Filter tips

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Spanner Monkey
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Name: Billy Belfont
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Filter tips

Postby Spanner Monkey » Wed May 25, 2016 1:15 am

Carrying on from my lengthy tome on plumbing, let’s look at filtration.

What’s the betting that you walk into a factors/speedshop/WalMart and just get ‘a fuel filter’ ‘an oil filter’ or ‘an air filter’ without considering what it really is/does.

Filters are measured in microns (a micron being 1/1,000,000th of a metre), and the measurement in microns is essentially the size of the ‘holes’ in the filter that the fuel/oil/air pass through.

Fuel filters are possibly the least serviced items on any car, but one of the simplest items that will allow you to make – or lose – power, so looking at the fuel system first, it’s essential to have a pre-filter (before the pump) and a post filter (between the pump and carburettor/injector) Typically the pre-filter is around 100 micron – a bit of a ‘rock catcher’ to stop large particles entering the fuel pump, and post filter between 10 and 40 micron (depending on application) to trap finer particles and stop them blocking the carburettor jets and drillings or fuel injector nozzle.
Most in-tank fuel pumps have a nylon pre-filter. These (whether they are factory-fitted or aftermarket) are predictably only serviceable if the pump is removed from the tank. Usually the servicing of these involves cleaning and blowing the accumulated particles off the filter, but occasionally they do get damaged and require replacement.
With external fuel pumps there is often a coarse gauze around the fuel pick-up, but not always. On older carbureted production cars there is usually one ‘obvious’ filter – usually between the tank and pump, and then (unless someone has snuck in one of those cheap, plastic in-line filters) apparently nothing. But unscrew the fuel inlet fitting from your Holley/Carter/Rochester and you’ll find a filter hiding in there. These normally take the shape of a solid bronze filter which clogs easily and can be the cause of quite a few niggly carb running problems. Throw it away and fit a decent 40micron filter in-line between pump and carb.

On injected vehicles you’ll find the main filter between pump and fuel-rail. Usually a metal canister type that is totally unserviceable, and can only really be checked if you remove it and cut it open. Because these are generally hidden where they cannot be hit by road detritus, they are easy to ‘miss’ when dealer servicing calls for them to be changed. I’ve seen quite a few vehicles where, despite a full service history the fuel filter is clogged to the point of cutting the fuel off altogether, and when cut apart are packed with a substance resembling mud. It’s probably mud.

The advice then with fuel filtration is:
1. Check what you have.
2. On a carbureted vehicle make sure there is a decent filter between pump and carb, and (preferably) another between tank and pump.
3. On an injected vehicle either make sure the factory filter is changed at regular intervals or replace it with a serviceable high-flow aftermarket filter and check that at regular intervals.
As with all things you will get what you pay for with filters, good ones will cost you a couple of hundred dollars or more, acceptable ones around the hundred buck mark, and ‘trouble’ any much less than that. If you’re building for power check the manufacturers flow-rates for the filters. If they don’t have flow-rates then…. Well, it tells you all you need to know.

This shows you the difference between an 'acceptible' filter, and a good one. Both are 40 micron, both a -6AN size. One on the left is $100, one on the right is $200. I wouldn't use the one on the left for an injected system.

Air filters use the same measurement system. Performance filters generally have either a larger micron rating or larger surface area or both, which will allow more airflow. Look at the surface area of a CAI as opposed to the standard panel filter in your late model Mustang. Now look through both of them. The CAI has a larger area and bigger ‘holes’, that’s what gives the airflow that enables more power to be made. The more power you want to make, the more airflow you’ll need.
On older carbureted cars the increase in surface area is easy to attain. If you’re air cleaner is a ‘standard’ aftermarket chrome 14” diameter x 1.5” high, plates are available to fit up to 4” high filters in increments of around ¼” (depending on what room you have). K&N (and others) now also produce a filtered ‘lid’ for these filters, which adds even more to the surface area.
Remember always though, if you are adding more airflow it will affect the tuning of your engine so you will need to get your carb retuned or ECU ‘flashed’.

Oil filtration is possibly the most important to ensure engine longevity. Just about everyone on the planet uses spin-on canister filters now, and despite peoples preferences and the odd horror story these really are all much of a muchness. There are aftermarket serviceable filters available if you’re dropping your oil every weekend, but these aren’t really necessary on your average road/competition vehicle.
It is good to cut the old filter apart on change and take a look at the size and amount of particles it’s trapping as this is a very good indication of the mechanical condition of the engines reciprocating parts and bearings. Just as with air filters though, adding surface area by fitting a larger filter or tandem filters will trap more particles without compromising flow.


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