Ask our Experts, we're here to help!
Often lessons on how to build a performance car are learned from the mistakes of others. A fabricator buddy was hired to install a supercharged LT4 engine in a ’67 Chevelle and to remove the impressive 540ci big-block Chevy that was in the car. The owner had the original drivetrain installed by a different shop but almost immediately it suffered from a lack of power, and no amount of tuning seemed able to alleviate the problem. The owner claimed the car would barely spin the tires. So he decided to dump the Rat in favor of late-model power.
Our buddy Scott Gillman took on the project and within a day discovered the real problem with the original BBC motor. The previous shop that built the car had fitted the 700hp Rat motor with a fuel filter that had a total filter area of about the size of a silver dollar. When the filter was soon plugged with debris, it proceeded to radically restrict fuel flow to the engine. The restricted fuel-system pump filter supplied enough fuel to run the engine at part throttle but was way short of the volume needed to make power or melt pistons.
A larger, higher capacity fuel filter solution would have saved the car owner something like $25,000 by providing about 10- or 20-times the filter area of the original unit. By using a filter with a much larger area, it could still trap debris but wouldn’t restrict flow. As an example, Holley offers a multitude of different inline filters in a wide range of sizes and filter media. It’s important to know the style of filter in terms of its efficiency. These filters are rated by micron size. A micron is a millionth of a meter, or 0.0000393-inch.
Generally, high-performance fuel filters are rated in several different micron capacities from the finest size, 7 microns, to 100 microns. A 100-micron filter is generally used as a pre-filter for electric fuel pumps, while 10-micron filters are used as a final filter for EFI engines because larger particles can clog fuel injectors. A common mistake when building a fuel system is to position the more restrictive 10-micron filter in between the fuel tank pickup and the electric pump. The larger 100-micron filter is the correct choice because this filter minimizes the restriction on the inlet side.
All pumps generally work much better with the least amount of restriction on the inlet side. That’s why on water pumps for example, the inlet hose is larger, to present less of a restriction to flow. So if you are plumbing a pair of filters in your fuel delivery system, make sure the larger 100-micron filter is on the inlet side of the pump and the smaller, more restrictive 10-micron filter is placed on the pressure side leading to the engine.
Simple things like using a larger capacity fuel filter can make all the difference between a lame hot rod and one that runs like a banshee. Which would you rather have?
Check out Holley’s full line of fuel filters now!
That small, round filter is roughly the same size culprit used in that carbureted big-block example in the story. This is much too small a filter to adequately feed any decent street performance engine. Filter area is just as important as micron size – perhaps even more so.
This is a selection of filters featuring much larger surface area that can properly filter yet not become a restriction to proper fuel flow.
This etching at the end of the filter indicates this is a filter that only captures debris down to 100 microns. This is sufficient for carbureted applications but for EFI engines, a much finer, 10-micron filter is necessary.
This Holley schematic illustrates the proper placement of filters and pumps for a typical return fuel system. Don’t place a 10-micron filter in front of the fuel pump since this will cause a restriction on the inlet side of the pump, which will radically decrease its output. Position a 100-micron filter ahead of the pump and a 10-micron filter downstream of the pump.
