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Old 11-16-2007, 06:56 AM
HankL HankL is offline
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Default RE: mileage V6 vs. V8

don't make the mistake that the main way to improve MPG is with engine mods. Most gas engines are already about the same efficiency at their best spot. The easiest way to improve MPG is to improve tire rolling resistance, and to improve cutting through the wind with better aerodynamics. With those two improvements the engine won't have to make as much horsepower and will automatically improve MPG.

Bearing the above warning in mind
here is a cut from the FAQ
on where a gasoline engine is most efficient at turning fuel into horsepower:
There are drivetrain mods that will allow your engine to operate where
it turns fuel into horsepower most efficiently based on the two important
guidelines: (1) having its throttle nearly open but just below where an extra
wasteful percentage of fuel starts getting injected, and
(2) keeping piston speed below the range of 1000-1400 feet per minute
so that friction of the engine's moving parts does not begin to rise at a
dramatically faster rate than most engine owners realize. When these two
needs are met an engine turns fuel into horsepower most efficiently even
before the owner makes expensive engine modifications.

Note in the above paragraph we talk about large throttle openings and "piston
speed" rather than rpm. The dedicated pickup owner will have to be willing to
drop some old myths - myths his father's and grandfather's generation
disproved but that still hang around among those who have not operated
and measured an engine 'hands on'.
Myths like:
some magical "Powerband,"
or trying to run at high vacuum gauge readings,
or that engines "don't work as hard" at higher rpm.

You may have heard the saying:

"It's all about Torque"

For acceleration you want high torque all the time no matter what rpm
your engine is at. Engine builders strive to have a 'broad' torque
curve from low rpm to high.

Consumers brag to one another about their engine's maximum horsepower
but 99% of them don't know what (or why) the Scottish mechanical genius
James Watt invented the term "horsepower" or how it tells you the maximum
torque that can be generated at the rear wheels when a correct ratio set of
gears has been used after the engine's output shaft.

Engine designers brag to one another {when having drinks at the SAE convention
center hotel bar} about the range over which their engine develops 90% or more
of its maximum torque, because they are all aware of how this shows true
design skill.

It is all about Torque in fuel economy too. You want to generate high
torque at as low an rpm as possible without going into fuel enrichment.
This is because low piston ring speed against the bore walls
creates less wasteful internal engine friction. Most vehicle owners don't
know that if you double the RPM the friction inside the engine does not
just double - IT GOES UP ABOUT 8 TIMES.


Before discussing gearing, one tip to beginners is to drop all
confusing talk of 3.55/3.92/4.10 differential ratios, tire heights,
'effective' ratios, and so forth. It is much less confusing in the long run
to talk about gearing in terms of vehicle speed per 1000 rpm. This is
especially true if the tires have been changed out from the typical 30
inches of factory installed rubber to the 34-38 inch diameter that some
"My self esteem is low but my truck is the tallest"
Ram owners install on ridiculously lifted & unsafe pickups that seldom
if ever leave the blacktop.

If your stock Ram in top gear was showing 1500 rpm on the tachometer
when the speedometer was showing 60 mph, then your gearing was (60/1.5)
or 40 mph per 1000 rpm. This way it is easy to see that if you speeded up
to where your tachometer was showing 2000 rpm, you would be at 80 mph.

In a similar way if you turned your OD off and dropped down to 'direct drive'
third gear you can find the mph/1000 rpm for that gear too. If the tach now
read 2250 rpm at a speed of 60 mph, the gearing for your truck's third gear
is (60/2.250) or about 27 mph per 1000 rpm. You can quickly see that at
4000 rpm you would be going about 108 mph.

It is worthwhile finding this out for all your transmission's gears.
Even better, find out for yourself
where your engine's rpm of
Maximum Torque, then
Maximum Horsepower,
falls in speed (MPH) for each of the transmission's gears.

At this point after figuring out your truck's gearing you should notice that
Dodge engineers felt that to get best MPG at highway cruise the top gearing
needs to be at least around 40 mph/1000rpm or more,
and for acceleration in most "real world" conditions second gear should be
about 17 mph/1000 rpm - which puts second gear at about 30 mph at 1800
rpm where the unlocked torque converter is designed to 'stall' and goes up
to around 90 mph in the 6000 rpm range when you take into account
the percent slip of the unlocked torque converter at high horsepower.

Keep these numbers of 40 and 17 in mind if you intend to modify your
truck's tire sizes or differential gears later. If you have an interest, I
suggest you figure out for yourself what MPH/1000 rpm you should have
in 2nd gear if you want your 0-60 time to be the fastest possible - here
60 mph should occur near your "redline" rpm but you should figure in about
8% slip in the torque converter. In a similar way you can guess where
your quarter mile terminal speed will be and what MPH/1000 rpm
would be best for overall gearing if winning at the dragstrip was at the top
of your wishlist. Should that gear be 3rd or Overdrive 4th?

Now lets discuss gearing and fuel economy.
In city driving gearing doesn't matter nearly as much as not having a
'heavy foot' or hauling a lot of weight in the bed of the truck.

A vehicle that is either overgeared or undergeared will lose MPG at highway

An engine is numerically overgeared if it is not cruising at about
60-75% open throttle.

An engine is undergeared and "too weak for the job"
if it has to operate with its pistons going faster than an average of about
1200 feet per minute when at 75% throttle.

Note that you could reduce engine weakness by turbocharging
as well as the much more common thought to just make the engine larger in
cubic inches. This is the reason nearly all diesels have become turbocharged
in the last 20 years.

How do you calculate this average piston speed?
Multiply the stroke of the engine in inches by two (because the piston goes
both up and down on one rpm) and then divide by 12 to convert inches to
feet. Multiply this by the rpm.

So where does 1200 feet per minute end up for various engines?
Here some examples:

For the 3.58 stroke of 3.7, 5.7, 5.9 Dodge engines:
2011 rpm = 1200 ft/min /( 2 x 3.58 inches/12 inches per ft)

For the 3.405 stroke of the Dodge 4.7V8
2114 rpm = 1200/(2x3.405/12)

For the 3.00 stroke of the Ford 5.0L V8
2400 rpm = 1200/(2x3.00/12)

For the 4.72 stroke of the Cummins Inline 6 diesel
1525 rpm = 1200/(2x4.72/12)

Is it possible for an engine to be made to get its best fuel economy at a
piston speed above the typical 1200 feet per minute?
Very thin piston rings, using less than the normal 3 rings,
slippery coatings on piston skirts, extremely hard but slick coatings
on bore walls like "NikoSil" and keeping the bore walls very hot
so that the oil there will be thin and less viscous can all allow the
'best economy' piston speed to be raised but even 'state of the art'
giant marine diesel where nearly every trick is used seldom exceed
1500 feet per minute.

Honda has announced that over the next 5 year period that the major
part of their engine research $ will be spent on ways to reduce internal
engine friction, and this is from a company that NASCAR engine builders
already admit has the best rod and crankshaft bearing material for sale.

Which is more important: having the rpms near this 1200 foot per minute
speed for the pistons, or having the throttle in the 65-75% open range?
Answer: it is more important to be in the 70% throttle range and you should
slow down the rpms to get there. There is not much change in efficiency
when piston speed drops from 1200 down to 800 ft/minute but there is a big
change in efficiency when the throttle goes from 70% open to 40% open.


Imagine that you have a disassembled engine in front of you. Put a loose
piston in a bore and pull it down against the friction of the rings. It won't
be very hard. Now imagine that same piston in the bore has the top of the
bore sealed off and has a vacuum sucked of 12 inches of Mercury.
12 inches of vacuum is about 6 psi of negative pressure. If the piston is
4.00 inches in diameter it has an area of about 12.5 square inches
(remember pi R squared but most cherry pies are round?)
6 pounds per square inch times 12.5 square inches = 75 pounds would be
required to pull the piston down - you probably couldn't do it with the grip
of just a couple fingers! An engine running with a high vacuum in the intake
manifold has to do just this, and for eight cylinders!

Engineers refer to this as one of the two parts of "Pumping Losses". The
other part of Pumping Losses is due to pushing the exhaust out the tailpipe.
Most engine owners easily imagine that exhaust loss, accept it, and spend
a lot of time and money on mufflers, headers, Y pipes etc. These same
engine owners would do well to remember the "Pumping Loss" on the intake side
and try to keep it low during highway cruise conditions.

{Note to engineers: Yea, I know, 'negative pressure' does not really exist and
what actually happened is 14.7 psi of air pressure is pushing the underside
of the piston and there was just less pressure on the top of the piston to
create a delta P - but the general public thinks in 'vacuum' terms.
And it is not just the general public - talk to the biologists who think
-100 psi or more of 'negative' pressure pulls water up capillaries to tree

Setting Gear Ratios to suit the Customer's Expectations

Most auto manufacturers intentionally overgear their vehicles because they
know from experience that customers will bitterly complain about a vehicle
that downshifts to climb a hill.

Engineers know that to give their customers the best possible highway MPG
the transmission would downshift on EVERY hill, but the sales department
reminds them that that every employee at the automaker (including engineers)
will lose their jobs if customers {even ignorant ones} get mad
and buy someone else's vehicle that meets their expectation.

So vehicles are overgeared to 'feel strong' during the 20% of the time they
are climbing hills in Overdrive, even if it means this hurts MPG a bit during
the 80% of the time the vehicle is on level roads or going downhill.

An engine is numerically undergeared if it is trying to cruise in the
'Power Enrichment' zone above about 80% throtttle opening where the
PCM computer greatly enriches the air to fuel ratio from 14.7 to 11.
On most vehicles with automatic transmissions it is not possible to cruise
in OD on level highways in the power enrichment zone because the
transmission will downshift to a lower gear long before the accelerator
is pressed that far.

For the same reason anyone who presses the accelerator down all the way near
the floor whether they are driving in the city, highway, climbing a hill, or
pulling a trailer is going to get about 25% worse MPG than someone who
presses right up to about 70% throttle but never goes over it.

This is why the old advice to install a vacuum gauge
can certainly help MPG if the driver watches it out of the corner of his eye.

When the vacuum gauge reads about 1 to 6 inches of mercury
you are in the fuel guzzling 'Power Enrichment' zone that you want to avoid
except when you want the highest horsepower and accept the higher fuel usage.

When the vacuum gauge is showing about 7 to 9 inches of mercury vacuum
your engine is converting fuel into horsepower at its most efficient level.

Each engine goes into Power Enrichment at a slightly different MAP sensor
level, but if you listen closely to the engine sound you can hear it happen
as the engine gets a bit smoother with the very rich mixture.

Even better than just a vacuum gauge is to add a Air to Fuel meter than reads
the output of the O2 sensor. Most of the time this AFR gauge will read a
boring 14.7 to 1 air to fuel, but when it starts to drop as the accelerator is
pressed that is the 'fuel enrichment' start point for that rpm. The fuel
enrichment start point usually begins at ever more modest throttle settings
as the rpm rises because at high speed the piston tops, cylinder heads, and
exhaust valves begin to overheat faster and excess fuel is used to cool.

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