What is an acceptable Racor vacuum gauge reading?

[dfloeter]

Having just installed a vacuum gauge on the downstream side of the Racor I now wonder what to expect.  I could change the filter to calibrate the system but would rather ask you all instead. Mainly this option might save me an expensive filter. 
The current reading is 8.5 in Hg at cruising rpm of 2500 on this M35.  My gut feeling from a little reading is this is already a bit high.  Has anyone some data to share?

Thanks.

[Ron Hill]

Dietrich : The question to ask is - "how old is the filter you have on now"?  If it's a recent filter then you might be OK, but for the best (starting) reading I'd start with a new filter.  I know you didn't want to hear that, but that's what I would do.

Also I'd use 2400rpm as the check RPM because it is closer to 80% power - which I believe is the optimum power setting for cruising. 

A few thoughts

[dfloeter]

Thanks Ron, and yes, I am just trying to save a filter.  The filter in place isn't very new and I suppose I should just suck it up and swap out the filter.  I used 2500 rpm as the target because on this engine wot is 3200 with 80% being pretty much that.  Thanks. 

[KWKloeber]

So switch the filter to determine the clean/new baseline. 
Keep the clean one on if the numbers make sense to do that.
If the numbers say that the current one is ok, put it back on - empty the new cartridge, put in some diesel treatment, dump out, store it in a double freezer bag to use next change.
Really, how much is a filter cartridge in the big picture?  Seems more of a PITA than it's worth to avoid a normal maintenance change out but you have to weigh your time against the cartridge cost!

[Jim Hardesty]

Dietrich,
I've put a vacuum gage on Shamrock, I really don't find it that useful. My check is to run full throttle if it runs at full rpm I call it good, if not the filters need replacing.  For some reason I don't understand it never works for me to only change the Racor, need to also change the spin-on engine fuel filter.  I think that if you want a true vacuum reading you need to start with both filters new.
Jim

[Ron Hill]

Guys : Like I've posted before - I wired the electric fuel pump into the blower switch. The engine would normally run on gravity fuel feed.  If I turned on the blower/fuel pump and the RPM increased, I change out the filter!!    I later change it back and just change the filter every spring!!  Bad fuel or a clogged filter isn't worth $$ the hassle!!

A thought

[dfloeter]

Thanks guys for the input.  I will go ahead and swap out the filter and carry on. 

[Stu Jackson]

And every time I read a fuel filter "issue" I include this reminder, just in case it's not the filter itself:

Fuel Starvation and The Obscure Check Ball Valve
https://c34.org/bbs/index.php/topic,10681.0.html
and
https://forums.sailboatowners.com/threads/fuel-starvation-and-the-obscure-ball-check-valve.1249925776/?highlight=Fuel%20Starvation

[waughoo]

Stu... thanks for posting these reminders.  I just replaced my racor but glad to know about the possible obstruction.

[Noah]

Not an issue with the Racor 500 Turbo, I hope?

[Stu Jackson]

Not an issue with the Racor 500 Turbo, I hope?

The article explains that it is for the Racor R24 series housing.  Other filters recommend check valves but most others do not have them built in.  RTFM.