How To Choose a Fuel Filter
An important part of building a performance car for either the street or the race track is ensuring the fuel delivery system operates at its peak potential. You can’t make horsepower if there’s no fuel to burn. In the old days of small carburetors and mechanical fuel pumps, car builders relied on those tiny little brass filters used in the inlet of a Holley carburetor or a small inline fuel filter.
Those parts worked sufficiently for engines that barely made 400 horsepower. However, today’s supercharged EFI engines have pushed the horsepower peak well into the four-digit range. A 1,000 horsepower pump gas street engine would have been unthinkable 30 years ago, but today hardly raises an eyebrow. More importantly, those engines can inhale an astonishing amount of fuel in a very short time. The days of 5/16-inch fuel line and fuel filters no larger than the size of your thumb can be assigned a spot in automotive museums alongside one-barrel carburetors and red plastic fuel line.
The point is that while horsepower has never been easier to make, you must be able to feed the engine with the fuel it demands. So a mild 500 hp EFI street engine places sufficient demand on the fuel delivery system – which includes the fuel filter. A tiny filter stuck in the middle of the fuel line running to an EFI fuel rail won’t get the job done.
The best way to illustrate the importance of designing and constructing the proper fuel system is with a short, anecdotal story. This is not a hand-me-down urban myth. We watched this happen. A good friend was hired by Galpin Auto Sports (GAS) in Van Nuys, California to perform an engine swap. The owner of a ’67 Chevelle was frustrated with his 621 hp 572ci big-block carbureted crate engine. According to the owner, the car was lazy and could barely spin the tires. Several shops had failed to remedy the problem. The owner was so frustrated that he purchased a supercharged EFI LT4 engine and asked GAS to perform the swap.
Gillman inherited the project just after the Rat motor had been removed and sold. He was curious why the big-block didn’t perform since those engines have an excellent reputation for making good power. While removing the original fuel delivery system, he discovered the culprit. The original builder arbitrarily installed a tiny screen fuel filter with the area roughly equivalent of a silver dollar and tasked that small filter with the impossible task of removing debris and flowing 600 horsepower’s worth of fuel. When dirt and junk clogged the small filter, fuel flow dropped precipitously which seriously handicapped horsepower.
The moral of our fuel flow equivalent of an Aesop fable is that every part of the fuel delivery system – including the fuel filter – must be sized properly to ensure proper fuel flow. But as we’ll reveal, the size of the filter is only half the story. The other half is the micron rating of the filter. Let’s jump right into that.
In the old days of carbureted engines, no one talked about micron ratings of fuel filters. But with the advent of electronic fuel injection and high pressure fuel delivery systems, this became an important aspect. The main reason is that the rather large orifices in a carburetor do not demand a fine filter capacity. But high pressure fuel pumps operate because of very tight clearances. Plus, electronic fuel injectors can be quickly plugged with material that would have easily passed through a typical carburetor. This places a fine point on improving filtering capacity. Enter the micron rating.
A micron is defined as one millionth of a meter. Expressed as a decimal it comes in at 0.000039-inch. Yes, that’s pretty tiny and invisible to the naked eye. Even with this microbial diameter, electronic fuel injectors can quickly become fouled unless protected by a 10 micron filter. That 10-micron rating means that the filter is capable of catching 90-plus percent of all dirt and debris measuring 10 microns or larger In diameter. That’s a piece of debris measuring only 0.00039-inch or larger. To put that in perspective, a high output big-block Chevy main bearing clearances are generally close to 0.003-inch, so that 10micron filter will remove dirt one-tenth this dimension.
But the story now gets a bit more complicated. While a 10 micron filter does a great job of removing dirt, this fine filtering capacity also means it creates a bit of a restriction to flow. All fuel pumps are designed to push and pressurize liquid fuel, but in order to do so, they demand an unrestricted inlet. The most important point in any fuel delivery system is the inlet to the pump. Creating a restriction on the inlet side of the pump is guaranteed to reduce volume and pressure and make the pump work harder which will reduce its life expectancy.
But electronic fuel pumps still need to be protected from debris that could reduce efficiency or even render it inoperative. Since a 10 micron filter is too restrictive on the inlet side, a less restrictive filter is preferred. For inline pumps where it is located outside the tank, a 100 micron pre-filter is recommended. This larger filter minimizes the restriction into the pump and finer grit material will still be filtered by the 10-mircon filter located downstream of the pump before it reaches the injectors.
A great pre-filter for in-tank pumps is Holley’s HydraMat system. This unit performs double duty as an excellent pickup for fuel for EFI system and it also works as a great pre-filter with its 15 micron rating.
Now you might be thinking “Wait, that HydraMat is way too restrictive,” given the information in the previous paragraph. On the surface that may appear to be the case, but according to Holley fuel system engineer Matthew Sosa, the HydraMat’s large surface area reduces the restriction and employs a coarse outer layer to prevent large debris from clogging the filter. Combined with the weight of fuel sitting on the mat to help push the fuel into the pump, the HydraMat and even the white colored pre-filter “socks” (which are rated at 10 microns) do not represent a major restriction again because of their large surface area.
Along these lines however, we have seen several instances where the 10 and 100 micron filters have been accidentally reversed, and this can reduce the fuel pump’s effectiveness although it’s not a deal breaker.
One place where Holley has reduced the complexity of plumbing the fuel system is with the EFI Filter Regulator. This unit combines a 175 gph, 10 micron filter with a pre-set and non-adjustable 59.5 psi regulator. This unit can be placed downstream of the EFI fuel pump. This package eliminates two AN fittings between a separate regulator and filter which also reduces the same number of potential leak paths. The filter is completely serviceable and can be easily disassembled for cleaning.
The main tasks for any EFI system filter is to both cull out the dirt before it reach the fuel injectors while also offering sufficient flow capacity so that the engine is not starved for fuel. This is best accomplished by increasing the filter’s total surface area. This brings us back to the original example of the 572 Rat motor starved for fuel. The filter installed on that Chevelle offered less than one square inch of total filtering surface area.
Conversely, a typical 175 gallon-per-hour (gph) Holley inline filter with a 10 micron paper element offers 37 square inches of surface area. That’s a 50x increase in filtering area. Let that sink in for a moment. If the original 572ci Chevelle builder had simply used the appropriate 40 micron stainless mesh version such as a Holley 175 gph filter, the fuel delivery system would have flawlessly fed that big-block with enough fuel to annihilate those rear tires.
We mentioned the 40 micron stainless filter because that micron rating is more than enough to filter fuel for a carbureted application. So when we combine a large surface area like the stainless steel mesh filter with a 40 micron filtering rating, this allows the fuel pump to deliver its maximum potential flow rate with only a minimal pressure drop through the filter. Ideally, this would be installed in a return style fuel delivery system.
Holley’s HydraMat is also a great option for a fuel pickup in a custom or one-off application where a Sniper fuel tank or Holley in-tank conversion is not available. The HydraMat acts almost like a siphon or sponge where if any portion of the mat comes in contact with fuel, the HydraMat will direct the fuel into the pump inlet. This will help to minimize that sag or hesitation with electronically fuel injected engines where the fuel sloshes away from the pickup, creating a serious drop in fuel pressure. Plus, as we’ve mentioned, it also works as an excellent pre-filter for a high-pressure EFI fuel pump.
This should serve as a great introduction to fuel system delivery and how important filters are to any high performance engine. While delivering fuel to the engine is critical, the fuel also needs to be clean – which also serves to reduce other maintenance problems. It’s also best to think of the entire fuel system as integrated components rather than individual parts. A combination of the right parts and a professional installation will create a system that will deliver lots of fuel, horsepower, and fun for a long time.
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