Had a discussion with my bro in law about efficiency vs. HP gains yesterday. As with most conversations with him (he is a bowtie minion) things got heated.

In any case, when a gain in MPG is observed does this ALWAYS mean that HP goes up also? I have my opinion, but need some good info to back me up.

Any feedback, links to articles, etc is greatly appreciated!!!

It really depends on why there was a MPG increase...

Quote:

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...when a gain in MPG is observed does this ALWAYS mean that HP goes up also?

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No.

Short simple answer:
if a fuel economy gain comes from
less friction inside the engine
or a faster burn in the cylinder that requires less degrees of ignition timing
then there is usually also a power increase too.
Recently, with many engines getting Variable Valve Timing, this feature usually adds better fuel economy 1500 to 2500 rpm and increased HP 3500 to 6500+ rpm.

Long answer

MPG
is for an entire vehicle
travelling on a specific road.

Horsepower for an engine
is usually used refering to a "peak" power
measured by a set of rules such as
SAE or Standard Dyno,
but can also be "instantaneous power" under different comditions.

Transmission/differential gearing
ties the engine to the vehicle
and certainly affects both
MPG and Torque at rear wheels.

If the pavement changes from
rough new blacktop to smooth old concrete
the MPG goes up,
but peak hp is unchanged.

If air temp goes up,
mpg goes up
but instantaneous power available goes down,
and so forth.

If a different gear is used
on a specific section of road
MPG may change
but peak rated HP is not.

To get away from such difficulties,
engineers create various styles of
maps,
comparing more than two things at once,
such as:

Power versus
RPM versus
Throttle Opening versus
Fuel Flow

Torque produced vs RPM vs Fuel flow

Perhaps the best such map
is the one that compares

Brake Mean Effective Pressure ( gives both Torque and HP)
versus
Average Piston Speed ( depends on rpm and crank throw)
versus
Fuel Consumption
expressed as mass of fuel used up during an hour of power produced,
which is called in techno-speak as
Brake Specific Fuel Consumption
or shortened to initials
BSFC

[Edit: See examples several posts down from this one]

While the above sounds complicated
it is a
"universal"
map that can be used to compare
engines of any size, stroke, cycles from
tiny weed whacker engines
to automotiveV8s
all the way up to
huge 100,000 HP ship engines

Any change that
improves fuel economy
in an area within such a
universal map
usually also improves
torque and HP
but only near that
specific piston speed.

At higher or lower
piston speeds
a change might cause
the BMEP to go up
but the BSFC may not
... could go up, down or stay the same.

Top Fuel dragsters dont get 100 MPG but they have 1500 HP

360view, thanks for the info!!!

Let me pose this question...if I gain 1mpg from changing spark plugs (which I did on my daily driver, the old plugs were not "old", I just swapped to a different design)did/do I have to be making more power?

Obviously efficiency went up, but did power HAVE to go up as well?

Yes, kudos to 360View for explaining the incredibly complicated world of MPG.

Re: spark plug change. In this particular case, yes, even though I don't believe it really happened. But if it did, the mpg increase resulted from an increase of combustion efficiency. If it did happen, you probably had one or two plugs that weren't firing all the time. Getting those cylinders to fire all the time with new plugs increased power and mpg.

Without changing ANYTHING ELSE, increasing combustion efficiency will always result in an increase in power. Always.

R.

Depends upon how the mpg increase was obtained. If the vehicles resistance us reduced, mpg will go up and no more fkywheel hp is produced, but rear wheel hp will go up.

If the engine is leaned out, mpg will go up, but usually hp will go down.

Astrobuf

Did the new sparkplugs have:

longer projected nose
sharper tip electrodes
bigger gap?

the above 3 design features "usually" result in slightly faster combustion and bring the stock degrees of ignition advance closer to the ideal setting of " mean best torque" or MBT

this usually results in a little bit better fuel economy, torque and power all at the same time

most Chrysler engines seem to be retarded from MBT
by anywhere from 6 to 10 degrees or so
for safety sake to reduce warranty claims for detonation damage
when an unlucky tank of less than 85 octane gasoline is bought
on hot dry summer days

this means faster burn sparkplug features
can gain benefits
if you are willing to to reduce the factor of safety
against detonation

the rough rule of thumb is
10 degrees of retard from MBT timing
results in 5 % less torque at full throttle
but allows burning gasoline
5 Octane points lower before audible pinging

Fuel Economy-wise
it is always better to wipe out degrees of retard before MBT
with better burn sparkplugs ( or cyl head chamber design)
than by simply advancing the ignition setting

when the burning begins before the piston reaches top dead center it creates "negative work" that the other cylinders have to overcome

ideal cylinder heads and sparkplugs combos
would provide MBT
at 0 degrees top dead center

one guy doing a Masters degree in Engineering a MIT
added a second sparkplug to the side valve CFR engine cyl head
and found gains in
fuel economy,
torque and
power
after the degrees of ignition advance
were reduced to a new MBT setting.

He decided to do his thesis
on how many sparkplugs
would be needed to get best results.

He got up to 13 sparkplugs
yet found there were still measurable gains from 12,
but now there was no more room
on top the side valve cylinder head for another threaded hole.

attached JPEG picture is a graph of a typical 1960s automotive engine performance map with
BMEP
(brake mean effective pressure which is similar to torque per cubic inch of displacement)
plotted against
Piston Speed in Feet per minute
(convert to RPM using stroke in feet instead of inches)

From Volume I of Charles Fayette Taylors classic book on engines "The Internal Combustion Engine in Theory and Practice" figure 12-16.

Modern engines have improved about
7% in fuel economy BSFC for each ten years,
so the ovals of best fuel economy
would be smaller numbers than the 0.500 pounds of mass per horsepower hour here,
except that this is for "pure" 120,000 BTU per gallon gasoline
and today it it blended with ethanol
down to as little as
107,000 BTU per gallon in some winter blends.

The upper line for BMEP
which tops out in this 1960s data at 120 psi, would now go up to the
138 psi to 160 psi range.
Seems like I remember that both the 1995 cast iron Magnum 8L V10 and Magnum 5.9L V8 maximum torque values calculate out to about 138 psi.

Reading this makes my head hurt! Very good info!

Here is a more modern engine performance graph which is unfortunately metric, from page 559 of the Fourth Ed of the Bosch Automotive Handbook.

The ovals are now total thermal efficiency fractions. 0.34 would be 34% efficiency

Torque is in Newton Meters

Instead of RPM
the nutty Bosch-co-vites
use the odd number
"Radians per Second".
There are 6.28 Radians in each shaft rotation
and 60 seconds in a minute, so the
400 R/sec figure on the bottom middle of the graph
is the same as
3,821 RPM

Do you think that an "overdrive" transmission is one
where the driveshaft turns faster than the engine RPM?

Well the Bosch-co-vite engineers define a 'overdrive' ratio
as one where the combination of engine and transmission gears can or can not
bring the vehicle up to its theoretical top speed,
so if you think your
2.76 diff gear 727 acts in all practical terms like a
3.92 diff gear 518 trans then you are in their camp.

The lines on the graph running bottom left to upper right
with numbers at their top like
0.71, 0.85, 1.0, 1.12
are 'overdrive' ratios.

Another graph from page 328 of the
Fourth Ed of the Bosch Automotive Handbook
showing how a change in gear ratios
can get a vehicle/engine combination
closer to the best engine efficiency oval
(sometimes known as the "island of best fuel economy")
but slows its full throttle acceleration in that gear.

i guess another common factor
when the fuel economy goes up
but torque and max HP goes down
is when the
air to fuel ratio
is leaned out.

Somewhere between
18 and 28
air to fuel ratio
missfiring begins to increase
and fuel economy and torque both
go down

Very lean a/f burn engines
are similar to Atkinson cycle engines
in that their expansion ratio
per lb of fuel
is high.

Also very lean mixtures 20 to 28 a/f
can be more highly compressed
without detonation
than the "worst to ping"
14.7 to 17 a/f ratios can

Half throttle Atkinson Cycle gasoline engines,
Lean Burn gasoline engines,
and quarter rack position loaded diesel engines,
are all operating in
somewhat similar conditions
inside the cylinder
come to think of it