Mercruiser Vacuum Gauge

[IslandExplorer]

Hello, does anyone happen to know what the old mid 70's Mercruiser Vacuum guage on the instrument panel is all about? I'm guessing fuel economy optimization but curious if anyone knows just how these were used and how they were hooked up to the engine from the factory.

I really like 70's tech stuff like this so the more detail the better. If anyone has any it'd be greatly appreciated!

 

[IslandExplorer]

Here is an example picture of the guage I'm asking about:

 

[Scott06]

Here is an example picture of the guage I'm asking about:View attachment 390682

Pontiac used to offer a vacuum gauge in the 60s it is hooked to manifold vacuum with a small tube. Generally it is used for economy indication as vac drops with throttle opening. Not sure what place it has on a boat as part throttle cruise doesn’t exist in a marine application like on a car at highway speed

 

[flashback]

Like Scott06 said, it was part marketing and could give a reference point for fuel consumption and engine health..

 

[flashback]

You need to remember, back then folks liked fins and a cockpit full of gauges!

 

[kenny nunez]

They also help with having the correct propeller. The sweet spot will be where the needle is between 0 and 5 at full throttle and trimmed out.

 

[Scott06]

They also help with having the correct propeller. The sweet spot will be where the needle is between 0 and 5 at full throttle and trimmed out.

Would Anni correct pitch show lower or higher vacuum

 

[kenny nunez]

If there is too much pitch and the rpms are under what the engine is rated at the gauge will be at 0 which is not good. Also if the boat is overloaded with the correct pitch the engine is taking a beating.
I have seen main and rod bearings hammered out from overloading.
Picture A 24’ Bayliner, one of those “floating bedroom models“, 5.7 engine, 20 people with a drunk owner holding the throttle wide open. This was one of my customers I saw when I was trying out another boat, this idiot asked me why the boat would not plane off! The boat ended up at my shop a week later for a short block.

 

[IslandExplorer]

Awesome, thanks guys! I like that there is some purpose mixed in with the gimmicky style too. I think I'll be adding one on if I can. Would be interesting to see if it helped me increase economy at times if not at least just be an overload indicator. I'm definitely not going to have any fancy electronic fuel flow meters or anything for the time being but a vac gauge would be cheap and super easy for my old boat.

 

[H20Rat]

Personally, it is of almost no value in a boat, and eventually is going to cause a vacuum leak... I guess if you really want to fill a spot on the dash, maybe, but there are about 10 other gauges I'd put in first.

 

[IslandExplorer]

Personally, it is of almost no value in a boat, and eventually is going to cause a vacuum leak... I guess if you really want to fill a spot on the dash, maybe, but there are about 10 other gauges I'd put in first.

I hear ya. I'm just a hapless victim of nostalgia sometimes. I know a lot of people want a fuel flow meter showing exact gph but I think I'm going to skip that one as not to monetize and ruin my throttle stabbing fun! lol I figured the vac gauge would more vaguely let me know when I'm being an idiot without putting a digital dollar sign on it. My existing gauge cluster doesn't have a vac guage but it does have a trim indicator which is surprising bc I do not see any electrical sensors on the old MC-1 outdrive. I have Speed, Fuel, Trim, Amps, Temp, Oil, Tach. If you were going to add some- what would it be? Curious what other ones you'd add in order of importance if possible. I do have a standalone depth sounder guage that I will be adding as optional backup to my chartplotter readout.

 

[Scott06]

I hear ya. I'm just a hapless victim of nostalgia sometimes. I know a lot of people want a fuel flow meter showing exact gph but I think I'm going to skip that one as not to monetize and ruin my throttle stabbing fun! lol I figured the vac gauge would more vaguely let me know when I'm being an idiot without putting a digital dollar sign on it. My existing gauge cluster doesn't have a vac guage but it does have a trim indicator which is surprising bc I do not see any electrical sensors on the old MC-1 outdrive. I have Speed, Fuel, Trim, Amps, Temp, Oil, Tach. If you were going to add some- what would it be? Curious what other ones you'd add in order of importance if possible. I do have a standalone depth sounder guage that I will be adding as optional backup to my chartplotter readout.View attachment 391058

Depth might be useful. You can live without the trim and speedo....

 

[IslandExplorer]

Depth might be useful. You can live without the trim and speedo....

Totally agree, depth is super important especially in the shallow areas I frequent. I'm thinking I will keep my plotter reading depth below hull (no offset for transom depth) and calibrate the standalone to depth below outdrive if I can.

 

[kenny nunez]

The trim gauge is operated by a hydraulic cylinder, it has 2 clear hoses and some wires going to it. If it still operates the gauge you are lucky.

 

[nola mike]

I hooked a vacuum gauge up when I was initially tuning my carb. Never took it off. It's helpful for setting idle, can give you a warning if you have valve problems. In combination with my a/f gauge I can tell when I go from lean-->rich on my carb rods, and also what spring strength I should be at for a given throttle position (Edelbrock 4bbl). Yeah, pretty niche uses, but it's a fun toy. Certainly more use than a trim gauge.