr.) by multiplying by 6.2 pounds per gallon to come up with 731 lbs./hr. This appears to be roughly a 40-percent increase in capacity so you should have plenty of pump capacity.

Your question relates to whether a -6 (3/8-inch) fuel line would be sufficient to feed your 700-hp engine. As we’ve shown, the pump has the capacity but there are some flow losses involved with pumping fuel through a line. Brett Clow at Aeromotive was kind enough to clue us in on some testing they have done in terms of pressure drops that occur with different fuel line sizes based on the volume of required fuel.

-6 Fuel Line

Flow rate (lbs./hr.)
10 feet
15 feet
20 feet

Referencing the -6 fuel line chart, we will assume the fuel line is 10 feet (120 in.) long since we don’t know the specific application.

Since your pump can produce over 700 lbs./hr., we’ll just use the 600 lbs./hr data to illustrate the point. According to the chart, the pressure drop for 600 lbs./hr. at 10 ft. for a -6 fuel line will be about 10 psi. This means if the pressure at the regulator was 50 psi, the pump would have to be working at 60 psi to maintain that 50 psi at the regulator.

If you look at the -8 fuel line chart with a 10-ft. long fuel line at 600 lbs./hr., the pressure would drop only 3 psi, so you can see there is a substantial improvement that may make the conversion to a -8 line worthwhile. This might come down to packaging issues and price as -8 fittings and hose is more expensive than -6.

-8 Fuel Line

Flow rate (lbs./hr.)
10 feet
15 feet
20 feet

One other thing to consider is that under boost, the regulator will be boost-referencing the pressure.

You said that your engine will run 8 psi of boost. That will put additional load on the fuel pump. Let’s look at what happens under boost. When using a -6 line, the pump at full boost must start at 60 psi and then overcome the 10 psi pressure drop which means it will run at nearly 70 psi. The TI Automotive chart shows that the pump is less efficient at this elevated pressure.

This is where your concern about pressure vs. volume becomes important.

TI Automotive’s chart reveals the pump loses roughly 12-13 percent flow when the pressure is pushed up to 70 psi. This is still more than enough to feed the engine. With a -8 line, the pressure drop is drastically lower, making life easier on the pump. So this may affect your decision as to the size fuel line.

While this required a little bit of math and some rather specific information in order to make this decision, you can see how applying this data makes the decision much easier because now you have the numbers to back up the choice. Essentially you are not making the pump work nearly as hard by using the -8 fuel line.

Your concern over pressure vs. volume is only related to pumps. For a given size pump, as the pressure increases, the volume has to decrease so it often becomes a situation where a larger pump with more pressure capacity is necessary rather than just raising the pressure to compensate for injectors that are too small.

Lower operating pressure is always a better way to go.

As an example, let’s say our fuel injectors are slightly on the small side. One solution is to increase the line pressure to increase their output. But this places undue burden on the fuel pump. It’s actually better to choose larger injectors so the pump will operate at a lower pressure even with the boost-referencing for the supercharger.

Boost referencing is only required when the injectors are located in the intake manifold where the boost pressure is present. If the injectors are located above the supercharger, then boost referencing isn’t necessary because the injectors aren’t exposed to the boost pressure.

All of this is based on operating the pump at 13.5 volts continuous power. This is an important consideration because it is essential that the pump is wired properly and that the vehicle’s charging system is capable of sustaining a solid 14 volts in the charging system so that there is a minimum of 13.5 volts at the pump. Anything less than this voltage and the pump capacity will drop radically.

This package was only using six injectors instead of eight, you said. This does make a significant difference in terms of horsepower. Running six injectors at 72 lbs./hr. equals 432 lbs./hr. of fuel at 100-percent duty cycle.

To avoid burning up the injectors, the maximum duty cycle should be closer to 85 percent. That means we have to multiply 432 x 0.85 = 367 lbs./hr. of total fuel.

To determine how much horsepower these injectors can feed, we then divide the 367 lbs./hr. by the BSFC number for a supercharged gasoline engine.

Aeromotive establishes this BSFC range between 0.60 and 0.75. If we place the BSFC in the middle at 0.67, then 367 / 0.67 = 548 hp.

This unfortunately is far short of the 700 horsepower goal. However, if it’s possible to add two more 72 lb./hr. injectors, then the numbers fall into place with 72 x 8 = 576 lbs./hr. x 0.85 = 489 / 0.67 = 730 hp.

If adding two more injectors is not physically possible, then you will have to increase the existing six injectors to a minimum of 96 lbs./hr. — with 110 to 120 lbs./hr. injectors being an even better choice.

We hope this clears up some of your questions and opens up the path to big horsepower numbers.

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.