Aero My Accord??

[Ecky]

It's not all that straightfoward. If you're on the highway with cruise control on, you want to have higher vacuum because this means you're burning the least fuel. However, consider this example BSFC chart for a Ford 2L:



How to read this chart: The contour lines show how many games of fuel are required to make one kilowatt-hour of energy, or in other words, how effectively the engine is converting gasoline into usable energy. More grams to make 1kWh means more waste, so lower is better on this chart. Notice that the engine is most efficient at high load (close to wide open throttle) at lower RPM - so, even though it's burning more fuel than if you used less throttle, it's burning it more efficiently.

This is part of why pulse and glide works - you're running the engine in a more efficient range (low vacuum), but that would cause you to accelerate out of your target speed, so you cycle the engine to maintain speed.

Another example, taller gearing improves fuel economy, but also causes vacuum to decrease.

[aerohead]

Originally Posted by Ecky

It's not all that straightfoward. If you're on the highway with cruise control on, you want to have higher vacuum because this means you're burning the least fuel. However, consider this example BSFC chart for a Ford 2L:



How to read this chart: The contour lines show how many games of fuel are required to make one kilowatt-hour of energy, or in other words, how effectively the engine is converting gasoline into usable energy. More grams to make 1kWh means more waste, so lower is better on this chart. Notice that the engine is most efficient at high load (close to wide open throttle) at lower RPM - so, even though it's burning more fuel than if you used less throttle, it's burning it more efficiently.

This is part of why pulse and glide works - you're running the engine in a more efficient range (low vacuum), but that would cause you to accelerate out of your target speed, so you cycle the engine to maintain speed.

Another example, taller gearing improves fuel economy, but also causes vacuum to decrease.

Remember,that the higher the intake manifold pressure,the less pumping loss for filling the combustion chamber.Also,if you have an intake runner sized specifically for cruise velocity,it's butterfly valve can be wide open,reducing pumping losses as well.Honda did this with their 3-barrel carbureted CVCC engines in the early CRX series,and their mpg has basically not been matched ever since.

[RoadCyclist]

I got a best tank today! 33.4 MPG. Very exciting!

I am launching an experiment: I am going to run 5 gallons shooting for a low Hg/in2 pressure; gliding in drive between throttle pushes. Then I will fill up again and drive 5 gallons trying to maintain high pressures.

I may repeat for accuracy. Hopefully the results will be instructive.

[morningglory]

Looks good. Keeping an eye on this for my own project....

[RoadCyclist]

Finished first five gallons. Not good at 27mpg, though it was mostly city driving. Now I'm doing minimal throttle/high pressure. Feel free to take a look at my fuel log for more details.

[California98Civic]

I don't think gliding in drive has much likelihood of being helpful for FE. But it is important to have a consistent technique for measuring fuel use in order for your test to yield information that can help you understand what is happening. How are you measuring fuel?

[RoadCyclist]

I always remember the fill up gallons and drive it down to a certain point on the fuel gauge.

[California98Civic]

Originally Posted by RoadCyclist

I always remember the fill up gallons and drive it down to a certain point on the fuel gauge.

Well, that won't be precise enough to help you. The fuel gauge is too imprecise. And different gas station pumps vary a little. Your relatively small refueling amount (5 gal) all but guarantees more data noise than necessary because any variation has a larger effect on such a small total volume. Put it all together and you may never learn if your experiment is working.

Here is a more reliable method, under the circumstances: pick a specific pump at a specific station. Do all your refueling at that pump. When you fuel, set the flow rate to the slowest rate. Never top off. Always refuel on a close to empty tank if you can (safely). It does not matter precisely, but maybe wait until the low fuel light comes on.

If you do those things you experiment will take longer but you'll have somewhat better controlled for the imprecise instruments and hardware we have to use.

[RoadCyclist]

That seems logical. I was actually kind of worried about my margin of error in measurements. I will have to tighten up my procedures. Rats....

[freebeard]

Accuracy of the pumps is certified by the State.

Compare your odometer mileage over a route known to Google maps (for example, or GPS) and determine the 'fudge factor' to reconcile the various methods.