Vacuum gauge

[Boat]

Hi all,

just wondering about a blank spot in my panel. im thinking i want a vacuum gauge in there for some reading for fuel economy.

What range of gauge are you guys running?
mechanical or electronic senders?

Thoughts on install? anything you want to try?

thanks

[Marian Claire]

0 to 30 Hg.
The only thing to hook up is the hose. Because I have an early manifold I used the spacer sold by MMI that mounts between the carb and manifold. With the Moyer kit you can attach it where the scavenge tube goes if you have a late manifold. http://www.moyermarine.com/cgi-bin/s...rebuilder.html
Dan S/V Marian Claire

[joe_db]

I am using the Moyer gauge (Isspro). it works well and I find it very useful. I used a length of fuel hose and then the clear plastic hose. The clear plastic it comes with will melt if hooked right to the hot manifold. Also note the light bulb can contact the gauge mechanism and cause odd readings. I did a little surgery on the light fitting to solve that.

[Dave Neptune]

When mounting the vacuum gage I suggest using 1/8 copper instead of the 1/8' poly-tubing. Just put a vibration coil in the line and it will last many many years.

Dave Neptune

[Boat]

Thanks all for the info. What kind of a reading am i looking for? How do i use it for optimal performance.

[joe_db]

This isn't an easy thing to answer in one post, but basically if you know vacuum and RPM you can infer power.
If on a flat clam day it takes 5" vacuum to turn the prop 2300 RMP and next week it takes 3"* to turn 2300 RPM something has changed. Most likely guess is a foul bottom.
Example - going to the yard I did not tow my dinghy. At 5 inches I was turning at least 100 RPM more than when towing.

* most info you will find online about manifold pressure and RPM is about airplanes. While the basic principles are the same, you need to mentally reverse the numbers. Airplane manifold pressure gauges (vacuum gauges) are absolute, not relative to ambient pressure, and read the other way. Sitting still at sea level they read 30 inches, not 0. So assuming an airplane at sea level, 25 inches for that gauge is 5 inches for our gauges. Sitting still engine off we read 0 and they read 30.

Thanks all for the info. What kind of a reading am i looking for? How do i use it for optimal performance.

[Marian Claire]

Boat.
The main reason I installed the vacuum gauge was to help me choose the correct prop for the MC. This thread, http://www.moyermarine.com//forums/s...ead.php?t=6046, goes all over the place so no need to read the whole thing. Post # 69 gives some numbers that show how under-propped I was. Post #111 shows the much better #'s after installing the new prop.
For how I use my boat I wanted a descent cruising speed at a good "continuous use" RPM with more power to spare for bad conditions. I did not want to be running very high RPM all day (under-propped) or having the engine maxed out, low vacuum #, low RPM and HP (over-propped) to obtain a good cruising speed.
It is important to note that the MC has the 2 to 1 reduction gear.
The gauge I installed was not "marine grade". I did not think I would use it after the prop issue was settled. Wrong!! Just the other day I noticed my speed was less than normal for the RPM I was running. My first thought was "must be the head wind" but then I checked the vacuum gauge and saw that the # was much lower than it should of been. This lead to checking/cleaning the prop.
Dan S/V Marian Claire

[Dave Neptune]

A Caution

A caution while using vacuum gages. I have found that unless you step up and pay premium money for a premium unit they vary quite a bit from unit to unit.
Use the "gage" to develop your own parameters and don't be concerned about matching someone else's numbers unless your swapping gages too. The gage is a good tool on YOUR boat and will be consistent to your boat and engine!!! If the numbers being compared are within an inch or so I would not concern myself with the "other one" just my own tuning!

Dave Neptune

[romantic comedy]

old car repair manuals had pages about how to use a vacuum gauge. Chiltons.

[joe_db]

To expand on that, if you get a liquid filled gauge, you need to mount it upright with the rubber plug snipped to make an air vent. If you do not do this for the liquid filled pressure and vacuum gauges on FleaBay the readings will vary a lot with temperature. I spent hours tracking down a non-existent fuel issue because of this

A caution while using vacuum gages. I have found that unless you step up and pay premium money for a premium unit they vary quite a bit from unit to unit.
Use the "gage" to develop your own parameters and don't be concerned about matching someone else's numbers unless your swapping gages too. The gage is a good tool on YOUR boat and will be consistent to your boat and engine!!! If the numbers being compared are within an inch or so I would not concern myself with the "other one" just my own tuning!

Dave Neptune

[Boat]

thanks all for the replies. been doing alot of motoring this summer due to lack of wind in the puget sound. my motor burns a tick more fuel than most on here i have seen. i do have more pitch in my prop. my max rpm i can get is 2100 i think i have a 12x8 prop everything on the engine has been either replaced or serviced. i do need to do the timing though, maybe there is something to my fuel use problem. but i digress, i wanted to play around with a vac gauge and see maybe where the sweet spot is for cruising.

[sastanley]

To expand on that, if you get a liquid filled gauge, you need to mount it upright with the rubber plug snipped to make an air vent. If you do not do this for the liquid filled pressure and vacuum gauges on FleaBay the readings will vary a lot with temperature. I spent hours tracking down a non-existent fuel issue because of this

joe, I recently bought a liquid filled fuel pressure gauge...it is useless for my application because of all the reasons you mentioned...totally useless in the engine area when the temp goes from ambient to 150°+ next to a hot engine!

As it turns out, the old 1561 Mr. Gasket mechanical fuel pressure gauge has worked just fine for me, and I've switched back to it because I've found it is more reliable than any of the others I've tried.

edit - not totally related to a vacuum gauge, but i think in most A4 engine "rooms" it might apply.

[joe_db]

What I found was the fuel pressure would slowly fall to 0 when underway. The engine still ran fine, so it was driving me NUTS!
Then I was on the boat in February on a very cold day and the fuel pressure was 3 PSI! The engine had not been run since December so I was even more confused. I did some research and it all became clear - the glycerine in the gauge expanded in the heat and made the internal gauge pressure rise, thus making the reading drop and then the opposite in winter. I snipped the top off the rubber plug and now all my readings are perfect

Likerwise on a friend's boat we have the vacuum version on the filter and we had to remount it facing upright and snip the plug for accurate readings. If you can't mount them upright, as you discovered, you need a dry gauge.

joe, I recently bought a liquid filled fuel pressure gauge...it is useless for my application because of all the reasons you mentioned...totally useless in the engine area when the temp goes from ambient to 150°+ next to a hot engine!

As it turns out, the old 1561 Mr. Gasket mechanical fuel pressure gauge has worked just fine for me, and I've switched back to it because I've found it is more reliable than any of the others I've tried.

edit - not totally related to a vacuum gauge, but i think in most A4 engine "rooms" it might apply.

[dvd]

Vacuum Guage

I use a vacuum gauge and I can tell you it's helpful. I usually run about 12 on mine for cruising speed, however my engine can withstand 10. I have a 2 to 1 gear.

[hanleyclifford]

I use a vacuum gauge and I can tell you it's helpful. I usually run about 12 on mine for cruising speed, however my engine can withstand 10. I have a 2 to 1 gear.

Can you tell us what prop you run, cruise rpm and type of boat? That is impressive vacuum.