Fuel Filters - Good read
Discussion
I receive a bi-monthly news letter of these people: http://www.robbmcperformance.com/about.html
Here's todays and I thought someone on here must be interested:
FUEL FILTERS
A fuel system should have two filters; One before the fuel pump and one after the fuel pump. This is how the factory did it. They generally had a "sock" filter in the fuel tank and then another filter in, or just before, the carb.
The smaller the micron rating of a filter, the smaller the particle it will catch. However, for a given size filter, the smaller the micron rating, the more restrictive the filter. All things equal, the filter before the pump should be freer flowing than the filter after the pump. This is because the fuel before the pump is under a vacuum and is more likely to vaporize or cause pump cavitation.
One way to make a filter flow more is to make it larger so it has an element with more surface area. The other way to make a filter flow more is to use an element with a larger micron rating. So, if you use two filters with the same micron rating, the one before the pump needs to be bigger. The alternative is to use two filters of the same size but with the filter before the pump having a larger micron rating.
Fortunately, the filter before the pump generally doesn't need to filter as finely as the filter after the pump because the pump is not as sensitive to contaminants as the carb or injectors. A 90 to 200 micron filter will filter well enough for mechanical pumps and rotary vane style electric pumps. Gerotor style pumps have tighter internal clearances and thus require a prefilter of about 30 to 50 microns. The filter after the pump needs to filter out smaller particles than the prefilter. A 30 to 50 micron will work for a carb and 5 to 10 microns will work for FI.
To summarize, the prefilter needs to flow more than the post-filter. To accomplish this, the prefilter needs to either be much larger than the post filter, or have a larger micron rating, or both.
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Phil
Here's todays and I thought someone on here must be interested:
FUEL FILTERS
A fuel system should have two filters; One before the fuel pump and one after the fuel pump. This is how the factory did it. They generally had a "sock" filter in the fuel tank and then another filter in, or just before, the carb.
The smaller the micron rating of a filter, the smaller the particle it will catch. However, for a given size filter, the smaller the micron rating, the more restrictive the filter. All things equal, the filter before the pump should be freer flowing than the filter after the pump. This is because the fuel before the pump is under a vacuum and is more likely to vaporize or cause pump cavitation.
One way to make a filter flow more is to make it larger so it has an element with more surface area. The other way to make a filter flow more is to use an element with a larger micron rating. So, if you use two filters with the same micron rating, the one before the pump needs to be bigger. The alternative is to use two filters of the same size but with the filter before the pump having a larger micron rating.
Fortunately, the filter before the pump generally doesn't need to filter as finely as the filter after the pump because the pump is not as sensitive to contaminants as the carb or injectors. A 90 to 200 micron filter will filter well enough for mechanical pumps and rotary vane style electric pumps. Gerotor style pumps have tighter internal clearances and thus require a prefilter of about 30 to 50 microns. The filter after the pump needs to filter out smaller particles than the prefilter. A 30 to 50 micron will work for a carb and 5 to 10 microns will work for FI.
To summarize, the prefilter needs to flow more than the post-filter. To accomplish this, the prefilter needs to either be much larger than the post filter, or have a larger micron rating, or both.
__________________________________________________________________________________________________________________
Phil
Interesting topic for those of not wishing to make our new fuel pump 
Now off out to garage to check my microns. There is more than one thinks to putting a car back together that is if you want to avoid expensive mistakes
Spose Trev or Peter must have sat down and worked it out for us numpties before banging it all together!
Now off out to garage to check my microns. There is more than one thinks to putting a car back together that is if you want to avoid expensive mistakes Spose Trev or Peter must have sat down and worked it out for us numpties before banging it all together!
So the Gauze "strainer" thing I got from RS is probably OK then, as all it has to do is prevent big bits of crud getting into the pump.Gassing Station | Wedges | Top of Page | What's New | My Stuff