The following articles provide more information on the Gen II LT1. This may help most of you 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 cylinder barrels first heat the coolant, 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 that is full of air and foam reduces cooling system performance and can even lead to engine overheating.
LT1 Coolant Flow
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 small-block 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 small block 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.
Dry Intake Manifold
The LT1 has absolutely NO water running through the intake manifold! Conventional cooling systems have passages in the intake manifold that 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 that 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.
Gear Driven Water Pump
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. The excessive side loads placed on the bearings and seals of a conventional water pump through the belt drive mechanism have traditionally caused this. 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 drivable 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.
Reverse Flow Radiator
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.
Low Operating Pressure
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 that 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 that 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 a mechanical belt driven thermostatic clutch fan replaces the 150-watt electric primary 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 that 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 that 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 pursuit.
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 that 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 that 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 that 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:
Mueller Technical Research
21718 Mayfield Lane
Barrington, IL 60010-9733
Compuserve ID: 73145,1566
Rebuilding the Chevrolet LT1 Engine, Doug Anderson, Automotive Rebuilder, September 1999
Photos follow article
Thirty-seven years after the birth of the small block Chevy V8; the Generation II engine was introduced in the 1992 Corvette as the LT1. Although it shared many common dimensions, looked much the same and even had a few common parts, it was totally redesigned to provide more power with lower emissions and better fuel economy.
Compared to the 1991 Chevy 350 L98 with TPI, the LT1 made 20% more horsepower, got better fuel mileage, and had a much broader torque band with 90% of itís peak torque available from just over 1,000 rpm all the way up to nearly 6,000 rpm.
GM Powertrain accomplished all of this by reverse cooling the engine so they could bump the compression ratio up to 10.5 to 1, tweaking the airflow in and out of the engine, and using sophisticated electronic controls for both fuel and ignition. This combination gave the LT1 300 hp in 1992 and ultimately led to the 1996 LT4 that used better heads, more cam timing, roller rockers and sequential fuel injection to make 330 hp. Although the LT1 was only around for five years, there were two-bolt and four-bolt blocks, aluminum and cast iron heads, regular and H.O. cams that came with long and short dowels, and three different front covers. There was also the "Baby LT1," the 265 cid version that was the standard engine in the Caprice from 1994-Ď96. With all that in mind, letís take a look at this family of engines and see what goes where.
350 - There are two blocks, one with two-bolt mains and one with four-bolt mains. They both have the same 10125327 casting number, so thereís no sure way to know which one you have until you get the pan off. However, if it came out of a Corvette, it should be a four-bolt block, and if it came out of anything else, it was supposed to be a two-bolt. GM used the two-bolt block for everything but the Corvette because it had plenty of strength and it weighed a little bit less.
265 - There was only one block used for the 265 cid version of the LT1. Itís a 10168588 casting that had the numbers "4.3" cast on the side, too. Itís real easy to spot if the heads are off because of the small 3.74" bore. Getting the right cam in the right engine can be a little bit tricky because there were several variations over the years. There are essentially two different grinds used with two different snouts, depending on which distributor was used on the engine.
350 - The crank for the LT1 looks just like the one in the late 350 and has the same casting number 14088526, but itís balanced for the lightweight pistons that were installed in the LT1. Be sure to keep these cranks separate so they donít end up in a regular 350, and donít ever use a regular 350 crank in a LT1. In fact, if you are short of LT1 cranks and donít have a balancing machine in your shop, you would be better off using a crank from a 305 instead of a 350 because itís actually closer to the balance specs for the LT1 crank.
265 - The 265 has itís own unique crank with a 3.00" stroke. Thatís the same stroke the original 265 had back in 1955; itís funny how things go around and come back full circle. Itís a 10168568 casting.
350 - The original LT1 came with regular forged 350 rods, that were shot peened for localized hardness under the head of the bolt and nut. Powdered metal rods were phased in for the Corvette around 1994 and used in all of the LT1 engines by 1995. GM made the change because the powdered metal rods were cheaper to make and were much stronger than the GM high performance "pink" rods. In fact, they are supposed to be good for up to 450 hp. They are machined at the parting line so they can be reconditioned.
265 - The 265 rods are 0.240" longer than the ones in the 350. Both blocks are the same height, but the stroke for the 265 is 0.480" shorter, so the rods have to be longer to make up for half the difference. These rods can be identified by the single, raised dot on both sides of the shank.
1992-í95 350 WITH ALUMINUM HEADS - The 1992 Corvette had a steel roller cam with a shallow hole in the snout that measured .450" in the front and tapered down to .240" at the bottom. It had a short dowel (.320") that was used to locate the timing gear and a hole with 16 splines in the center of the gear for the stub shaft that drove the early distributor. The 1993-Ď94 H.O. cam had a few subtle changes, but all of the early H.O. cams are the same for all intents and purposes. They can be identified by the number "241" stamped on the barrel in front of the first lobe.
1994-í96 350 WITH IRON HEADS - The distributor drive was changed on the iron-headed motors only in 1994, so the front of the cam and the timing gear were changed, too. The cam had a pilot hole that was bigger and deeper (0.500" x 1.0625") and it had a longer (.685") dowel pin that stuck out beyond the timing gear to drive the new distributor. This iron-headed motor was used in the Chevy Caprice, Buick Roadmaster and Cadillac Fleetwood, so it came with a milder cam that improved low end torque and reduced valve train noise. These cams have the long dowel pin and either "600" or "779" stamped on the barrel of the cam in front of the first lobe.
1995-í97 350 WITH ALUMINUM HEADS - In 1995, the aluminum-headed motors got the late, pin-drive distributor, so thereís a second version of the H.O. cam with the big pilot hole (.500" x 1.0625" ) and the long (.685" ) dowel pin. Look for a cam with the long pin and either "242" or "705" stamped on the barrel in front of the first lobe.
1994-í96 265-INCH MOTORS - All of the 265 engines came with the later, pin-drive distributor, so they all had the later style cam with the big pilot hole and the long dowel pin. The 265 used the same mild cam that came in the iron-headed LT1. Look for the long dowel pin and either "600" or "779" stamped on the barrel of the cam in front of the first lobe.
The cam gear had to match the cam and the distributor drive, so there were two different gears used, depending on the year and the application.
The original cam had a small, tapered hole in the center and a short dowel pin. It was used with the cam gear that had the small hole in the center with 16 splines in it. It was connected to the distributor with a short drive shaft that was splined on both ends. The cam gear is a GM p/n 10128349. This combination was used from 1992-Ď95 on the aluminum-headed motors.
GM had some problems with the early distributor due to both carbon tracking and moisture, so a new sealed distributor with a vacuum port was introduced on the iron-headed 265s and 350s in 1994 and used on all LT1s in 1995. The new distributor was located with a pilot shaft and driven by a pin, so both the cam and the gear were changed. The cam had a large, deep hole in the center for the pilot shaft and a longer dowel pin to drive the distributor.
The cam gear had a bigger hole and it didnít have the splines that were found in the early gear. The pilot shaft for the distributor extends through the hole in the cam gear and seats in the hole in the cam; the long dowel pin that sticks up through the cam gear drives the distributor. The cam gear is a GM p/n 10206039.
350 ALUMINUM HEADS
There were two versions of the aluminum heads used on the Corvettes, Camaros and Firebirds. The later ones have less material around the top of the intake ports and weigh about 2- 1/2 lbs. less than the earlier ones, but they are identical otherwise. Look for a 10128374 and possibly a 649.
The front covers have been changed three times, once because of the changes that were made to the distributor and once due to OBD II.
The original cover had three holes, one for the crank, a small hole (@ 0.70") for the water pump drive and a second one for the small drive shaft for the distributor. Itís a 10128289 casting.
The second front cover still had the small hole for the water pump shaft, but it had a much larger hole (@ 2.63") that sealed on the outside of the distributor housing itself. Itís a 10214196 casting.
The second cover was modified again in 1996 to accommodate the crank position sensor that was located in the lower corner of the cover on the passenger side. This same cover was used for the few engines that were installed in 1997, too. Itís a 12550032 casting.
There were two heads used on the 350, one aluminum and one cast iron, along with one cast iron head for the 265. GM claimed that the original LT1 aluminum head had a 15% increase in airflow as a result of revised port angles and higher port ceilings when compared to the 1991 L98 head. That was impressive in 1992, but the iron LT1 head that came out in 1994 was even better. It flowed 20% more on the intake side and made more horsepower on the dyno.
All LT1 heads used small combustion chambers to get the higher compression ratios with flat top pistons; the 350s had a 10.5 to 1ratio and the 265s had a 9.8 to 1 ratio.
350 IRON HEADS - All of the full-size cars came with iron heads. They were 10125320 or 12554290 castings.
265 IRON HEADS - The 265 had its own unique cast iron head with a 10208890 casting number. The chambers are smaller, so these heads cannot be interchanged with any of the 350 LT1 heads.
Thatís the story on the parts and pieces for the LT1. Chart 2 on page 35 shows how they all fit together year by year, but there are a few more things every rebuilder should know in order to avoid some possible problems.
The 350 LT1 head gaskets are not interchangeable with regular 350 Chevy head gaskets because they have different water passages due to the reverse cooling. The original head gaskets on the LT1 were wider and had holes that held the pushrods in place for assembly, but the replacement gaskets look a lot like the ones used on a regular 350. Make certain not to mix them up.
Use the correct key for the year of the engine. Donít try to get by with a regular 350 key; the hub will hit it before it bottoms on the crank gear and it will cause the balancer and belts to be misaligned.
The drive shaft for the early style distributor is reversible by design, but it may have a machined lead on it that will cause it to pump oil past the front cover seal if it is installed backward. Unfortunately, the shop installs it, so all you can do is wait for the phone call when the seal leaks and theyíre looking for someone to blame.
There is no keyway in the hub for the harmonic balancer, so GM used a special cutback key that was flush with the front of the timing gear on the 1992-í95 engines. Itís p/n 10128303.
When the crank position sensor was added in 1996 for OBD II, the cutback area on the key was shortened so it stuck out far enough (about 0.100") beyond the face of the timing gear to index the notched disc that was used for the crank position sensor.
EXTERNAL COOLANT LINES
There is an extra hole that goes into the water jacket on both ends of the heads. These should be plugged when theyíre in the front, but left open when theyíre on the back. Thereí s an external coolant transfer line that connects the holes on the backside to a reservoir that vents the air and steam vapors that would be trapped in the head and cause hot spots.
FRONT TAPPET CUP PLUGS
Both of the cup plugs in the front of the lifter galleries have a 0.030" hole drilled in them. These prevent air pockets from forming in the front of the galleries and provide added lubrication to the water pump gear drive.
WATER PUMP DRIVE SHAFT
Be sure to check the seal surface on the geared shaft that drives the water pump. If itís grooved, it will leak and it will be your fault. New ones are available from GM (p/n 10219554) for around $40. Thatís cheap insurance when you consider that the timing cover, chain and gears have to come off to replace it.
The LT1 powerplant uses the late model 350 oil pump with the 3/4" pickup tube. This should be fairly easy to identify.
HUB AND BALANCER
The balancer is a two-piece assembly with a pulley that bolts onto the hub. Separating the two made it easier to install the Optispark distributor on the assembly line and out in the field. The holes in the hub are offset, so the balancer only fits on it one way, but thereís no keyway in the hub to index the hub on the crank. This shouldnít be a problem unless the damper was drilled at the factory to "trim" the final engine assembly.
If it was drilled a lot to compensate for an engine that was out of balance, you could end up with a shaker, depending on how everything stacked up with the remanufactured engine compared to the original engine. If you encounter a balance problem on a remanufactured LT1, try rotating the balancer assembly on the crank 90į at a time to see if it eliminates the problem.
THE LT4 CHEVY
There is one more version of the LT1 out there that could cause some confusion if you get a core and donít know what it is. The LT4 was the high performance version of the LT1 that was standard in the 1996 Corvette "Grand Sport" and optional for any 1996 Corvette with a manual transmission. There were also 100 1997 SS Camaros built by SLP that came with the LT4. Chances are most rebuilders will never see one, but just in case you do, the following are a few things that make the LT4 special.
BLOCK - Should be the same LT1 casting with the four bolt mains.
CRANK - The LT4 has a special nodular iron crank with undercut and rolled fillets for added strength.
RODS - Powdered metal rods are used with a "cracked cap" instead of a machined parting line.
CAM - The LT4 cam was similar to the ones used in the high performance LT1, but it has had considerably more lift at the valve because it had 1.6 to 1 rockers.
HEADS - The heads had smaller 54cc chambers that gave the LT4 a 10.8 to 1 compression ratio. The intake ports were raised 0.100", too. The LT4 used larger intake and exhaust valves (2.00" x 1.55"). The intake stems were hollow to reduce weight and the exhaust stem was sodium-filled to improve heat transfer. The valve springs were made of oval wire to prevent coil bind with the higher valve lift and fitted with lightweight retainers to help the engine rev to its 6,300 rpm redline.
All of these changes enabled the LT4 to make 330 hp at 5,800 rpm instead of 300 hp at 5,000 rpm.
Thatís about all you need to know about the LT1 family. Theyíre out there and theyíre old enough to show up on your dock. Although the LT1 isnít just another Chevy 350, itís pretty straightforward when you know what to expect.
Doug Anderson is vice president of Grooms Engines, Parts, Machining, Inc., located in Nashville, TN. He has authored numerous technical articles on engine rebuilding for Automotive Rebuilder for more than 12 years. Anderson also writes Automotive Rebuilderís regular Shop Solutions column.