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LT1 engine cooling
information
LT1 engine used
in late model Camaro's, Firebird Formula's, Corvettes and Impala SS's
This may help
most of you with these late model F-body cars appreciate what you have under
the hood and you'll understand the advantages of the LT1 engine vs. the L98
predecessor and other conventional engines.
The
LT1 Reverse Flow Cooling System
|
One of the greatest
features of the '92 and up Chevrolet LT1 engine is the reverse flow cooling
system. In fact it is reverse flow cooling that is truly the key to the incredible
performance of the modern LT1. Reverse flow cooling is vastly superior to the
conventional cooling systems used on virtually all other engines. This is because
it cools the cylinder heads first, preventing detonation and allowing for a
much higher compression ratio and more spark advance on a given grade of gasoline.
A fringe benefit is that cylinder bore temperatures are higher and more uniform,
which reduces piston ring friction. Because of this new cooling system, the
LT1 can easily meet ever increasing emissions standards with significant gains
in power, durability, and reliability.
Conventional
Coolant Flow
|
In a conventional
engine design, coolant enters the front of the block and circulates through
the block's water jacket. The coolant is first heated by the cylinder barrels,
and then hot coolant is subsequently routed through the cylinder heads and intake
manifold before returning through the thermostat to the radiator.
Because the
coolant from the radiator is first directed to the cylinder bores, they run
at below optimum temperatures which increases piston ring friction. The heads
subsequently get coolant that has already been heated by the cylinder block,
which causes the heads to run well above optimum temperatures. The hotter
cylinder heads promote detonation (spark knock) and head gasket failures.
To combat the increased tendency to detonate, compression ratios has to be
lowered and spark advance reduced, which significantly reduces engine power
output and efficiency.
Besides promoting
detonation, causing gasket failures, forcing reduced compression, spark advance,
and significantly reduced power output, a conventional cooling system causes
several other problems. Since the thermostat is on the exit side of the system,
it does not have direct control over the cold coolant entering from the radiator.
This is especially true when the thermostat first opens after reaching operating
temperature. As the thermostat first opens allowing hot coolant to exit the
engine, a rush of very cold coolant enters the block all at once, shocking
the engine and causing sudden dimensional changes in the metal components.
The extreme thermal shock experienced by the engine causes head gaskets and
other soft parts to fail much more quickly.
Conventional
cooling system design also allows isolated engine hot spots to occur, which
lead to the generation of steam pockets and coolant foaming. Coolant which
is full of air and foam reduces cooling system performance and can even lead
to engine overheating.
The LT1 is completely
different since it uses reverse flow cooling. The incoming coolant first encounters
the thermostat, which now acts both on the inlet and outlet sides of the system.
Depending on the engine coolant temperature, cold coolant from the radiator
is carefully metered into the engine. This allows a more controlled amount of
cold coolant to enter, which immediately mixes with the bypass coolant already
flowing. This virtually eliminates the thermal shock present in the old system.
After entering
through one side of the 2-way thermostat (at the appropriate temperature),
the cold coolant is routed directly to the cylinder heads first, where the
combustion chambers, spark plugs and exhaust ports are cooled. Then the heated
coolant returns to the engine block and circulates around the cylinder barrels.
The hot coolant from the block re-enters the water pump, and hits the other
side of the 2-way thermostat, where it is either re-circulated back through
the engine or directed to the radiator, depending on temperature.
The main concept
behind reverse flow cooling is to cool the heads first, which greatly reduces
the tendency for detonation, and is the primary reason that the LT1 can run
10.5 to 1 compression and fairly significant ignition advance on modern lead-free
gasoline. Reverse flow cooling is THE KEY to the Generation II LT1s increased
power, durability, and reliability over the first generation smallblock engine.
All LT1 engines utilize
a special 2-way acting full bypass thermostat. This means that the thermostat
regulates coolant flow both in to as well as out of the engine, while the bypass
portion of the thermostat circuit supplies the water pump with a full flow of
liquid coolant at all times. This is unlike a conventional engine thermostat,
which only regulates coolant flow at the engine outlet, and which does not allow
full flow through the water pump when the engine is cold and the thermostat
is in bypass mode.
Both sides of
the 2-way thermostat used in the LT1 are linked together, and a single wax
pellet actuator operates the spring loaded mechanism at a pre-set temperature.
When the designated temperature is reached, the wax pellet expands, opening
the dual acting valve. All current LT1s come from the factory with a relatively
low 180 degree temperature thermostat. Most conventional engines today use
195 degree thermostats in order to meet emissions specifications at the expense
of power, durability, and reliability.
It is important
to note that the 2-way thermostat is unique to the Generation II LT1 and is
not interchangeable with older Chevrolet smallblock engines. This is particularly
important if you decide to change to a colder 160 degree thermostat, make
sure it is the proper dual acting type required by the modern LT1.
Additional LT1 Cooling System Improvements
|
In addition to reverse coolant flow, there are several other improvements
in the LT1 cooling system over conventional engines.
The LT1 has absolutely NO water running through the intake manifold!
Conventional cooling systems have passages in the intake manifold which
allow coolant to crossover from one side of the engine to the other. In
the LT1, coolant crossover occurs in the water pump, which is also where
the thermostat is located. Since there are no coolant passages in the
intake manifold, a major source of leaks has been eliminated. Overall
engine reliability is improved since an intake manifold leak allows
coolant to enter the top of the engine which can quickly wipe out the
camshaft, lifters, and other major engine components. Designing a dry
intake manifold without either coolant passages or a thermostat housing
also allows a much lower profile. The LT1 engine is 87mm (nearly 3.5
inches) lower than the previous L98 Corvette engine.
One big problem with conventional cooling systems is the water pump, which
simply cannot last a targeted minimum 100,000 mile reliability figure
without experiencing leaking gaskets or seal failures. This has
traditionally been caused by the excessive side loads placed on the
bearings and seals of a conventional water pump through the belt drive
mechanism. In the LT1 this problem is solved by driving the water pump
directly via a spur gear driven by the camshaft sprocket. This results in
a dramatically more reliable water pump that should easily last 100,000
miles or more.
Since the water pump is no longer belt driven, the vehicle will still be
driveable even if the serpentine belt fails. This is a major safety
factor as it allows one to drive the partially disabled vehicle to the
nearest service center.
The LT1 has strategically placed steam vents at the back of both cylinder
heads. Since the heads are the hottest part of the engine, pockets of
steam can be more easily generated there. The steam vents are connected
together by a crossover vent tube at the back of the heads, which directs
any steam and a small flow of coolant to the front of the engine where it
flows through the throttle body, warming it for improved cold weather
performance. After passing through the throttle body, most of the steam is
condensed back into liquid coolant and returned to the system.
In LT1 B/D-cars, coolant exiting the throttle body is passed directly into
a pressurized coolant reservoir where any air remaining in the coolant is
completely scavenged. In LT1 F-cars, coolant from the throttle body
connects to the heater outlet via a vented "tee" connector, where any
trapped air in the system can be bled off manually. Eliminating steam
pockets and foam in the coolant allows for more uniform cooling system
performance, preventing hot spots and potential overheating.
Unlike a conventional cooling system, the thermostat coolant outlet is
connected to the bottom of the radiator. This forces the coolant entering
the radiator to push up through the radiator core and eventually emerge
through the top radiator coolant outlet. This helps to eliminate air
pockets in the radiator, and provides a more even distribution of cooling
through the core and improving radiator efficiency.
Precision Machined Thermostat Housing
|
The thermostat housing is a precision machined component that fits
directly onto the top of the water pump without a gasket. Instead, an
O-ring is used to seal the thermostat inside the housing. This precision
design reduces the tendency for leaks, plus it makes thermostat
replacement a very simple job since there is no old gasket material to
scrape off. Servicing is further simplified because the thermostat housing
is situated directly on top of the water pump, and access is unobstructed.
I dare say that the LT1 thermostat is the easiest to change I have ever
experienced. Finally, an air bleeder valve is located on the top of the
thermostat housing, which allows one to quickly and easily bleed out any
trapped air after cooling system maintenance has been performed.
The entire cooling system on the LT1 is designed to operate at lower
pressures than conventional cooling systems. The maximum operating
pressure in the LT1 cooling system is 15 psi for B/D-cars and 18 psi for
F-cars, limited by a pressure cap. These limits are similar to other cars,
but in the LT1, these maximum pressures are rarely reached. Running at a
lower pressure drastically decreases the number of leaks and significantly
improves overall reliability and durability.
Corvette and B/D-car LT1 applications use a pressurized coolant recovery
reservoir instead of a non-pressurized overflow tank used with
conventional cooling systems. All of the coolant flows continuously
through the pressurized reservoir, which is an integral part of the
cooling system. The pressurized reservoir in the LT1 B/D-cars is connected
to the cooling system in three places. One inlet hose connects to the top
of the RH radiator tank, a second inlet hose is attached through a "tee"
connection on the heater inlet hose, and a third outlet hose is connected
to a "tee" connection in the throttle body heater outlet.
The pressurized reservoir is mounted at the highest point in the system,
and provides a place where all air can be continuously scavenged from the
coolant. Any steam and bubbles are allowed to rise to the surface,
eliminating foam and providing pure liquid coolant back to the engine.
Pure liquid coolant is returned to the system via the heater outlet hose
connection. The pressure relief/vent cap in these systems is rated at 15
psi and is located on the reservoir rather than the radiator.
LT1 F-cars use a conventional coolant recovery system which consists of a
non-pressurized coolant overflow tank connected to the radiator by a
single hose. These cars use an 18 psi rated pressure relief/vent cap on
the radiator like most conventional systems. Since these cars cannot
scavenge air from the coolant as well as the B/D-car or Corvette systems,
they have two air bleeder valves for manually bleeding trapped air from
the system. One is in the thermostat housing, which is the same as all
other LT1 engine vehicles, and the second one is located in a "tee" where
the coolant from the throttle body connects to the heater return hose.
B/D-car LT1 (Caprice/Impala/Roadmaster/Fleetwood) Cooling Systems:
Standard equipment for all LT1 equipped B/D-cars is a dual electric fan
setup with a 150-watt primary (RH) fan and a 100-watt secondary (LH) fan.
The electric engine coolant fans are independently operated by the PCM
(Powertrain Control Module) based on the inputs from the Engine Coolant
Temperature (ECT) sensor, A/C Pressure Sensor, Vehicle Speed Sensor (VSS),
and various other inputs.
The B/D-car coolant fans operate under PCM control at the following engine
temperatures and A/C system pressures:
Fan Mode Temperature A/C Pressure
====================================================
Primary (RH) Fan ON: 107 C 225 F 189 psi
Primary (RH) Fan OFF: 103 C 217 F 150 psi
----------------------------------------------------
Secondary (LH) Fan ON: 111 C 232 F 240 psi
Secondary (LH) Fan OFF: 107 C 225 F 210 psi
Additionally, the PCM will turn off the fans at higher vehicle speeds
(above 48 MPH I believe) since running fans can actually impede airflow
through the radiator at high speed. Each fan also has a minimum running
time. Once activated, the primary fan will run for a minimum of 50
seconds, and the secondary fan for a minimum of 26 seconds. Finally,
certain Diagnostic Trouble Codes (DTCs) may cause the PCM to turn on one
or both fans.
All LT1 B/D-cars have two transmission oil coolers and an engine oil
cooler as standard equipment. The transmission coolers include a primary
oil to water type inside the RH radiator tank, and a secondary external
oil to air cooler (KD1) mounted in front of the radiator on the RH side.
The external KD1 cooler is an aluminum stacked plate type cooler painted
black with metal tube lines linking it in series with the other cooler in
the radiator tank. LT1 B/D-cars also include an engine oil to water cooler
(KC4) mounted in the LH radiator tank.
Optional B/D-car LT1 Cooling Systems
|
There are two optional cooling system upgrades for LT1 B/D-cars, called
V03 (Extra Capacity Cooling), and V08 (Heavy Duty Cooling). Performance
models such as the WX3 (Impala SS) and 9C1 (Police) cars automatically get
the upgraded V03 (Extra Capacity Cooling) system. V03 includes a larger
radiator, an increased capacity A/C condenser, and an upgraded secondary
electric fan. V03 is also optional on most B/D-car models.
Note that the '94 V03 (Extra Capacity Cooling) option uses a 150-watt
primary (RH) fan, and an upgraded 240-watt secondary (LH) fan. In '95-'96
the V03 package was revised and no longer included an upgraded 240-watt
secondary fan. Instead the standard 100-watt secondary fan was used, which
is the same as the base cooling system.
B/D-cars other than the Impala SS or Police package Caprice also have an
optional V08 (Heavy Duty Cooling) package which is part of the V92
(Trailer Towing) package. V08 includes the larger radiator, increased
capacity A/C condenser, and upgraded secondary fan as in the V03 system,
however it differs in the primary cooling fan. With V08 the 150-watt
electric primary fan is replaced by a mechanical belt driven thermostatic
clutch fan. To drive the mechanical fan, the V08 system includes a crank
pulley, belt tensioner and bracket, and a large radiator shroud in
addition to the mechanical fan itself. This package is not available on
the WX3 (Impala SS) or 9C1 (Police) cars since the mechanical fan is
driven by an additional pulley and belt on the engine crankshaft, which
draws engine power thus reducing performance.
The mechanical fan used with the V08 cooling system contains a
built-in thermostatic clutch which senses the temperature of air that has
been drawn through the radiator. When the temperature of this air is below
66 degrees C (151 degrees F), the clutch freewheels and limits the fan
speed to 800-1,400 rpm. When the temperature rises above 66 degrees C (151
degrees F), the clutch begins to engage, and the fan speed increases to
about 2,200 rpm. The RH radiator hose in V08 equipped vehicles has a steel
tube section near the fan designed to prevent damage in case of fan
contact.
There are several SEO (Special Equipment Option) B-car cooling options
which are included as standard only with 9C1 (Police) package Caprices.
These include the following:
In addition to the standard inclusion of the V03 (Extra Capacity Cooling)
package, all LT1 Caprice 9C1 (Police) cars also include SEO 1T1 (Silicone
Radiator and Heater Hoses). SEO 1T1 consists of special green radiator and
heater hoses made out of pure silicone rubber. These hoses are designed to
last the life of the vehicle and never need replacement unlike the
standard black rubber hoses. SEO 1T1 also includes heavy duty stainless
steel worm gear hose clamps which replace the standard squeeze type hose
clamps. The clamps have a solid full perimeter band, which prevents the
hose from extruding between the slotted area where the screw fits. This
also prevents the hose from being cut or damaged by the clamp, and allows
a more even sealing force around the entire clamp perimeter.
The 9C1 Police package also includes SEO 7P8 (External Engine Oil to Air
Cooler). This is an unpainted aluminum stacked plate type cooler which is
mounted in front of the radiator on the LH side opposite the external
transmission cooler. This heavy duty engine oil cooler replaces the
standard engine oil to water cooler found in the LH radiator tank of other
LT1 B-cars.
Also included with the Police package is SEO 7L9 (Power Steering Fluid
Cooler). This consists of a loop of metal tubing installed between the
radiator lower support and the front stabilizer bar. This cooler prevents
the power steering fluid from overheating in rigorous driving situations
such as high speed persuit.
F-car LT1 (Camaro/Firebird) Cooling Systems
|
Standard equipment for all LT1 F-cars with A/C is a dual electric fan
setup with primary (LH) and secondary (RH) fans. There are two different
wiring schemes used for these fans, an early design that was used in
'93-'94 and a late design that has been used from mid-'94 up. Note that
non-A/C F-cars have a single primary fan which operates at a fixed high
speed.
In '93 and early '94 models with A/C, the two cooling fans are
independently operated by the PCM (Powertrain Control Module) at a high
fixed speed by using a single relay for each fan. Late '94 and newer F-car
models operate both fans simultaneously in either a low or a high speed
mode by using 3 relays. In low speed mode, the fans are powered in series.
In high speed mode, the relays operate to power both fans in parallel,
resulting in a higher speed of operation.
One way to tell which setup you have is by looking at the alternator. If
an F-car is equipped with the 124 amp alternator (KG7), then the vehicle
has the early design setup and the fans are operated independently. If the
vehicle has the 140 amp alternator (KG9), then it also has the newer
design configuration which operates the fans simultaneously in low or high
speed modes.
The PCM operates the coolant fans based on input from the Engine Coolant
Temperature (ECT) sensor, A/C Pressure Sensor, Vehicle Speed Sensor (VSS),
and various other inputs. The F-car coolant fans operate at the following
temperatures and pressures:
Fan Mode Temperature A/C Pressure
==========================================================================
Primary (LH) or Dual Low-speed Fan(s) ON: 108 C 226 F 248 psi*
Primary (LH) or Dual Low-speed Fan(s) OFF: 105 C 221 F 208 psi*
--------------------------------------------------------------------------
Secondary (RH) or Dual High-speed Fan(s) ON: 113 C 235 F 248 psi
Secondary (RH) or Dual High-speed Fan(s) OFF: 110 C 230 F 208 psi
*Note - this information is probably incorrect, although it is quoted from
the service manual.
Additionally, the PCM will turn off the fans at higher vehicle speeds
(above 70 MPH I believe) since running fans can actually impede airflow
through the radiator at high speed. Each fan or fan mode has a minimum
running time. Once activated, the primary fan or dual low-speed fans will
run for a minimum of 50 seconds, and the secondary or dual high-speed fans
for a minimum of 30 seconds. Finally, certain Diagnostic Trouble Codes
(DTCs) may cause the PCM to turn on one or both fans.
All LT1 F-cars with automatic transmissions also have a transmission oil
cooler as standard equipment. The transmission cooler is an oil to water
type mounted inside the RH radiator tank.
Optional F-car LT1 Cooling Systems
|
There is only one option in an LT1 F-car with respect to cooling, and that
is an engine oil cooler (KC4). The engine oil cooler is an oil to water
design that is mounted in the LH radiator tank. The KC4 oil cooler is
included with various other combinations of options on the F-cars.
Operating Characteristics and Observations
|
I have an accurate digital temperature gauge installed in the RH cylinder
head water jacket on my '94 Impala SS. I installed a brass "T" fitting in
the RH cylinder head, in the tapped hole where the factory temperature
gauge sender was originally installed. This allowed me to install both the
original analog gauge sender as well as the sender for the new digital
gauge. With the stock 180 degree thermostat, cruising at 80 mph on a cool
night I would routinely measure coolant temperatures in the head as low as
167 degrees! If I slowed down, the temperature would climb up into the
170-180 degree range depending on ambient temperatures and cruising speed.
The temperature would run in the 180s-190s cruising more slowly on a hot
summer day. In heavy stop and go traffic, the temperature would quickly
climb up into the 220-230 degree area, which is where the primary fan
starts to come on.
Many have noticed as I have that the engine will actually run cooler in
traffic with the A/C on. This is because turning on the A/C will also
cause the PCM to activate at least the primary fan, and possibly the
secondary fan (depending on A/C system pressure) as well.
The radiator and A/C condenser in B/D-cars equipped with the RPO (Regular
Production Option) V08 (Heavy Duty Cooling) or V03 (Extra Capacity
Cooling) systems are extremely large, perhaps the largest of any passenger
car on the market today. The cooling and A/C system performance on these
cars are outstanding, in fact the best I have seen on any vehicle.
Recommendations for Cooling System improvements
|
If you have a B/D-car, there are several easy improvements you can make by
simply adding the cooling related SEOs (Special Equipment Options) from
the 9C1 Caprice Police package. For example, I have installed all of the
Police package cooling upgrades in my '94 Impala SS. This includes the 1T1
silicone hoses, 7L9 power steering fluid cooler, and 7P8 external engine
oil cooler. Combined with the already powerful V03 cooling system, these
factory upgrades combine to form the most extreme duty factory cooling
system present on any automobile I have seen.
If you have an F-car which was not factory equipped with the optional KC4
engine oil cooler, then I would highly recommend installing it as an
upgrade. The KC4 option consists of a different radiator with the engine
oil cooler located inside the LH tank. An adapter installs on the oil
filter pad between the filter and the engine, and lines run to the cooler
in the radiator tank.
There are two other cooling system improvements that can be applied to any
vehicles with the LT1 engine, including the Corvette and F-cars
(Camaro/Firebird). These are to change to a colder 160 degree thermostat
(180 is standard), and to alter the electric cooling fans to come on at a
lower temperature. This latter function can be accomplished by adding an
external thermostatic switch to the fan circuit, or by re-programming the
PCM fan operation settings.
As mentioned earlier in this article, the stock fans do not come on until
at least 225 degrees, which I feel is too hot. To prevent the engine from
heating up this high in traffic or while moving slowly, I installed a 203
degree GM thermostatic switch (p/n 3053190) in a pre-existing tapped hole
in the LH cylinder head water jacket, and wired it to both the primary and
secondary fan relay via a 3-position toggle switch.
When the coolant temperature reaches 203 degrees, the primary or secondary
fan (depending on the setting of the toggle switch) will run. This
prevents the engine from running hotter than about 200 degrees or so. I
have tested this modification in 100 degree ambient temperatures, while
trapped in stop and go traffic, and never saw coolant temperatures higher
than 205 degrees. I wired the toggle switch to operate either the primary
or secondary fan, as well as to disconnect the thermostatic switch from
the circuit, thus disabling this function. No matter what the toggle
switch setting, the PCM still has control over the fan relays, and will
continue to operate the fans oblivious to the additional thermostatic
switch function.
I have more recently purchased the Hypertech Power Programmer, which
re-programs the PCM to turn the primary fan on at 176 degrees (instead of
225), and the secondary fan on at 191 (instead of 232). At first I
installed the Hypertech program without the recommended 160 degree
thermostat in order to observe the operation of the fans. I found that the
primary fan would run continuously once the engine had warmed up, and even
the secondary fan would be on most of the time. This is due to the overlap
between the high thermostat setting and the lower fan activation
temperatures programmed in by Hypertech. The new settings were turning the
primary fan on at a setting lower than the thermostat itself would open.
After installing the recommended 160 degree thermostat, the fans worked
normally, and would only begin to run after the car was not moving which
allowed the temperature to rise. In actual operation I saw temperatures
while moving about 10 degrees lower than what I observed with the 180
degree thermostat. While moving very slowly or sitting stationery, the
engine would never climb above the low 190 range, no matter how high the
ambient temperatures was or how slow I was moving. After observing this
operation, I would wholeheartedly recommend the 160 degree thermostat and
the Hypertech Power Programmer. If you use the Power Programmer, then the
160 degree thermostat MUST be installed or the fans will run continuously,
which is not good for either the fans, alternator, or battery.
If you do not want to purchase the (fairly expensive) Power Programmer,
then I highly recommend installing the 203 degree thermostatic fan switch
I listed, which will prevent the excessive temperatures encountered in
traffic that are allowed by the stock PCM program settings. The fan switch
will work well with either the stock 180 degree thermostat or a 160 degree
unit, and will limit the maximum coolant temperatures to 205 degrees or
less.
GM Vehicles Featuring the Generation II LT1:
Chassis Models Years
-------------------------------------------
Y-car Corvette '92-'96
F-car Camaro/Firebird '93-'96
B-car Caprice/Impala/Roadmaster '94-'96
D-car Fleetwood '94-'96
Note that the D-cars are really a slightly stretched version of the B-car
and are virtually identical except for the wheelbase.
If you have any questions or comments concerning this article, I can be
reached at:
Scott Mueller
Mueller Technical Research
21718 Mayfield Lane
Barrington, IL 60010-9733
(708)726-0709
(708)726-0710 FAX
Compuserve ID: 73145,1566
Internet ID: 73145.1566@compuserve.com
E-mail:
Josh
