GAGES, where?
• For a vacuum motive system ....
tank --> filter---> vac. gage ---> lift pump ---> engine, etc.
As the filter begins to 'clog' the flow will be restricted by the clog and it will take MORE vacuum to move the fuel. You see the *increased* vacuum on the gage as the filter begins to clog. You see 'decreased' vacuum if the filter 'breaks' a pleat, or the filter is 'unloading' etc. !!!!!!

•For a pressure fed RECIRC. 'polishing' system ....
Tank --> pump --> pressure gage ---> filter ---> tank.
As the filter begins to 'clog' the flow will be restricted and it will take MORE pressure to move the fuel. Pressure 'dropping': broken or 'unloading' filter.

• For a pressure fed engine fuel system ....
Gages get complicated! A pressure gage in-line with this system configuration is of little value because the pump is 'deadheading' against the 'regulated/metered flow' of the injection pump. So, to use a pressure gage to assay the differential pressure drop across a filter you have to add a "2 Tees and a cock valve" .... a valved 'branch' from the outlet of the filter which 'crosses over' to the 'return line.
Tank --> pump --> pressure gage ---> filter ---> Tee ("A"**) ---> engine.
**From "A", a cockvalve --> another "Tee" connected to the 'return line".
To operate, run the engine at a reference high rpm with prop turning, open the cock valve and read the gage (now both delivering to the engine and also discharging to the tank which is at 'atmospheric pressure' (0 psig); close the valve when done. If the engine begins to 'stumble' during the gage check, slowly close the cock valve to remove the 'engine stumble', then read the gage.

You read, and record. the gages when the engine is running with a load (in gear) and at near wide-open- throttle!

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VACUUM systems VS. PRESSURE systems. Where does the pump go before the filter or after the filter????

There are many reasons for putting a filter in 'pressure mode' filtration .... vastly superior for 'mechanical', hydrodynamic, and 'particle deposition' at the filter.

Mechanical/Hydrodynamic considerations -
• A single stage lift, etc. pump (non-constant displacement pump) can only develop about 16" hg. vacuum or about (minus) 8 psig. With higher vacuum the pump will begin to 'cavitate' and the pump will 'slip' (pump less liquid), then 'stall' with NO flow.
• Put the same pump in pressure mode service and it can deliver up to 30psi (or more) *pressure* .... or about 4 times the pressure as when in vacuum motive service. There is no 'cavitation', etc. at increased operational pressures.
• The flow through a filter is dependent on the pressure drop (fluid friction) through the filter material .... and is always rated at gallons per minute/hour/etc. VS. differential pressure (pressure drop or ∆P) across the filter. On Filter flow performance charts this is usually a plotted line with flow (Q) on the x-axis and Pressure drop (∆P) on the y-axis .... (corrected for the 'viscosity' of the fluid, although) fuel oil filter charts are usually viscosity corrected for #2 or diesel fuel. If the chart is for 'water', just divide the flow (Q) by 4.
Simple speak: the higher the pressure 'across' the filter the more volume will flow.
• EPA, USCG, fines, etc. .... The *caution* in pressure filtration is that if the system develops a leak you wont starve/stall the engine, you'll fill the bilge with 'oil' --- you need a 'bombproof' piping system for pressure filtration; certainly NOT copper tubing and 'compression' fittings.

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Particle deposition in/on a filter ... inside the filter vs. ON the upstream surface of the filter material.
Vacuum motive systems will develop the maximum (negative) pressure on the downstream side of the filter material. This tends to cause the particles to 'get pulled' INTO the 'pores' of the filter material, the higher the (negative) pressure the deeper the particles penetrate 'into' the filter material, all filter material has thickness or 'depth' ... this particle migration deeply into the filter material causes a rapid plugging of the filter in comparison to 'pressure feed'.
Pressure motive systems typically will deposit the particle mostly ON the surface of the filter material 'and maybe' only penetrate upwards of 10%+ of the thickness of the material ... leaving the other 90% of the filter 'unchallenged' - the result is more 'open flow', less pressure drop (artificially caused) .... PLUS as in the 'estimated' data from the 'mechanical discussion' you have 4 times the pressure (Q/∆P) or a potential increase of 300% more pressure/force to do the 'work'.

On a practical basis most 'plugged' filters used in vacuum service (ignoring the deposition anomalies) are simply thrown away and would still have about 2-3 times the 'capacity' remaining; if operated in pressure mode service ... simply because the pump in vacuum service cant deliver the force as it can in pressure service.

There's a LOT more to this than my presented 'simplified version' as probably most of the particles in fuel oil are 'soft and deformable' particles that are easily 'extruded' through a filter ... but one thing for sure is that once the 'deformable' are entrained DEEP in just about any filter material and then dont 'extrude', the filter will VERY quickly 'plug' .... to best filter these 'softies' you need to keep them on the 'upstream surface', use as LARGE a surface area or BIG a filter as you can afford (Racor does a great job in their recommendations on this 'surface area' sizing vs. engine fuel consumption at max. rpm) and of course choose *pressure mode* filtration. On a pressure feed system, you probably should put a pressure relief valve branched from the 'pressure side' into the 'return line' in case the pump 'goes postal' (my lawyer told me to add this).

Why vacuum systems on recreational boats?
Cheap and easy/brainless to install. Besides the boat builders would then have to 'alter' (including addition of a strainer 'before' the pump and a 'relief valve') the fuel system all the way to the injector pump .... engine warrantee issues.
Most transfer pumps have an integral screen already 'built-in' to protect the pump.

Putting a filter in front of a pump and putting the pump in vacuum service ... simply makes no sense on a mechanical or hydrodynamic or 'filter basis' .... other than 'cheap and dirty'. What should be upstream of any pump is a high capacity 'strainer'. A strainer with low pressure drop and with OPEN mesh of about ~100-150+µM to remove 'rocks, stick and feathers' in order to "protect the pump". Many such pumps already have integral strainers.

hope this long rant hasnt been too boring, and hope this helps.