This discussion was lifted from the SSCA board , cruising sailors , not marine motorists , but the info might be a help to some.
I agrees sort of with the last comments , my preference is for a Gravity fuel Day Tank, not a pump, but its a minor thing.
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[/list]<h3>Re: Mystery Power Loss
╗ Tue Oct 26, 2010 4:04 pm
Whats the ÁM retention size of the filters installed ??????
Your symptoms suggest that you have a Racor 2ÁM installed in place of the normally recommended 10ÁM in the 'secondary' position of your fuel distribution system; and, that the OEM selection of the size (surface area of the filters installed) may be undersized, *especially* if you have a 2ÁM in the secondary position.
Filters take WORK to operate and they are sized/selected based on the ~150+++% demand of the engine at WOT and are 'sized' based on whats known as the "flow rate vs. differential pressure (vacuum) curve" - go to http://www.parker.com
----> Racor division ----> your specific filter mod. and ÁM .... and search/look-up the specific 'flow vs. P' curve for the filter ÁM being used. Then validate that the filter will be able to pass the required flowrate at the 150%-200% demand of fuel. Figure that a engines 'lift pump' will only be able to 'draw' about 4-5" of maximum vacuum if you dont have the exact value from your engine tech manual.
The 'typical' secondary filter will normally be selected/designed at 10ÁM rating .... if you erroneously change a 10ÁM to a 2ÁM, the flow rate at constant pressure/vacuum will *decrease* by a factor of 5. .... and if the OEM filter set is marginal in capacity, you now with a 2ÁM will be 'starving flow' to the engine guard filter (small engine mounted filter) and the engine. So, if you have a 2ÁM anywhere in the filter set, replace it with a proper 10ÁM and see if there is any change in engine rpm. The small engine 'guard' or 'last chance' filter on a Yanmar is normally 15-17ÁM (97% efficiency) .... even if it has a 2 or 10ÁM preceding it.
Note1: If your somehow erroneously think that a 2ÁM rated filter is better and the system was OEM designed for 10ÁM .... you will need to 'upgrade' the surface area of the filter. For a 2ÁM in place of a 10ÁM you will need FIVE TIMES more surface area to deliver the SAME AMOUNT of fuel (at the same pressure/vacuum) - a filter that is 5 times as 'large'.
Note2: Unless you are running an independent on-board constant-on 'recirculation polishing filter' ---- DO NOT use a 2ÁM filter anywhere in the system.
When was the last time you 'decontaminated'/cleaned your fuel tank??? Most biological growth/debris in diesel fuel is from common resin forming fungus (cladosporiuim resinae, etc.) which will be visible on a filter as a BLACK slime. However if you have a very rare BACTERIAL contamination, and the species does not use (much) the oil as its nutrient source then it is possible that the (invisible) bacteria are prematurely blocking the filter media. You change to a new filter and very quickly the fuel delivery seems to 'starve'. If so, the Remedy: Add a 'bacteria specific' fuel conditioner.
Recreational boat fuel systems are usually CRAP, made of the cheapest and most easy to assemble components - copper tube and COMPRESSION FITTINGS. Under vacuum motive service compression fittings are notorious 'leakers', especially vulnerable to SUCKING AIR, and the air may NOT be visible until the pressure changes in the lift pump or injector pump allow the air to 'come out of solution' from the oil. If this 'problem' continues and is not found by correction of blocked fuel flow (filters and delivery line) then suggest you consider to change to a PRESSURE motive delivery system ..... put a 12vdc fuel pump ON or immediately 'AFTER' the tank. The fuel filtration will instantly become vastly more efficient, any micro-air bubbles (common in diesel fuel) will tend to 'collapse' due to the higher pressure ..... you'll need double flared stainless tubing/connectors, etc. to do this.
If you have any TRANSFER PUMP that controls the fuel level in MULTI TANKS ...... transfer pumps are notorious for 'whipping up' the oil and entraining AIR/gas in the fuel. Much of this air will become similar to an 'emulsion' or bi-phasic mixture with the air being invisible, until pressure changes in the system (lift pump) or the down-flow side of a 'tight' filter will coalesce the air OUT of the fluid, the air collecting in 'high spots' in the delivery system. Typically you get 'surges' in engine rpm, but if the conditions are 'just right' the air 'slug' (in a vacuum motive system) may retard the flow of fuel.
If you have a TRANSFER PUMP ... turn the damn thing off and/or change its 'position' from a suction device to a pressure device by changing its location. You may have to add a small 'throttle valve' to the terminal end of the transfer circuit - to keep the pressure HIGH in the transfer lines so that the entrained air/gas bubbles are 'compressed'.
The typical Racor set up is 30ÁM--->10ÁM---->engine guard @ 15-17ÁM. Be sure to change out the engine guard filter when changing the primary/secondary because the 'guard' at 15-17ÁM although may be larger in ÁM 'rating' has VASTLY smaller surface area and is (as designed) will tend to 'choke' when receiving less than 10ÁM particulate due to the 'higher velocity' of fuel through this small surface area filter. The higher velocity causing the particulate being 'driven' very deeply into the filter media which tends to promote the much smaller than 'rating' sized particles to quickly build up and block/"bridge" the 'effective pore size' even when the particles are much much smaller than the filter's (arbitrary) 'rating'.
Fuel filtration and delivery is BEST done under PRESSURE, Boat builders choose vacuum type delivery system because they are the cheapest and least labor intensive. A vacuum system can only develop a 'few inches' of vacuum; in contrast, a pressure motive system with a 12vdc pump (immediately downstrem of the tank) can easily develop up to 6 psi PRESSURE ---- longer lasting filters, more efficient filtration, dont 'starve' under high differential pressure across a filter (when partly plugged) .... but takes some quality hardware and detailed installation to accomplish so you dont inadvertently fill the bilge with diesel oil if you do develop and oil leak.
hope this helps.