Carb Class: Fuel Requirements / Fuel Flow

Carb Class: Fuel Requirements / Fuel Flow

Admittedly a scary title, something every racer should ensure is an adequate fuel system. Sometimes simple things stand in the way of glory and fuel delivery can be the gremlin before the finish line. Here at our tech support office we receive a number of calls each day varying in complexity and nature. One repeating call seems to revolve around a car that seems to "nose over" before the stripe every pass. After running through fuel pressure and ignition parameters it is often determined that the customer might not have a fuel pump that is up to the task and fuel bowls become empty when it counts. Though this problem can mask itself in several variations, having the right pump and regulator will eliminate a lot of potential issues. 

Fuel delivery has several key factors affecting it. One factor that determines total fuel flow is lateral acceleration. Most racers mount fuel cells and pumps in the back of their car; this configuration requires the pump to "overcome" the force of acceleration that the car is producing in its pursuit of E.T. One "g" (32 feet / Sec2) will require roughly 10 P.S.I. of pressure to push fuel forward in the line. A larger fuel line means more there will be more fuel in line which is more weight and in turn requires more pressure to move forward - you can see where this is going by now. Modern race engines making the horsepower numbers we see in drag racing today have the potential to be severely underfed by the fuel system.

So if more is better then why not get the biggest pump and turn it all the way up? Have you ever heard the phrase "Too much of a good thing can be a bad thing"? What's true in most cases proves true in the world of fuel pumps. Increased fuel pressure is generally misconceived as increased volume, in fact it is the exact opposite. Excess fuel pressure means there is more amperage draw on the pump. Higher amperage draws will heat a pump up and lead to cavitation which shortens the life of a pump. Cavitation usually goes undetected and can leave you stranded between rounds with failed fuel pump. The lesson here is that a modern fuel pump does not have to operate at maximum output to meet the demands of most racing engines except in extreme cases. Our advice to the customer is to use the recommended pressure provided by the manufacturer of your particular fuel pump. 

 

Here is an example: A standard 220 GPH = 1,364 lbs. per hour = 2728 HP 

In perfect conditions this pump can support more horsepower than the track would see in both lanes!

 

All of our carburetors at Quick Fuel Technology utilize a float system to regulate incoming fuel. The float moves according to volume in the fuel bowl and as the bowl fills up it has to shut off the inlet to prevent flooding (think of the tank on a toilet). If you have your fuel pressure set at 9 p.s.i. then it will require 9 p.s.i. from the float to shut off the inlet (needle & seat). Floats in a carburetor, no matter what brand, are simply not designed or capable of operating at this pressure. Ideally fuel pressure needs to be between 5-7 p.s.i. Setting your float at a pressure of 6.5 p.s.i. is usually a good start and will give you plenty of float travel.  

Put simply, more float control leads to a more stable fuel system. A carburetor that can sense and react to variations in fuel level will always be first to the finish line; combine this with a pump and regulator setup that can get the fuel forward and you have a winning fuel system. 

Marc Shankweiler