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Old 14-02-2018, 05:01   #1
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Progression of fuel filters

I have a Volvo MD7A diesel. I have two fuel filters at the moment. The one on the engine and a racor pre-filter.

My question has to do with the proper filter size. Shouldn't the pre-filter be a courser grade than the engine filter ? Such as a 20 micron in the racor and a 2 micron on the engine ?

I appreciate any wisdom on this. Thank you.
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Old 14-02-2018, 06:03   #2
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Re: Progression of fuel filters

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Originally Posted by OldManMirage View Post
I have a Volvo MD7A diesel. I have two fuel filters at the moment. The one on the engine and a racor pre-filter.

My question has to do with the proper filter size. Shouldn't the pre-filter be a courser grade than the engine filter ? Such as a 20 micron in the racor and a 2 micron on the engine ?

I appreciate any wisdom on this. Thank you.
The question has been debated often hereabouts and elsewhere often without consensus. Conventional wisdom does call for a course "pre-filter" and finer after or "final" filter. However, we and many others, have found over the years that a large capacity (in terms of flow through capacity) coupled with a fine filter element in the "pre-filter" works very well. In our case, as with most of the folks we know, we use a Racor FG-500 with 2MC filter elements on a 50hp Perkins as our "pre-filter" and don't bother much with the secondary filter, and have yet to have a problem with fuel. Of course, we are obtaining fuel in the US from sources that move a lot of the stuff, so it tends to be quite clean and free of water contamination. Elsewhere that might not be the case, which is why most voyaging still rely on Baha Fuel Filters or similar when refueling. So...the choice is yours.

FWIW...
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Old 14-02-2018, 07:31   #3
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Progression of fuel filters

I go with the prefilter advice, or if you just go to a super fine pre filter, then having two filters is illogical.
My pre filter is 30 micron, and the final is Yanmar and you have a hard time finding out what that really is, but I donít think itís a 2 micron, I think itís bigger, of course fuel cannot be too clean, filtering finer canít be anything but good in the long run.
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Old 14-02-2018, 11:19   #4
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Re: Progression of fuel filters

Thanks for the replies.

I believe I will follow conventional wisdom and go with the 2 micron high flow pre filter and just change the engine filter on a regular schedule or as needed. I'll be looking for a Racor FG500.
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Old 14-02-2018, 11:29   #5
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Re: Progression of fuel filters

Wow, $30 on Ebay and you can get a case of 2 micron filters for $109. That seems like a deal !
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Old 14-02-2018, 11:41   #6
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Re: Progression of fuel filters

Yes, that's what I would do. coarser first, finer second. Too fine first and it may plug up fast.
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Old 14-02-2018, 11:42   #7
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Re: Progression of fuel filters

$30 is not Iím sure a real Racor, there are knock offs, I donít know their quality. Be wary, maybe post the link?
If you donít have a Racor now, be advised the fittings that go to the filter are only available through Racor, so order the fitting too.
I case of filters (12) and they have been real Racor filters.
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Old 14-02-2018, 12:12   #8
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Re: Progression of fuel filters

With "Miami Boat Show Pricing", a Racor FG 500 Turbine Filter retails for $173.99 at Defender.com (See Racor Filters). There is no way you're going to get that for anything less. And, Racor Aquablock filter elements sell for $10.99 each. Don't even think about going with a cheap knock-off.

FWIW...
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Old 14-02-2018, 12:33   #9
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Re: Progression of fuel filters

These are genuine Racor filters, itís how I buy mine,
https://www.ebay.com/itm/NEW-RACOR-2...1WsfRU&vxp=mtr
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Old 14-02-2018, 12:38   #10
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Re: Progression of fuel filters

2 micron is overkill. It will clog relatively fast. 10 micron is good enough for any Diesel.
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Old 14-02-2018, 12:52   #11
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Re: Progression of fuel filters

This same debate rages among chemical engineers and refinery engineers as well. The answer is that there is no single right answer. Believe, I've used both strategies and gone through many thousands of filters. A few things to consider:
  • Is the secondary filter hard to change without shut down? With engines, yes, and that can be a problem. You NEVER want a single critical filter to foul between scheduled outages. With twin filterers, you can change one at a time.
  • Is the fouling surface or in the pores? Fouling on the surface is sometimes actually reduced by having larger particles present which add porosity to the filter cake. In other words, just the fine stuff may quickly make an impermeable slime, where as larger particles might allow some flow.
  • Can you add more surface? With Raycors, they can be much larger and can be in parallel.
  • Do run both at the same time or one at a time? The advantage of swapping is that you have one fresh filter ready to go. However, by running them both, you will get at least 4x the filter life, because the surface area is greater and pressure drop is less. To change you may have to throttle all the way back.
So no single answer. A single fine pre-filter is probably bad (there should be two) and twin coarse filters may not be optimum either (they may allow the inaccessible engine mounted filter to foul underway, which is the worst outcome). Thus, I am no fan of 30 micron filters.



I believe 10 micron is fine enough for a number of reasons.
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Old 14-02-2018, 13:50   #12
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Re: Progression of fuel filters

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Originally Posted by a64pilot View Post
$30 is not Iím sure a real Racor, there are knock offs, I donít know their quality. Be wary, maybe post the link?
If you donít have a Racor now, be advised the fittings that go to the filter are only available through Racor, so order the fitting too.
I case of filters (12) and they have been real Racor filters.
Ok, it seemed to good to be true. I do have an older racor now, and I'm wanting to upgrade. Hence all the questions.
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Old 14-02-2018, 13:52   #13
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Re: Progression of fuel filters

OK, no cheap knock-offs, and that's 2 votes for 10 micron. I'm pretty sure that's whats on the engine itself. And access is not awesome, but access to the primary filter is a piece of cake.

What about spin-on filters for a primary ? Any thoughts ?
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Old 14-02-2018, 13:56   #14
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Progression of fuel filters

Price good spin on, they are usually much higher, so real soon there is no savings. Racor anyway.
I went from a tiny Racor S120 on my generator (a spin on) to installing a 500, which is way oversized for the job, cause the filters themselves were about half price, and way, way more available.
Thatís the thing with a Racor 500, everybody has one, so the filter replacements are easy to find.
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Old 14-02-2018, 14:39   #15
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Re: Progression of fuel filters

I always went with a 10 micron in the Racor until some evidently bad fuel in the Bahamas clogged the Yanmar on-engine filter (which I think is 5 micron) in the middle of the Gulf Stream.

Somehow the fuel contaminate was smaller than 10 micron Racor but bigger than the Yanmar filter. I couldn't get more than 1200 RPM even though the Racor filter was clean and the suction gauge on the Racor showed no problem. Worse, my racor is positioned for easy changing but the Yanmar filter on the side of a hot engine is downright hazardous to change in an eight foot sea. The moment the Yanmar filter was changed, everything was fine. I then put 2 micron in the Racor and went through six of them on the bad fuel getting to my destination.

Ever since, I just run 2 micron filters in the Racor. I'd always heard about the "frequent fouling" with a fine Racor but I haven't seen it in practice. The Racor 500 filter housing can handle 60 gph. I'm running 2 GPH. I have a gauge and change the filter at the first sign of suction buildup which on US fuel is over 200 hours. I carry a dozen of them in case I get some bad fuel again.

And this also means that the very expensive Yanmar filter hardly ever never needs changing as the Racor is finer. I do change it about every 500 hours just to be safe but looking inside the old one it seems clean as can be.

The only wrinkle for Yanmar owners is that Yanmar specs 30 micron and threatens to not honor your warranty if you run finer (I guess because they worry about damage to the fuel pump). Fortunately, my engine's out of warranty so I don't need to worry about it.
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