All the talk of manifold vacuum and how it increases fuel consumption relates back to throttle position.

The more you have the throttle open the more fuel the engine requires. The RPM will relate but not as directly, as you will find to hold an engine at 5000rpm requires more throttle than to hold an engine at 2000 rpm.

The second thing that will effect your fuel economy is the speed you travel at. The fast you go the more resistance the air produces against you moving through it.

Air resistance works like this
F= 1/2 CApv^2

F=force of air resistance
C=car's drag cooefficient
A=cross sectional front area of car
p=density of air (effect by temperature and pressure)[PV=nRT]
v=Car's velocity

So you can see that the air resistance increases with the square of velocity.

That's why cars have a top speed....pushing through air is hard work.

So the slower you drive the less force it takes.

And with the manifold pressure idea - you select a gear which allows you to maintain speed with the minimum throttle position you will use the least amount of fuel.

If you have to accelerate more in 5th to keep the car going, than in 4th or 3rd...you are probably using more fuel.

Hope some of that helps - i worked a lot of this out when making a 'street dyno' - I could calculate hp or torque from just an RPM and time data log.

I'm not really discovering that many drawbacks. I have a really small fuel tank, but that's my fault for being lazy and not wanting to chop up the rear of the car and mount a big tank.

LPG at 12:1 is perfect. I was always planning on going turbo once my registration problems are solved.

Worst drawback: 0.3 less energy per gallon and you have to 'find fuel'
undecided: fuel is 1.95/gallon and always full serve.
Best thing: More BTU/LB than gasoline

i am getting 15.5 MPG on lpg at the moment. Not great, not too bad. I've just been trying to get my calculations refined so I can plan fuel stops.

currently LPG is cheaper than petrol even if i use 1/3'd more. I'm saving about $0.04 per gallon now.

I'm still learning how propane behaves differently. The octane is high enough that you can idle around town in 5th gear and the car is happy. No acceleration, but it'll put from light to light.

I'm currently trying to not be a hoon and see how it goes if I keep the rpms down and don't mash on the throttle.

the range is 15 -17 mpg ( 19.5 - 22.1 petrol mpg - not too bad ) So far.

I remember being taught that if you convert an existing engine to LPG, you will get [as a rule of thumb] aboiut 10% less power & about 10% poorer fuel economy, but since the fuel was about 1/3rd the price of gasoline, then it didn't matter.

Then I was told that if you reguild the engine as a dedicated LPG engine, then you can raise the compression up to about 12 to 1 & this would pretty much restore the missing 10% in both power & economy, which makes it even cheaper to run.

LPG hes a number of very real advantages, but one or two drawbacks, as you are now discovering.

Lpg is similar in AFR.

What i was reading last night says that 12:1 compression would be ideal for propane.

I do have too much time to think about these things. I'm going on a 1000 mile trip soon so I've got to be thinking about how to get to fuel stops that are at the far end of my current range.

I feel like i have to make a flight plan before i go anywhere.

that's all very good thinking ... but i think you have too much time on your hands.

Or in our case, a little air pump


The air/fuel ratio is by weight. So one kilogram of fuel to 14 kilograms of air. For gasoline (14.7:1) that means something like 10,000 gallons of air to every gallon of gasoline.

For LPG won't the air/fuel ratio will be different?sure.

I've been thinking lately about gas milage. I've noticed that my lpg car seems to get about the same milage no matter how i drive. got me to thinking...

if the AFR is 14:1 needs 14 parts air and 1 part fuel ( by weight... 1 lb fuel 14 lbs air. )

the cylinder draws about 1.5 liters of this 14:1 mixture every rotation. this is true, but at a pressure less than atmospheric except at WOT. So we really draw 1.5 liters at a pressure of 15" vacuum so the density is way lower than if we mixed that AF up in a big bucket.

the amount of fuel is always the same for a given volume of air at a given pressure.

Is what i'm seeing that a tank of fuel is good for N rotations of the engine and what gear you use those in has more to do with how far you go than 'being a lead foot'

it's an air pump right? what makes it slow down is the vacuum in the manifold makes the density of the AF mix in the cylinder less dense and so the engine slows. Opening the throttle makes the mixture denser by keeping it closer to atmospheric pressure ( as long as the flow stays up, otherwise that would be 'lugging'.

That process would have an effect on fuel usage but only during transitions to and from idle.

that means there's really 3 fuel usage numbers ( compression braking, idle, acceleration ) the first is somewhat useless, it's how much fuel the engine can suck with the throttle closed and a high vacuum, the second is somewhat relevant a high idle wastes fuel ( wastes a few miles idling ), and the last is where most of the fuel would be used.

how much does the density change between 15" and 1" of vacuum?

it seems like the rpm has several effects, an engine at 3000 rpm pumps less than an engine at 5000 at the same pressure. But an engine at 5000 rpm and high vacuum pumps less than a engine at 3000 rpm at moderate vacuum.

so does fuel economy come from balancing low rpm with mild vacuum? You want the most vacuum that allows you to maintain a given RPM.

Which is why a turbo makes more power, by stuffing as much perfectly mixed air into the cylinder as possible.

let me know where i'm wrong...