I wonder if I could get you to bite on this topic HankL. You have a good point on whether you are going from stoplight to stoplight or pulling out from a stop sign and getting up to speed. In my case, it is getting up to speed. One thing to keep in mind is that with my type of driving where I have a fairly long stretch between stop signs, the acceleration is a small portion of the overall fuel consumption, so it probably doesn't make a huge difference in fuel mileage.

Your comment on timing the redlights is good, but often that depends on what the driver in front of you is doing, although I know I've accomplished this many times. Of course I often time the light to beat the idiot next to me who went screaming up to the light and is sitting there waiting for it to change. I'll hit the light at about 20 mph just as it turns green and go zipping past him/her.
Thanks for answering this question. I'm sitting here trying to digest all this. I know this opens another debate, but item number 4 seems to support why guys claim that a CAI will give them better fuel mileage. A CAI intake with a straight line of airflow to the TB and with larger pipes will reduce friction losses, which decreases the negative pressure in the intake. But maybe it isn't enough of a reduction to help with mileage. I don't know.

Of course, the biggest factor in the creating the vacuum is the throttle body blades. Opening the throttle body blades further will reduce the vacuum. This seems to correlate with your mention above of using 75% throttle to stay below the "fuel enrichment kick in" in order to hit cruising speed. BUT- I would consider 3/4 throttle on acceleration a "jack rabbit start", which is what people are told to avoid. Why does that conflict with the information you are providing here? F=ma comes to mind. The engine may operate more efficiently at the more open throttle setting, but now you are trying to push the heavy truck up to speed at an acceleration rate that is consuming more power. There has to be a sweet spot on the throttle where you get up to speed just fast enough to reach cruising speed most efficiently. Or is that setting at 75% throttle?


Interesting information on the piston speed. I've done some calculations on the piston speed. The 5.9L has a 3.58 inch stroke. So at the 1200 fpm mark the engine is running 2000 rpm. 1400 feet per minute is 2350 rpm. This is about where my truck shifts when I run it, but I'm using less than 3/4 throttle.

Also on the O/D issue, from what I understand, it seems that you would be better to keep the O/D on, let the engine pull down and use more throttle. I read one of your posts on another forum where an O/D poll was taken and you had data showing that it really didn't make a difference on fueld mileage whether it was on or off. Not sure what the topography is like where you were doing your driving. I still think I'll keep it off until I get on open, level roads. I prefer the quicker acceleration that I get with the lower gear.

HankL, Have you ever studied the Nebraska tractor tests? They run tractors through various tests where they apply varying amounts of load to the tractor through both PTO and drawbar. They measure the amount of fuel usage at these loads at full throttle and at 75%throttle. You might find the data intriquing.

 

Ash writes that:

>I know this opens another debate, but item number 4 seems to support why guys claim that a CAI will >give them better fuel mileage. A CAI intake with a straight line of airflow to the TB and with larger pipes >will reduce friction losses, which decreases the negative pressure in the intake.

Keep in mind the throttle 'butterflys'.

When you are driving around, the throttle does away with any effect of a lower restriction air intake.
Only if you were at Wide Open Throttle would a lower restriction intake have any effect on fuel economy, horsepower or torque.

I know from past conversations that the effect of the throttle is the hardest thing for people to understand as they learn about gasoline engines.

It is so much easier for people to understand diesel engines which have no throttle.
In diesels a cold air intake CAN have a slight effect on fuel economy because it does slightly change the air to fuel ratio by allowing a bit more air to flow with the same fuel, as well as slightly decreasing the 'pumping loss' due to vacuum in the intake.

A key to understanding gasoline engine fuel economy is that the throttle 'strangles' the engine down to whatever horsepower is needed at that moment.

If you install a lower restriction air intake
(or intake port or valve)
the throttle butterflys will close slightly and just 'strangle' the engine a bit more to make up for this.

But it is slightly different about the 'Cold'.
Colder air is slightly easier to compress - which should slightly improve fuel economy if nothing else happens.
Unfortunately this colder air also will be measured by the IAT (intake air temperature) sensor and the PCM computer will then slightly advance the ignition timing at part throttle because cooler air (and lower pressure air) does not burn as fast. At part throttle with cool air the ignition advance can be as high as 40 degrees BEFORE top dead center in order to get the burning to create peak pressure to push against the piston at 10 degrees PAST top dead center.
This can slightly hurt fuel economy at PART THROTTLE.

But at WIDE OPEN THROTTLE the cooler air will be more dense with oxygen and increase the fill of the cylinder which will produce more Torque and show improved fuel economy. The cool air may prevent detonation, which also decreases fuel economy by building peak pressure too early.

Confusing?
Yes it is.

The key thing to remember:
What happens at part throttle is not always the same as at full throttle.

95% of USA driving is done at part throttle.
EPA's own study showed this, and Michigan Congressman John Dingle used it to stop the EPA from regulating the pollution of engines at full throttle so that they would continue to have 'emergency passing power' and called this a safety feature. Once you go past 80% throttle you can pollute all you want in the USA. Surprised by this? You can't do this in a car in Japan or Europe where they have different laws.

Is there something about Full Throttle that someone interested in Part Throttle fuel economy should know?

Yes!

Generally the way to get better fuel economy,
is to get CLOSER to full throttle !

How do you do this?

Shift into a higher gear.

When you stay at the same vehicle speed,
but shift to a higher gear,
the horsepower needed to move the vehicle stays the same,
but the engine produces that same horsepower
by opening the throttle a bit more and slowing down the rpm some.
This improves fuel economy.
It also makes the torque a cylinder produces higher.

Getting better fuel economy is actually simple if you think of it right.

You want high torque in the cylinder,
but low friction of the moving parts.

This is not that different than wanting an engine to produce high power for racing.

For high power,
you want the best 'compromise' of good torque in the cylinder at the same time as the engine shaft rpm is as high as possible so that you can use gear reduction for maximum torque at the wheels.

It is about 'compromise' in where the engine operates.

For highest horsepower it is the best compromise of rising rpm and declining torque.

For highest fuel economy it is the best compromise of torque and low friction.

 

So theortically then, if you put in a tall enough gear that would require you to run at nearly wide open throttle just to maintain speed, you would get better economy?

HankL, can you explain or reference a good expaination of torque as it relates to an engine. I undertand torque as it relates to a lever, but never could grasp the concept of how it applies to an engine. Since torque is made up of two parts, force and distance, the force is the only variable in the engine, correct? How does the force change with engine rpm. I've seen torque curves and have always wondered why they start going down after a certain rpm.

 

Thanks for ringing in HankL. When you say:

This seems academic, but what are your thoughts on going from say 50% throttle @1900 rpm to 25% throttle @2200 rpm when the motor in question does not start to produce any signifigant torque until say 2100 rpm?

I was @ ~14 mpg. Then I had my pcm reprogrammed and went to ~15.5mpg. Same road, same trip, wind and weather may have been different, but not drastically. I then installed a M1 intake. Mpg fell to ~14.5; again same road, same trip, etc. etc. Am I way off base in thinking that the drop was due to loss of low end torque, and that a cruising rpm that gets back towards torque production will improve mpg?

I guess I will find out for sure this weekend as the gear install will be finished today/tommorrow, but, like ASH, I am trying to learn here. I have 5 dodges and if I can figure out how to spend a few bucks on each to get a 1.5-2 mpg gain, it would significantly improve year end numbers as we work all over the state of Va.

 

I've had a few days to mull over what HankL wrote and I think he has some valid points, but I would like to see some charts or tables that provide hard data. The point that he made about the butterflies creating a vacuum is interesting. He made mention of a diesel engine and that makes a good comparison. Unless I am mistaken, a diesel motor does not have butterflies in the intake, so you have an unrestricted flow of air. The accelerator only controls the fuel pump. Even though they run a much higher compression, a diesel motor does not "hold back" like a gas engine, that is why they run jake brakes on trucks to help slow them down. The vacuum created in a gas engine provides "braking horsepower" that a diesel does not have. Also, you need an air pump on a diesel to provide vacuum for such things as your A/C vent controls, since you cannot get it from the manifold.

So keeping that in mind, I can understand why it takes extra fuel to overcome the vacuum force created when operating at a less open throttle. The more you open the throttle, the less vacuum you have in the manifold. This is also why some people complain that the A/C air starts coming out the defrost vents when they go to WOT or are working the motor hard. They already have a vacuum leak in the system and now that they open the throttle and lose manifold vacuum, the vacuum loss causes the vents to go to their default position, which is the defrost.

So, know that I understand that by opening the throttle you decrease the vacuum pressure and the engine is working more efficiently, the next variable is the amount of fuel pumped into the engine by the injectors. Basically, the more you open the throttle, the higher the gallons per hour of fuel that will be run through the injectors. Somewhere the fuel consumption rate of the amount of extra fuel required to overcome the vacuum pressure equals the amount of extra fuel required to drive the pistons at a higher speed. This is the data that I would like to see in a graph form.

I think mopowar has a good point too with losing torque. He made changes to his engine that changed the torque curve and experienced a mpg loss. We have all seen horsepower and torque curves. I would like to see a fuel consumption per hour chart combined with these curves. There has to be a specific rpm where the engine works most efficiently. I'm not sure if the flow injector rate can be measured by a scan tool, but it would great if you could. If you put your truck on a dyno, record the fuel consumption while it is on the dyno then you could get this data. Once you find that rpm where you get the best fuel consumption rate, choose gears that will put you closest to that rpm at your normal driving speed.

Now I'm sure that the auto manufacturer's already do this. I know that manufacturer's of heavy duty trucks have complex computer programs to help you spec out a truck for you specific needs, choosing everything from engine, tranny, gears, and tire size. Actually, maybe the program that mopowar linked to does this, I didn't download it yet.

Guess I started rambling here again. But hopefully I made a little bit of sense.