does anyone know where to plumb in a vaccum guage, i have hooked up the electrics, just dont know where to hook up for the vaccum pipe. its an old smiths one i found a a mates palce. he didnt know how to hook it up either.
Its just up to a stock a12 set up.

to the inlet manifold

That is right, to the intake manifold. Find any nipple there in the intake manifold,you can use a T fitting and share the nipple with another hose,on the intake manifold, not to the carburator.Intake manifold is not the same as carburator manifold, so do not hook it to the bottom of the carburator. I have a Vaccum gage installed in my A15 engine, it is excelent source of info , a healthy engine will show a steady 15 to 22 reading. While driving, you can save on gas, because when driving at any speed, there is a threshold of efficiency that without the vaccum gage you can pass all the time. At , for example, 70 MPH, you could be saving gas or burning more than you need. The thing is to keep the vaccum gage needle at the highest vaccum reading posible at any speed. YYou will see that, following the 70MPH example,you could go at 70 with 15 pounds, or at 70 with 5 pound, just by pressing the accelerator pedal a little more than needed. Intake manifold increases when the carb throatles are closed , the more they open , the more it drops,because when the carb throats are closed, the pressure in the manifold is higher, it is trying to suck air through the closed carb plates, (like trying to suck through a straw with thick milkshake)high vaccumm. Once they open, the resistance lessens(as if the straw was taken out of the milkshake glass, no more resistance)so the pressure drops,perhaps you know this, so I will leave it there, but it is an excelent tool for driving, and also tells you about the performance of the engine.
Jaime.
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Just curious about your gage, how did it go?
Is your readings all right?
Jaime
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fuel ecconomy would have to be a function of engine speed too wouldn't it?

For example I've found in my car that you can have a more vaccuum at a certain speed in 4th gear compare to a less vaccuum at the same road speed in fith gear. However the engine has higher revs in 4th.

Anyone know how to work out the optimum operating conditions to recieve best fuel ecconomy?

Best economy is found where the car is not accelerating.

So to maintain speed, it's a function of overcoming:
1. Drivetrain losses
2. Wind resistance
3. Road friction.

So long as the car is putting in enough effort to overcome these 3 forces..then it's moving as fast as it can based on using the minimum fuel to keep it moving. This assumes that your carby /fuel injection is setup to give a good mixture at this given RPM.

The Wind resistance is the big killer in a 1200, we'd need to plot the resitance vs speed. This may not be proportional. Then we could calculate the effort required to equal the wind resistance, thus maintain speed, thus maximise fuel use.

Having a heavy flywheel makes maintaining speed easier, thus producing better fuel economy.

So to answer you question, vaccum is not a good indication of fuel economy on a modified engine. It is a good indication on a factory tuned, well maintained car. Using Vaccum to determine fuel economy is only as accurate as the carburettor and distributor tuning.

Chris

Here's how to get exceptional fuel economy:

1. Drivetrain losses

- Use the highest gear for the speed

2. Wind resistance

- lower your 1200

3. Road friction

- Use the skinniest tires and high air pressure

And, keep your foot out of it. The vacuum gauge will tell you about this ... it's high when your foot is off the pedal, and low when you have it pushed to the floor.

With the Vac gage connected to your engine, forget about anybody else's cars, for your engine, maintain your vaccum reading the highest posible at any given speed, and you are maximizing gas usage at that speed. You can notice that as you accelerate, vaccum drops, because the plates open and gas comes in faster, so, keep the neddle (vac gage) as high as posible, then you are optimizing gas consumption at that given speed. The vac gage is just an indicator of the vacumm amount , in this case , in the manifold, it just tells you about the vacum , it does nothing about helping the performance of the car, like the speedometer, it tells you how fast you are going, the rest, condition, tuning, etc, the speedometer does not care. The vac gage helps you in the way that allows you to know if you have good vacumm(at iddle between 15 to 20), and while driving, (unless you are going fast, racing, driving fast for fun, in which case you are not concerned with saving gas)... allows you to know, at cruising or constant speeds, the difference between saving some gas and misusing your gas. Without the vac gage, (with yours on you can verify this)... you could be going at 40MPH at a 15 reading, or, with just a little extra foot on the pedal, at the same speed, 40, with 10 or 5 readings, in which case , you would be going at the same 40MPH speed, using more gas(less vaccum, more sucking in of mixture.). Sometimes I am driving in the freeway, at say 70MPH,then I see the gage, a 5 reading, inmediately adjust my foot on the pedal to bring the neddle down to a 15 reading, then I know I am going at the same speed, 70MPH, but using less gas, at a 15 reading. That extra gas I would be using at a 5 reading would be useless, because it is getting burnt with no increase in speed.It is a fine tuning aid while driving, an indicator, not part of the rest of reasons a car uses more or less gas as I read before, regardless of what you do to your engine, the vacum gage will tell you the condition of the vaccumm and gas use at that given time with that given engine in that given condition.If you have an excelent engine, you will have better readings overall.If your engine is bad, your readings will be bad, but still, given that good or bad motor, you can read the gage and save gas within the allowance that motor gives you.
Jaime.

havent done it yet, been working all week with overtime, too tired. will try tommorow. anyone know whick tube from the manifold? There are a few that just plumb back to each other via those chambers that do nothing. i will take a phot to explain after my coffee kicks in (might need another coffee).

Any connection to the inlet manifold can be used.