What is normal Racor vacuum

[bobk]

Hi Guys,

Remember my thread earlier this year about plugged filters. Well, I bought a case or Racor 2020PM's (30 mic) and changed several times 'just in case'. I also have a bunch of secondaries but have not changed them yet.

Last week I got around to installing vacuum gages with a drag needle on the Racors. My first run with the gages showed 9" vacuum on port side, 6" on starboard. I just changed the Racors again and ran out for a check and both are at 6". 6V92 TA's, running at 1600.

I thoroughly cleaned the bowls and turbine recently, but I'm seeing some slime collecting in the bowls. I run 2500 gal diesel each year and am surprised at the crud persisting over three years of this kind of use. By the way, no sign of water in the bowls.

What is typical Racor vacuum? And any ideas on the slime short of draining/cleaning the tanks which doesn't look easy in the 48MY.

Bob
Chateau de Mer
1981 48 MY

 

[Brian Degulis]

There is no typical Racor vacum. The vacum with clean filters is dependant on the fuel system and engine flow rate. For DDs I believe 15 is the max allowable vacum so 6 is fine. Use that as a basline for filter changes. If you been having fuel problems I would change the secondaries. I would also put a pressure guage (liquid filled) on the discharge side of the secondary fuel filter housing (there's usually an un used port that's pulgged off). Your vacum guages only tell you the condition of the fuel system up to the delivery pump they don't tell you fuel pressure at the fuel rack. So if your secondaries are getting clogged the fuel flow slows down that lowers the vacum reading leading you to believe all is well when in fact you don't have enough fuel going thru the rack. Adding a pressure guage gives you the whole fuel story at a glance secondary filter condition inculded.

Brian

 

[MikeP]

When I had Racors, they ran anywhere from 7-11" vacuum depending on the loading. They would also change vac depending on how full the tanks were - less vac when full, considerably more when low.

I replaced the Racs several years ago with the fleetguard system and they run about the same.

I have seen sites where they say that vacuum above 10 indicates a clogged filter but I have not seen that to be true with either the Racors or the Fleetguard.

I've had readings as high as 15" at WOT which had no affect on engine operation. This was when we first bought the boat and it sea-trialed at 18.5K. I replaced the filters shortly after purchasing the boat and have never seen it that high since but clearly 15" did not restrict fuel flow in any noticeable way on our 8V71TIs at WOT.

 

[Brian Degulis]

DD wants no more than 12 in a dirty system. 15 will restrict fuel flow but not enough to starve the engine it will still get enough fuel to operate but not enough to pass thru the injectors and cool them. At 15 your also aproaching the point where you will start to pull air out of the fuel and cause it to foam this happens even if there is no air leak there's a term for it that I don't remember. I would use 12 as a safe max with a dirtry system. Lower is better but not always possible it all depends on the lift lengh and size of the fuel lines # of valves elbows etc.

Brian

 

[Pete]

On the output of the secondary fuel filter, what are the PSI numbers you would look for during normal operation versus the lower limit for safe operation? Related to this, what full scale PSI rating gauge would you use for this purpose? Thanks

Pete

 

[MikeP]

The gauges on my still-present racor filter for the genny shows a red line at 15" HG (inches of mercury) which is 1/2 the gauge reading. So a gauge with up to 30" would be plenty. 20 would be fine if there are such gauges available. There is no "lower limit" per se, the lower the better though there will always be at least the amount of vacuum needed to pull the fuel up to the engine. On our 53 I'd figure there would have to be at least 4-5 inches of vacuum even if there were no filters at all.

 

[Brian Degulis]

On the output of the secondary fuel filter, what are the PSI numbers you would look for during normal operation versus the lower limit for safe operation? Related to this, what full scale PSI rating gauge would you use for this purpose? Thanks

Pete

DD calls for 50-70 PSI @ 1800 RPM and above. There is no spec for under 1800 but they normally run 15-25 PSI near idle. It's going to depend on what return line orifice is in your engine. The best way to determine what yours should be is the same as the vacum guages. Get readings with clean filters @ cruise RPM that gives you a basline to work with. A 100 PSI Liquid filled guage is fine Graingers has a complete line.

Brian

 

[bobk]

Thanks guys. On the trawlers that sip fuel we usually so no reading until the filters got fairly plugged and then had to change ASAP. I'll just monitor daily until I see it start to increase.

Bob