Anyone else striving for better MPGs by adding HPs?

[t vago]

Originally Posted by Lonesome Trail

You're climbing a hill, it requires 'x' amount of energy to maintain speed.

Car A is a 4 banger with typical 4 banger torque band with a curve and peak around 4k RPM, Car B is an efficient V8 with flat torque curve and peak around say 2500RPM as is very common in today's engines.

For sake of argument both cars are geared the same, same size tires, etc.

Now both cars hit the hill at the same time, and to maintain speed Car A must really open the throttle to achieve enough power production to climb the hill, Car B cracks its throttle.

By your logic Car A is using less fuel because it's a small engine making more power per displacement, hence is more 'efficient' because of that ratio, correct?

Let's revisit Car A and Car B going up the same hill. Let's hold all atmospheric conditions the same. Both Car A and Car B are maintaining the same speed up that hill, they hit the same pavement going up that hill, and the same driver takes each car alternately up that hill.

Let's also assume, for the sake of argument, that everything else about Car A and Car B are identical. They have identical bodies, identical trim levels, identical gearing, identical tire and wheel sizes, identical liquid capacities, have identical weight distribution, consume the same amount of electricity (identical alternator loading), an identical amount of drivetrain wear, and they both weigh the same.

If nothing else, Car B with the V8 will have a larger intake manifold vacuum than Car A with the 4-banger. Remember, Car A has a completely opened throttle, and Car B has it's throttle cracked open.

Generating an intake manifold vacuum requires work, and to generate a larger vacuum, a greater amount of work must be expended. Work requires gasoline in either Car A or Car B, so Car B requires more gasoline simply to push Car B up the same hill as Car A, everything else being identical.

[Mustang Dave]

Sorry about my late entry - I just found this thread.

Lonesome Trail, welcome aboard from a fellow Mustang owner and Coyote owner and Arizona resident.

In my experience, the old hot-rodding trick to increase available power to the wheels by under-driving the accessories does result in better fuel economy. It results in more power available to the wheels throughout the entire operating RPM range.

Last May, I installed an ASP under-drive crankshaft pulley on the 4.0 liter Cologne V-6 in my '07 Mustang. It under-drives the accessories (particularly, the water pump and power steering pump) by 25%. It gave me close to 1 MPG, according to my fuel log.

Most people don't believe that I can get over 32 MPG with a 4 liter Mustang.

[gumby79]

Generating an intake manifold vacuum requires work, and to generate a larger vacuum, a greater amount of work must be expended. Work requires gasoline in either Car A or Car B, so Car B requires more gasoline simply to push Car B up the same hill as Car A, everything else being identical.

DWL (driving with load) is driving with the highest possible Vacuum reading posable aka the best mpg per load.
There are many examples of this. The real world trumps the paper this time.
As to personal experience.
When I installed a MSD 6 Digital ignition system (from a stock HEI) and a RPM Air gap Edelbrock intake (from a stock aluminum) . my vacum went up 2"from 15"mg to 17 at idle (625rpm engine liked 425rpm but the alt. quit below 600rpm and the 800 stall tork converter would not allow a stock750-800rpm idle) and I saw an improvement in mpg of 2.5. (From 7.5 allday every day. to 9.5-14mpgdepending on how I drove.)
This was a 6000lbs tall (24"under the frame) off-road capable 4x4 that was also used to pull a 16,000lbs 5th wheel
More of the right kind of power = more efficiency = better mpg..
Gumby Stay Flexible .

[MobilOne]

I have a question. Take a Ford 4.6 and a 5.4. IIRC the 5.4 is a stroked 4.6. How would they compare, mpg wise with the same set-up? Then, how would they compare if the 5.4 had a taller rear end? Just thinking.

[oil pan 4]

I am going to just about triple the horsepower and double the torque on my suburban. Be interesting to see what it does to the fuel economy.

[t vago]

Originally Posted by gumby79

DWL (driving with load) is driving with the highest possible Vacuum reading posable aka the best mpg per load.

Uhhhh.... No.

Originally Posted by gumby79

There are many examples of this. The real world trumps the paper this time.

Oh, okay. I'm sure that must be the case. You sure showed me. And everyone who ever put in a taller final drive. And all the car manufacturers who invested in production variable cylinder technology.

Tell you what. Explain why the MDS 5.7L Chrysler Hemi gets better gas mileage than a non-MDS 5.7L Chrysler Hemi, between a 2005 Dodge Ram 1500 pickup truck with RWD and a 5.7L non-MDS engine and the 545RFE transmission, and the 2006 Dodge Ram 1500 pickup truck with RWD and a 5.7L MDS engine and the 545RFE transmission. The 2006 had an EPA rating that was 1 MPG better than the 2005 (city and highway and combined). No other significant changes were made to this truck from 2005 to 2006, so they both weighed the same, and both had the same aero.

2005 Ram -> https://www.fueleconomy.gov/feg/noframes/21104.shtml

2006 Ram -> https://www.fueleconomy.gov/feg/noframes/22186.shtml

Originally Posted by gumby79

As to personal experience.
When I installed a MSD 6 Digital ignition system (from a stock HEI) and a RPM Air gap Edelbrock intake (from a stock aluminum) . my vacum went up 2"from 15"mg to 17 at idle (625rpm engine liked 425rpm but the alt. quit below 600rpm and the 800 stall tork converter would not allow a stock750-800rpm idle) and I saw an improvement in mpg of 2.5.

So... you were able to idle all over the place while driving? You didn't touch your gas pedal once while driving? I'd hate to have been stuck behind you.

Originally Posted by gumby79

(From 7.5 allday every day. to 9.5-14mpgdepending on how I drove.)
This was a 6000lbs tall (24"under the frame) off-road capable 4x4 that was also used to pull a 16,000lbs 5th wheel
More of the right kind of power = more efficiency = better mpg..
Gumby Stay Flexible .

I think the confusion is coming from somebody here equating higher idle vacuum with higher efficiency.

[t vago]

Originally Posted by MobilOne

I have a question. Take a Ford 4.6 and a 5.4. IIRC the 5.4 is a stroked 4.6. How would they compare, mpg wise with the same set-up? Then, how would they compare if the 5.4 had a taller rear end? Just thinking.

Fuel economy by swapping in a taller rear end can be roughly guessed at by the rule-of-thumb that a percentage change in gear ratio will give you about half that percentage change in fuel economy. For instance, going from 3.92 to 3.64 (which is a 7% change) should give roughly a 3.5% increase in fuel economy.

Don't know about the change in displacement, though. Couldn't even venture a guess on that one.

[Ecky]

There seems to be some confusion here. Usable power gains and improved economy can be seen from:

-Reducing parasitic loads
-Increasing compression
-Fundamental combustion chamber redesign, for higher efficiency (e.g. longer stroke*)
-Weight reduction
-Aerodynamic drag reduction
-LRR tires

*Longer stroke may actually reduce peak power if you need to reduce redline RPM

However, many methods of adding usable horsepower have the opposite effect and will reduce economy, all else being equal, such as:

-Running rich
-Adding more displacement
-Higher RPM / shorter gearing
-Forced induction (on non-diesel)
-Cold air intake

Many modifications that improve economy, do so at the expense of power, such as:

-Smaller displacement (cylinder deactivation, smaller block, etc.)
-Cam regrind
-Warm air intake
-Running lean
-Taller gearing


So, it really depends on which approach you're going to take.

[t vago]

Originally Posted by Ecky

There seems to be some confusion here. Usable power gains and improved economy can be seen from:

-Reducing parasitic loads
-Increasing compression
-Fundamental combustion chamber redesign, for higher efficiency (e.g. longer stroke*)
-Weight reduction
-Aerodynamic drag reduction
-LRR tires

*Longer stroke may actually reduce peak power if you need to reduce redline RPM

However, many methods of adding usable horsepower have the opposite effect and will reduce economy, all else being equal, such as:

-Running rich
-Adding more displacement
-Higher RPM / shorter gearing
-Forced induction (on non-diesel)
-Cold air intake

Many modifications that improve economy, do so at the expense of power, such as:

-Smaller displacement (cylinder deactivation, smaller block, etc.)
-Cam regrind
-Warm air intake
-Running lean
-Taller gearing


So, it really depends on which approach you're going to take.

This post really ought to be a sticky, all by itself.

[gumby79]

https://youtu.be/MBYVZLRyQjk
On my 79 Chevrolet the vac gage acted like an mpg meter It got so as I could predict the mpg based on what the # of inmg vac had ben doing for the last 2 hrs . Hold it down at 7inmg you got 7mpg (85-90mph) holdit up at 14 inmg and you got 14mph (55-70mph). Get behind a truck and watch it pick up 3-5inmg. Pulling a grade I'd try to maintain 7.5inmg because my Edelbrock carburetor had the 7inmg enrichment springs.
Z---
As a side note when I put the MSD in I was able to switch from premium to bace grade . This improved my bottom dollar by 20-40cents per ga.+ a healthy bump on the amount of fuel consumption .
So bottom line. Proformance parts sold to hard core racers ,installed in place of stock parts, helped my cost of operations aka economy. By improving the efficiency of my approximately 400hp 350ci.

Working my truck at Tuttle Creek Off Road Park in Kansas
Gumby Stay Flexible