Filtering the Facts: How to Avoid Major Fuel Filter Mistakes

There are lots of options when it comes to fuel filters, but there are only a few smart choices for any engine making over 500 hp. Regarding filers, bigger is always better.

There are lots of options when it comes to fuel filters, but there are only a few smart choices for any engine making over 500 hp. Regarding filers, bigger is always better.

Photos and Words by Jeff Smith

In Shakespeare’s Richard III, in the middle of the battle, he cries “A horse, a horse! My kingdom for a horse!”

The 21st Century equivalent of that cry is a bit more muddled because when horsepower dies today, it might be more difficult to figure out where it went! We recently visited a shop where they had just removed a thumpin’ 572c.i. Chevrolet Performance big-block from a customer’s car because he had given up trying to make the engine run right. The solution was to swap in a late model LS engine.

Part of the conversion called for an upgrade from the original carbureted fuel delivery system to one designed for high-pressure and EFI. While removing the original fuel line, the guys discovered the real reason why the 620 hp Rat motor never ran right. The rear-mounted electric fuel pump was more than big enough to feed the engine, but was plumbed using a fuel filter with an area the size of a quarter. The filter was plugged with all kinds of debris and was obviously incapable of flowing anywhere near the amount of fuel the engine required. The engine didn’t burn a piston because the filter restricted flow so severely, it couldn’t make enough power to hurt itself!

This may sound like a simple oversight — which it obviously is. But, this is not an isolated case. Making big power in the 21st Century has never been easier. Magazine stories today don’t consider a 500-hp engine to be anything special. Attention seems to have focused now on street engines making upwards of 1,000 hp. What is often overlooked is the volume of fuel it requires to make this kind of horsepower. Yet, it seems many enthusiasts have not made the mental transition when it comes to filters. While most everyone’s attention is focused on high capacity fuel pumps, there’s more than a little thought and consideration that must be paid to the filter as well.

Let’s start with the more demanding EFI fuel delivery system. The best way to build a high-pressure EFI fuel delivery system is by designing it as a return system. This starts with a pre-filter ahead of the pump, along with another filter after the pump with a finer filter and the entire system plumbed with sufficient size lines and full-flow fittings. The return line must also be the same size as the feed line to prevent backpressure. We will focus this story on filters, but an emphasis on the entire system as a whole is the only way to create a successful and efficient fuel delivery structure.

The most critical location or position in any fuel delivery system is the inlet side of the pump. All pumps, including electric fuel pumps, will deliver maximum performance when the inlet to the pump is unrestricted. This is because pumps are designed to push fuel and are generally not efficient at pulling fuel into the inlet. This is why submerged, in-tank pumps are by far the most efficient. However, an inlet filter is still required because high pressure EFI pumps operate using very tight tolerances. If debris finds its way into the pump, the result is likely a quick failure.

We will be using Aeromotive’s recommendations for filters, and these same requirements will hold true with any high-output fuel delivery system. The idea is to create a balance between protecting the pump while minimizing flow restrictions. Aeromotive specifies a 100-micron filter for the inlet side of an EFI pump. A micron is equal to 0.00039-inch, which means a 100 micron filter is capable of trapping debris that’s at least 0.039-inch in diameter and perhaps slightly smaller. This will prevent large foreign material from harming the pump. As the filter’s micron rating number becomes smaller (trapping finer particles) it also becomes more of a restriction. A 100 micron rating on the inlet side still filters large particles but is a minimal restriction. On the high pressure side of the pump, the spec is far tighter to remove material that could foul a fuel injector. The Aeromotive spec for this filter recommends 10 microns (0.0039-inch), a full 10 times finer than the inlet side.

But, even more important than the particle size is the filter’s surface area. Common sense dictates that a large surface area for a filter is probably more important than its screen size. A large surface area allows the filter to do its job for an extended period of time without worry that debris will reduce fuel flow. That’s why these filters are so large. Keep in mind that during part-throttle operation, nearly all of the fuel is returned to the tank. This moves the fuel through these filters far more often than a non-return system.

There’s also a choice of filter materials. Aeromotive and others offer both paper (cellulose) or a cleanable stainless steel. Obviously, the stainless media allows the user to clean the filter and re-use it — assuming, of course, your cleaning solution (like dirty solvent) does not add debris to the filter!

One important point is to never use a cellulose or paper filter with E85 fuel. Aeromotive says they have seen issues where ethanol creates a gel when used with a paper fuel filter. Remember, ethanol is predominantly an organically-based fuel, as opposed to pure gasoline that is petroleum-based. E85 is no different that grain alcohol except for the 15-percent gasoline mixed with it. So, to prevent this “biological fouling” — essentially a bacterial slime — from growing inside your fuel system, stick with stainless steel filters. To support this, Aeromotive makes a special 40-micron stainless steel filter specially designed for use with E85.

Carbureted systems are no less vulnerable, as evidenced by the builder’s misuse of that tiny fuel filter on that 572c.i. big-block example in our story introduction. It’s easy to understand how this oversight could happen. While carbureted fuel pressures are significantly lower than EFI systems, fuel volume is still a crucial requirement. The engine’s horsepower potential determines the flow capacity required of the system. You can’t make horsepower without fuel.

A simple equation can help in determining capacity. We’ll keep the math to a minimum, but if a normally-aspirated big-block Chevy is capable of 750 hp, let’s assume a BSFC number of 0.5. BSFC is the acronym for brake specific fuel consumption — a fractional rating of the pounds of fuel required to make one horsepower for one hour. In this case, that comes to a half-pound of fuel per horsepower per hour. If the number were larger (0.65 for example), the engine would require more fuel to make the same horsepower. Conversely, if the BSFC number is smaller (0.35 as an example), it is more efficient at converting fuel into power, requiring less fuel.

So with a BSFC of 0.5, this means for a 750-hp engine, we divide 750 by 0.50, which gives us a minimum of 375 pounds of fuel per hour. In order to ensure we will supply enough fuel, we will add a little “head room” to the number, making it 400 pounds of fuel per hour as a safety margin. Divide 400 pounds by the weight of gasoline at 6.2 pounds per gallon and that equals almost 65 gallons of fuel per hour at a minimum pressure of at least 5 to 6 psi.

Despite the fact carburetors don’t demand the same pressure as EFI systems, flow is still important. With flow rates increasing with higher horsepower engines, a full return-style system is the best way to build a system, even for a carburetor. This can be easily accomplished with a return-style regulator.

If you’re not sure of the cartridge’s filter rating, it is stamped on the closed end of the filter. In this case, it’s a 100 micron filter.

If you’re not sure of the cartridge’s filter rating, it is stamped on the closed end of the filter. In this case, it’s a 100 micron filter.

Regarding filters for a carbureted application, a 100-micron inlet filter is still a good idea while the pressure side filter does not have to be quite so fine, so a 40 micron filter with a large surface area would be the best. Because of these lower pressures, these inline filters can be slightly smaller with no fear of flow restrictions. For example, the Aeromotive PN 12335 filter offers a slightly shorter 5.5-inch long body with a stainless steel 40-micron filtering ability that can easily accommodate even a dual-carbureted Pro Stock big-block application.

As street cars become more sophisticated and engines continue to make more power, the fuel filter is one of those few instances where bigger is better and where more surface area only means more flow. So at least here, you can’t go wrong by going big.

Sources: Aeromotive; 913/647-7300; aeromotiveinc.com; Holley Performance Products; 270/781-9741; holley.com; Edelbrock(Russell); 310/781-2222; edelbrock.com

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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