What is the deal with Vacuum reading ??

[ST2008]

I'm also wondering if the vacuum has any impact at fuel pressure, thus the mpg. I ask this question because the fuel pressure regulator is controlled by the vacuum and I have inconsistent mpg with my Honda Civic 2000.

See my other thread.
http://ecomodder.com/forum/showthrea...tml#post518225

[redpoint5]

Originally Posted by ST2008

I always think that the car is more efficient if I can drive it with less engine load for the same speed. Am I wrong ?

Higher load is better for any given speed. Here's the explanation...

There are 2 ways to make more power:

1. bigger explosions (more torque)
2. more frequent explosions (more RPM)

Consider that the biggest explosion the combustion chamber is able to produce happens when the throttle is wide open. With no restriction, the intake stroke fills the cylinder with the same density of air as the open atmosphere, and there is no vacuum (pressure difference between the open air, and the inside of the cylinder). This is the maximum amount of oxygen the combustion chamber can hold without forcing more air into it with a supercharger. Fuel is added to the combustion chamber in proportion to the amount of air drawn in, and since the cylinder has the maximum amount of air, the maximum amount of fuel that can be burned cleanly is injected.

Since we know that restricting the intake by closing the throttle is not efficient, it follows that bigger explosions are a more efficient use of fuel than smaller, more frequent explosions.

If low load were efficient, then it would be best to run in 1st gear on the highway, with the engine screaming away, and the accelerator barely being pressed. Instead, reducing the RPM and pushing the accelerator pedal more improves efficiency. You accomplish this by shifting to a higher gear.

Like I said earlier, none of this is of much practical use unless you pulse and glide, or drive a manual transmission. An automatic car is going to select the appropriate gear given the speed and engine load. There is a trick to get an automatic to shift to a higher gear sooner by briefly backing off the throttle a little, letting the car shift to the next gear, and then applying more throttle again. It's not going to save much fuel in the grand scheme of things, and it's too much effort for most drivers to be concerned with.

*In case anyone is curious, the horsepower equation is:

Horsepower = Torque (how big are the explosions) * Engine RPM (how frequent are the explosions) / 5252

You can arrive at the same horsepower by increasing torque and reducing RPM, or by increasing RPM and reducing torque, the former being more efficient than the latter.

[Ecky]

Originally Posted by ST2008

I'm also wondering if the vacuum has any impact at fuel pressure, thus the mpg. I ask this question because the fuel pressure regulator is controlled by the vacuum and I have inconsistent mpg with my Honda Civic 2000.

See my other thread.
http://ecomodder.com/forum/showthrea...tml#post518225

I would say, little to none. Fuel pressure should have no effect on economy, and vacuum should have no effect on fuel pressure. Your variable fillups more likely have to do with driving patterns than anything mechanical.

What you really need is instrumentation, so you can see what your fuel economy is while you're driving it. The cheapest option is an ELM327 bluetooth dongle (available on Amazonr or eBay for $5) paired with a smartphone or tablet. Other, better options include a Scangauge or Ultragauge.

[ST2008]

I have two ELM327 bluetooth dongles and I got my vacuum reading from these devices. The problem with Torque Pro is it shows only data, but it doesn't record or give me an average of specific parameter, like vacuum or map sensor reading.

I read somewhere that if the fuel pressure is higher, because of vacuum leak from the fuel pressure regulator hose, we will have crappy mpg because more fuel dumped into the cylinders. This is one of the reason why I try to find more info about the Vacuum.

[Ecky]

I think that only applies to old carbureted engines, but I'd get a second opinion.

Torque can give you instant and average fuel economy, you know.

[redpoint5]

Your car should be throwing a check engine light if there is an issue with anything relating to emissions. Running rich would be one of those conditions that should trigger a CEL.

You will use the short and long term fuel trim gauges along with the air fuel ratio to evaluate a vacuum leak or other problem with air and fuel delivery.

Snippet from this website:

For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.

Note that it's normal for an engine to constantly alternate between running slightly rich to slightly lean air fuel ratio as it constantly adjusts around stoich 14.7:1

[ST2008]

Originally Posted by Ecky

I think that only applies to old carbureted engines, but I'd get a second opinion.

Torque can give you instant and average fuel economy, you know.

I know, but it's very inaccurate. It shows always around 35mpg but I can get 44mpg with 40L.

What I need to know is the average of other parameters so I can verify if one of them has relationship with mpg each time I refill the tank.

[ST2008]

Originally Posted by redpoint5

Your car should be throwing a check engine light if there is an issue with anything relating to emissions. Running rich would be one of those conditions that should trigger a CEL.

You will use the short and long term fuel trim gauges along with the air fuel ratio to evaluate a vacuum leak or other problem with air and fuel delivery.

Snippet from this website:



Note that it's normal for an engine to constantly alternate between running a slightly rich to slightly lean air fuel ratio as it constantly adjusts around stoich 14.7:1

The LTFT and STFT are both fine and I don't have CEL either. The STFT bounce around but it rarely gets over +/- 2%. It will be great if Torque Pro can provide me the average of both fuel trims for my trip. Thus I can compare these with the mpg.

[oil pan 4]

Originally Posted by ST2008

I'm also wondering if the vacuum has any impact at fuel pressure, thus the mpg. I ask this question because the fuel pressure regulator is controlled by the vacuum and I have inconsistent mpg with my Honda Civic 2000.

See my other thread.
http://ecomodder.com/forum/showthrea...tml#post518225

Yes most engines use a fuel pressure regular that is vacuum referenced.

[serialk11r]

It's worth noting that the throttle is hardly the main source of inefficiency in many cases. Friction in the engine uses more power than the throttle by a long shot.

The Nissan 3.7L V6 engines and all BMW engines today use valve timing instead of the throttle to control output, but low load is still inefficient on those engines. When you only suck a little bit of air per stroke into a big engine, the fuel/air doesn't burn as hot (bad for efficiency), and the sheer amount of power it takes to overcome the friction in a big engine can be more than the power it takes to keep the car rolling down the road.

Anyways, I don't even use a vacuum gauge for pulse and glide, I just calculated the instantaneous in each gear corresponding to the max load before the engine starts running rich, and use the in-dash mpg gauge.

For example, in 6th gear, after all the arithmetic, 24mpg is 0.78 grams per revolution, 22mpg is 0.86 grams per revolution, enrichment comes on at 0.8-0.9g/rev in the cruising range, so that's where I shoot for when accelerating. In 2nd gear, it would be more like 9mpg on the dash, in 4th it is like 14mpg.