Q&A / Tech

Mailbag: Intro to Fuel Flow Ratings, Fuel Pressure & Fuel System Basics

Q: I have a street/strip 1969 Camaro with a 427 big block that puts out about 475 horsepower at 6,000 rpm. The engine has an 800 cfm Holley fed by a factory mechanical fuel pump running fuel through a 3/8-inch fuel line.

I have seen quite a few mechanical and electric fuel pumps that are rated by gallons-per-hour and line pressure. Sometimes the pumps are advertised as being good up to 400 horsepower, 500 horsepower, etc. But I’ve read that only 0.5 lbs./hr. per horsepower of fuel is required at wide-open throttle. Maybe you can clear up my confusion by answering these questions:

  • How do you determine what gph and psi ratings an engine will need?
  • What guidelines should be used to determine fuel line diameter?
  • Isn’t the needle and seat assembly orifice the real regulator of fuel flow? (I have windowed .110 seats in my carburetor.)

A: Fuel requirements depend on engine size, type of engine, and application. A 600-horsepower engine will require more fuel than a 350-horsepower engine of the same size. Fuel pressure will vary depending on application as well. A drag race car with a lot of initial G-force needs more fuel pressure on launch to keep the fuel from “stalling” in the fuel line.

We recommend you check out our earlier article that covered the basics of choosing a fuel pump. We took a closer look at fuel flow and pressure and how they relate to a particular engine. We even included a formula to help you calculate ideal flow, and we also looked at some different scenarios and how they relate to fuel pressure.

As far as fuel line diameter, we’d recommend another of our earlier posts, which was guest-written by Fuelab’s Jon Light. This article includes vital information on how fuel line size relates to fuel pressure drop — and why you should care.

The needle and seat in a carburetor is the smallest opening in the fuel system. However, higher line pressures will change the flow rate of the fuel entering the carburetor, sort of like a garden hose turned on only part-way instead of wide open.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

Tags:

Leave a Reply

Your email address will not be published. Required fields are marked *

*

This site uses Akismet to reduce spam. Learn how your comment data is processed.