How To Choose a Fuel Filter

By: Jeff Smith | 10/05/2020 < Back to Motor Life Home

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.

In the case of a fuel filter, too small can cause problems but there’s really no size limit for too large. There’s no need to go overkill on a filter as long as it provides adequate protection. The mid-size Holley 175 gph filters in the middle will likely never become a restriction to flow on most road-going vehicles.

We’ve disassembled a 162-554 Holley inline 175 gph filter so you can see the filter and spring. The body of the filter is marked to indicate the direction of flow. Fuel flows into the body of the filter housing, past the capped end of the filter cartridge and through the filter medium and exits from the opposite end. The spring allows fuel to bypass the filter should it become completely clogged.

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.

It’s important to keep track of the micron rating in any filer, but especially with inline filters such as this Holley 175 gph version. While the filter body is externally marked with the filter micron rating, you can also ensure the filter is correct by looking for the laser etched micron rating on the end of the filter. This is a 100 micron filter.

This Holley illustration shows the placement of the 100 micron pre-filter before the pump with the finer screen 10-micron filter after the pump and before the fuel injectors. It’s very common for installers to incorrectly switch the positions of these filters, which will reduce pump output and possibly make the pump run hot.

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.

This is the entire filter capacity of that 572ci big-block Chevelle example. This filter measured roughly ¾-square inch of filter area. That’s pretty small. This is after Gillman cleaned all the debris that had clogged it nearly shut.

We laid out these three face masks that represent the filter area of a typical Holley 175 gph filter such as Holley PN 162-554 using a cellulose 10 micron filter. The Kennedy half dollar we’re holding is actually 20 percent larger than the filter area of a 1-inch diameter flat screen filter used in that big-block Chevelle. It should be obvious which filter area would be best for a 600 hp performance engine.

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.

Most Holley inline filters are available with either tapered pipe thread or ORB fittings. The most popular has become the ORB (o-ring boss) filters using specific AN fittings that use simple o-rings to seal the fitting to the body of the filter (shown). These fittings are easier to remove and replace compared to national pipe tapered (NPT) fittings that require pipe thread dope to ensure a good seal.

High flow, high capacity fuel delivery systems need a large filtering area to minimize pressure drops in the system. Holley’s VR series filters place two of the smaller filters end-to-end to double the filtering capacity while minimizing flow loss.

High pressure EFI applications require a 10-micron cellulose filter to prevent debris from clogging fuel injectors. Pre-filters for EFI pumps use a stainless mesh 100 micron filter. However, if your plans call for running E85 or methanol fuel, make sure to always use stainless steel mesh fuel filters. Cellulose or paper elements can react to the organic elements in the fuel and create a gel-like material that can cause problems.

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.

Holley has combined a simple pressure regulator with a filter that allow you to place this regulator just after the fuel pump which will also filter the fuel with its included 10 micron filter. The inlet and outlet are along the long line of the filter while the outlet from the regulator is plumbed as return back to the tank. This eliminates two sets of connections which also reduces cost. The regulator is pre-set to 59.5 psi.

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.

Some Holley carburetors used small sintered brass fuel filters just inside the fuel bowls. These will work, but because of their small size will need to be monitored and replaced regularly. These are considered post or secondary filters since older, carbureted cars used a mesh screen as a pre-filter.

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 an excellent addition to custom fuel tanks because the material will pull fuel even if only a portion of the mat is contacting the fuel. Plus, HydraMat makes for a good pre-filter with 15 micron filter capacity. This does not represent a serious restriction because of the mat’s large surface area.

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.

One of the best ways to mount inline filters is with a set of Holley’s billet fuel filter clamps that will not only secure the filter housing to the car but do it fashionably at the same time.

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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