03-29-2008, 08:56 AM
Join Date: Mar 2007
Location: Ogdensburg, NJ
low tem T-stats not as god as i taught ?
i was about tCoolant temperatures and thermostat settings
There is controversy about MPG and thermostat temperatures
on the internet in automotive forums
but there is no controversy amoung auto engineers
who actually test their effects on engines.
There is a clear trend that higher coolant temperatures controlled by
195-203 setting thermostats improve MPG slightly.
Sadie Carnot's 200 year old thermodynamic theory says
that cooler air intake temperatures would give improved fuel economy
because cooler air takes slightly less work to compress, and if a
140-180 thermostat is used on an old fashioned iron or aluminum
intake manifold with coolant cross-over passages there will
be less temperature rise in the intake air as it passes through.
Modern intake manifolds such as the ones on the 4.7V8 and 5.7 Hemi
are made of insulating Nylon66 plastic so this does not
apply as much - although the incoming air can still pick up
heat from the metal walls of the cylinder head ports.
But hotter oil on cylinder walls has less viscosity and creates less
friction against the piston rings - which can also mean better MPG. The
Cummins 'Secrets of Better Fuel Economy' white paper listed at the top has a
graph showing this on page 12. Theory also predicts that hotter block walls
and cylinder heads will absorb less heat from combustion and permit a greater
pressure 'push' on the piston.
The new four cylinder 'World Engine' in the 2006 Dodge Caliber has two
thermostats: one to feed lots of cooler coolant flow to the cylinder heads
so that pinging will be reduced and an MPG improving higher compression
ratio can be used - but another separate thermostat setting to adjust the flow
to the engine block walls so that they stay hotter and ring friction will be
lower. This may be the best of both needs.
Some Dakota owners who switched to 180 degree thermostats have reported less
ping, peppier acceleration and about +1 mpg, although most reports like this
are just about what their next tank of gasoline yielded - not a careful test
that you can trust. Other Dakota owners reported no mpg change or a loss.
Four Wheeler magazine reported +0.8 mpg gain with a 192 to 180 deg
thermostat swap in a 454 Suburban. Take these reports with a grain of salt
considering who did them and how magazines live on advertising.
My own experiments with a failed thermostat that cracked and stayed open at
around 140 degrees, then later thermostats of 180, 195 (stock) & 205 showed
no significant improvement in MPG at steady 60 mph highway cruise
on a 1995 5.9V8 Ram CCab shortbed, although the 205 did measure a
0.2 MPG gain in one 300 mile test run.
The 180 degree thermostat also did not reducing pinging at least 'by ear',
nor did the 205 thermostat increase pinging by ear, but a better test would
have been to measure with a MSD aftermarket knock sensor, or better yet
read the memory of a J&S aftermarket anti-knock ignition system with its
twin knock sensors and better knock detecting software & chip.
Why would this be if you have read a thousand internet postings that
180 degree thermostats reduce detonation? This may be because the
Dodge PCM computer senses coolant temperature and either
advances or retards ignition timing as necessary according to the
tables in the memory of the computer's software. The cracked
thermostat that stayed open at about 140 did reduce pinging by ear.
When you read that 180 degree thermostats reduced pinging on old
carburetor engines that might be true, but consider that today's
computer controlled engines 'have a mind of their own'
inside the PCM that carb'ed engines did not.
The Fuel Economy Calculator from Performance Trends software predicts that a
change from a 195 degree thermostat to a 175 worsens MPG by about 0.20 at a
steady 70 mph.
The Engine Analyser 3.2 program from Performance Trends predicts
that when a 1995 specification Magnum 5.9 V8 engine is 'throttled back'
to 10 inches of vacuum and a coolant temperature of 195 degrees
at a simulated steady highway cruise at 1600 rpm it
will make 50 horsepower at a fuel economy of 0.604 lbs/hp-hr.
If the coolant temperature is raised to 250 degrees and the throttle
re-adjusted to make 50 horsepower once again at 1600 rpm
the fuel economy improves to 0.571 lbs/hp-hr.
This is about a 5.5% improvement.
Respected tech editor Marlan Davis of Hot Rod magazine has reported that
all things considered, fuel economy is better with coolant at 210 degrees F.
Perhaps that is why the factory thermostat on the 5.7 Hemi is now marked 203
F, which is were it begins opening. Note that on 4.7 and 5.7Hemi engines the
thermostat position and function has been totally redesigned to control the
coolant in, rather than the coolant out temperature. This would lead one to
guess that the coolant coming out of a 5.7 Hemi is hotter still. The 5.7V8
now also has a closing bypass post sticking out from it that closes off the
bypass passage and results in greater coolant flow to the radiator once the
engine is up to designed temperature. This allows the water pump to be run
slightly lower in rpm and saves a bit of fuel.
Someone who is up to doing experiments could remove their thermostat entirely
and cut/weld in an electrically controlled valve in the tall metal nipple
above the thermostat flange. With this electric valve and a temperature
(there are several adjustable ones for on sale for electric fan control)
you could then have an adjustable thermostat.
For much cheaper & simpler you could also just put in a cable operated valve
and keep a 180 thermostat in the housing. Another option would be to
install a set of shutters to restrict air flow through the radiator to bring
up coolant temperature. Either of these would allow you to raise the coolant
temperature while at steady highway cruise to something like 220 with
50/50 conventional coolant mix (or 250-300 with Evans NPG coolant)
which might improve highway MPG by maybe 4 to 6%.
Coolant temperatures also affect bore wall wear, according to experiments
run by aircraft engine manufacturer Continental Engines in the 1960s.
Bore wear is relatively high at 140-160 degrees but begins rapidly leveling
off at 180 and shows no improvement above 205. It is thought that this wear
effect is due to acidic liquids condensing on the metal bore walls and
corroding them. Above a certain coolant temperature the liquids never
condense and remain gases. A similar problem happens in the chimneys
of power plants.o get a 180 T-Stat wen i found this