Using vac. to determine mpg

[Matthew A.]

I am considering installing a vaccuum gauge to operate off of manifold pressure behind the carbs. in hopes of obtaining better fuel mileage. My question is: How does vaccuum pressure determine fuel mileage?

 

[KnottyBuoyz]

Re: Using vac. to determine mpg

We have one on our boat and it's labelled an "Economy" guage! *lol*

I suppose the highter the manifold vacuum the better the economy. That's the idea anyways. I use mine mostly to tune the carbs to get the smoothest idle. I can tell when the back two barrels (Holley 850 Double pumper) open up by the vacuum guage and use that as an indicator.

 

[Frank Acampora]

Re: Using vac. to determine mpg

Well, Matt: The more the throttle butterflies are closed, the higher the manifold vacuum. This means that manifold vacuum is highest at idle. As you progressively open the throttle, manifold vacuum drops. If you overcarburate any engine, when you go to full throttle, too little air flows through the venturis to atomize gasoline and manifold vacuum will drop drastically--the engine will sag and die for lack of fuel. Manifold vacuum controls engine speed by limiting the amount of fuel/air ingested and also by increasing the amount of horsepower it takes to injest air.

Of course, this can't happen with fuel injection engines since the computer supplies pulse width to the injectors to deliver the proper amount of fuel, and throttle body area can be much larger (resulting in more power for a given displacement). That is why, for example, if you have a high throttle setting on your car (hard acceleration), vacuum devices will not work until partial throttle is restored. That is also why the power brake booster cylinder has enough volume for 6 or so brake applications and why there is a check valve in its vacuum supply line.

SO: For a GIVEN speed, the higher the manifold vacuum, the further closed the butterflies are and the less air is flowing through the venturis. Thus FOR A GIVEN SPEED, the higher the manifold vacuum, the less fuel will be used--even in a fuel injected engine.

Typically, you get the boat on plane, set your cruise speed, then back off on the throttle slightly to (hopefully) maintain speed while increasing manifold vacuum.

Personally, I believe that with outboards, especially lower powered ones, it is an exercise in futility. Boat attitude, speed, and drag changes with wave action, turns, and people moving around inside it. There are only minimal gains to be had for a lot of monitoring. Boating is supposed to be fun, not work. And really, unless you are going 100 miles, what is the gain of say 5.1 miles per gallon versus 5.0 miles per gallon? That's only 2% if you can realize that much.

Also, I have never tried it but I believe that with the large bore venturis on these carbs, at higher speeds, the reeds are a limiting factor. Realize that your 72 cubic inch outboard has three 1 5/16 venturis while a V8 inboard at 200-400 cubic inches with a four barrel has only about the same approximate venturi area.

 

[JB]

Re: Using vac. to determine mpg

Are you talking about doing that on your Force outboard, Matthew?

That is a 4 stroke technique. I don't think it is of any value on a 2 stroke.

 

[Mark42]

Re: Using vac. to determine mpg

More important is to ensure that you have the max timing advance while cruising to get best fuel economy. Almost all outboard motors have a design where when the throttle is pushed to full throttle, the timing is also pushed to full advance. When the throttle is backed off a bit to close down the carb, the timing stays at full advance. This gives best fuel economy. The trick to taking full advantage of this design is two fold:

1) The throttle lever must be pushed to full (wot) throttle to reach maximum timing advance. Then back off throttle.

2) Know how far the throttle can be backed off before timing is also backed off. Generally this is mechanical linkage and can be easily determined by watching the linkage move on the motor while moving the throttle lever. Make a mark on the throttle control at the point where the timing is backed off so you can tell what the setting is from the helm. This will help you achieve best fuel economy.