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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 07-08-2012
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diesel filter size 2 micron or 10?

All things being equal, if I have clean fuel, would a 2 micron filter in a RACOR restrict the flow of fuel to the engine compared to a 10 micron filter ?

My 30HP Nanni Diesel has a 12 micron filter on the engine. I have a RACOR fuel-water separator and have been using a 2 micron filter.

I believe my fuel is clean, having installed two 6" access ports, removed the old fuel, hand cleaned the tank and replaced with new fuel.

I have not been able to achieve full RPM on the engine. It is rated for a max of 3600 RPM. But with a clean bottom and a re-pitched new prop, cant get more than 3200 RPM.
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Old 07-08-2012
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Re: diesel filter size 2 micron or 10?

Why would you want to achieve max rpm?

Would you drive your car with the rev counter on the red line?

A boat [ assuming fuel economy and engine life is important] will be propped so that in flat water and with a clean bottom and at full throttle it runs revs BELOW max. Approximately midway between max torque and max rpm is a good starting point.
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Old 07-08-2012
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Re: diesel filter size 2 micron or 10?

At constant pressure/vacuum a 10ÁM Racor will filter at a rate of 5 times the amount of fuel than with a 2ÁM. Filters are 'sized' based on GPM vs. 'differential pressure across the filter'. Filtering takes 'work'. So, in a system that develops a maximum vacuum (or pressure) by its lift pump using a 2ÁM instead of a 10ÁM .... you will get only 1/5th the flow of fuel oil. ..... and this relates to only clean filters and not filters that have trapped debris.

If for some reason you NEED (and you shouldn't) to filter with a 2ÁM instead of the 10ÁM to get the same amount of oil flow, you will need to increase the SURFACE AREA of the 2ÁM by 5 TIMES.
Without increasing the filter surface area AND if there is sufficient debris in the oil, that 2ÁM will also 'choke' with debris at 'more' than 5 TIMES FASTER than a 10ÁM .... simply because in nature there are exponentially more smaller sized particles than larger sized particles. Putting an undue load on your engine's lift pump (5 times more vacuum needed to deliver the SAME amount of fuel) by using a 2ÁM instead of the spec'd 10ÁM will increase the potential of breaking the diaphragm in your engines lift pump.

However in your case, the average RACOR is soooooo 'oversized' for small hp diesel engines that I would guestimate that the filters are not the cause of you not reaching max rpm in your engine .... check for too tight stuffing box, wrong prop pitch, errant rpm gage, etc.

Best way to check rpm is with an optical rpm gage vs. the flywheel; most 'tachometers' on small boats are usually in great error - either use a mechanical or optical tach.

There are other reasons why a 2ÁM should not be used, but the above should be sufficient 'for starters' on a sound technical filtration basis.

;-)
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Last edited by RichH; 07-08-2012 at 12:22 PM.
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Re: diesel filter size 2 micron or 10?

On my Universal M25 I have a Raycor R24P (30 micron), and a WIX 33390 (10 micron) secondary filter. IIRC, the engine requires filtering to 17 micron (it might be 12 micron).

Under NO load, the engine will rev up to 3000 RPM. Under load, with the boat moving, it will rev up to 2600 RPM. Under load, while tied to the slip, it will rev to 2400 RPM.
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Re: diesel filter size 2 micron or 10?

Quote:
Originally Posted by KentIslandSailor View Post
All things being equal, if I have clean fuel, would a 2 micron filter in a RACOR restrict the flow of fuel to the engine compared to a 10 micron filter ?

My 30HP Nanni Diesel has a 12 micron filter on the engine. I have a RACOR fuel-water separator and have been using a 2 micron filter.

I believe my fuel is clean, having installed two 6" access ports, removed the old fuel, hand cleaned the tank and replaced with new fuel.

I have not been able to achieve full RPM on the engine. It is rated for a max of 3600 RPM. But with a clean bottom and a re-pitched new prop, cant get more than 3200 RPM.
I'd also check the exhaust elbow..
http://www.nannidiesel.com/images/st...3.30%20ENG.pdf

Last edited by aa3jy; 07-08-2012 at 05:04 PM.
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