GM Chevy LT1 Engine and Reverse-Flow Technology

Chevy LT1 Engine and Reverse-Flow Technology

The Chevrolet LT1 5.7L V8 engine that was produced from 1992 to 1997
has some significant differences compared to the previous small block
Chevy it replaced, and the third generation LS1 small block that later
replaced it. The most obvious difference that distinguishes the LT1
from these other engines is the front-mounted Opti-Spark ignition
system. Other differences from the earlier small block Chevy include a
reverse-flow cooling system and mass airflow sequential fuel injection
system (though early LT1s were equipped with a speed-density multiport
fuel injection system).

The LT1 engine was used in a variety of General Motors models,
including 1992-’96 Corvette (Y-body), 1993-’97 Camaro and Firebird
(F-body), and 1994-’96 Chevy Caprice and Impala (B-body), Buick
Roadmaster and Cadillac Fleetwood (D-body).

LT1 Basics

The LT1 has a cast iron cylinder block with aluminum heads in the
Corvette, Camaro and Firebird applications, but cast iron heads on the
Buick, Caprice, Cadillac and Impala models. The Corvette block also has
stronger four-bolt main caps while the rest have two-bolt mains. Both
blocks have the same 10125327 casting number.

Compared to the 1991 Chevy 350 L98 engine with Tuned Port Injection,
the 1992 LT1 produced 20% more horsepower and a much broader torque
curve. It also got better fuel economy, making it a worthy successor to
the long-lived small block Chevy. Horsepower ratings for the various
LT1 engines range from 260 hp to 300 hp depending on the model
application and year.

Though the LT1 has a different block and heads than the earlier
small block Chevy V8, the engine mounts and bellhousing configuration
were kept the same, allowing the newer Gen II engine to be swapped into
older vehicles.

The crankshaft in the LT1 is similar to a small block 350 crank, and
has the same casting number: 14088526. But the LT1 crank is balanced
differently to accommodate lighter pistons. Consequently, if you are
rebuilding one of these engines with an exchange crankshaft, make sure
the crankshaft is the correct one for an LT1, not a 350.

The connecting rods in the 1992-’94 LT1 engines are forged steel, while those in the 1995 and up engines are powdered metal.

Several different versions of the LT1 were made, including a smaller
displacement 4.3L L99 (offered in the 1994-’96 Caprice for better fuel
economy. The block casting number on the smaller engine is 10168588 and
it also has “4.3” cast on the side. The block has smaller 3.74”
cylinder bores and a shorter 3” stroke crankshaft with longer
connecting rods.

There was also a more powerful 330 hp version dubbed “LT4” in the
1996 Corvette that featured roller rockers and redesigned cylinder
heads with smaller, higher compression combustion chambers, raised
ports and better flow.

During its production run, the LT1 used several different camshafts.
On 1992-’95 Corvette and Camaro engines with aluminum heads, the engine
had a steel roller cam with a hole in the front of the cam and a short
(0.320”) dowel to locate the timing gear, and a hole with 16 splines
for the distributor shaft. These cams have “241” stamped on them by the
first lobe.

On 1994-’96 LT1 engines with cast iron heads, the pilot hole in the
front of the cam is larger and deeper, with a longer (0.685”) dowel pin
to drive the revised Opti-Spark distributor. The cam also was a milder
grind with less lift and duration to improve low-end torque and fuel
economy. These cams have “600” or “779” stamped near the first lobe.

In 1995, the aluminum head LT1s also got the revised Opti-Spark
distributor and camshaft with the longer drive pin. These are marked
“242” or “705.”

The smaller 4.3L L99 engines all had cams with the longer dowel pin
for the distributor drive, and used the milder “600” or “779” camshaft.

We’ve heard of some instances of LT1 valvetrain noise caused by
roller lifters pitting and developing flat spots on some of these
engines. The underlying cause is usually too infrequent oil changes or
an oil contamination problem.

If you’re replacing the timing chain and gear set on an LT1, be
aware that two different cam gears were used depending on the version
of Opti-Spark distributor on the engine. The 1992-’95 aluminum head
LT1s with the short dowel pin setup used a cam gear with a splined
center hole (P/N 10128349). Later versions used a different cam gear
(P/N 10206039) with a larger hole and no splines for the revised
pin-drive distributor.

In addition to the camshaft and distributor changes, the front
engine covers were also different depending on which distributor was
used. In 1996, the front cover was again revised to accept a crankshaft
position sensor for OBD II misfire detection.

Reverse-Flow Cooling System

One reason why the new
engine performed better was the reverse-flow cooling system. By routing
the coolant to the cylinder heads first and then the block, the engine
could handle a higher compression ratio and maintain more consistent
cylinder head temperatures.

The water pump on these engines is driven by a small shaft off the
camshaft gear. This improved cooling reliability (no belt to slip or
fail), but it also creates a potential leak path for oil. The water
pump driveshaft seal typically leaks once the engine has about 60,000
miles on it. The fix is to replace the seal, which takes about two
hours and requires a special tool (J39087) to install the seal. The
tool prevents the lips on the seal from deforming when the seal is
installed. If you don’t use the tool, chances are the new seal will
leak — and you’ll have to do the job over again!

The cooling system on the LT1 tends to trap air when you are
attempting to refill it, so there’s a small bleeder screw on the
thermostat housing to help vent air. But even this may not be enough,
especially on the Camaro. If air gets trapped in the system, the
temperature sensor for the cooling fan may not be in contact with the
coolant, preventing the fan(s) from coming on, causing the engine to
overheat. To get the air out, you may have to raise the front of the
vehicle up so the radiator becomes the highest point in the system. Add
as much coolant as the system will take, then lower the vehicle, start
the engine, let it warm up, then shut it off. After the engine has
cooled, recheck the coolant level and add more coolant as needed to top
off the system.

Opti-Spark Ignition

The front-mounted Opti-Spark
distributor on the LT1 is also driven by the cam gear. Inside are two
optical sensors that read slits in a rotating disk to inform the PCM
about camshaft position and ignition timing. The PCM controls spark
advance, while high voltage is provided by a single coil and ignition
module mounted by the front of the right cylinder head.

Early Opti-Spark systems on the 1992-’94 Corvette and 1993-’94 used an
unvented cap that tended to trap moisture, resulting in internal
corrosion, arcing and misfiring. Later Opti-Spark systems used a vented
cap with two hose connections (vacuum and air vent) to keep moisture
out of the cap.

GM technical service bulletin 87-65-27 offers a fix for Opti-Spark
misfiring in the form of a distributor upgrade kit (P/N 10457293 for
the earlier applications, and P/N 10457735 for the later ones).

The distributor caps on these engines typically go bad after about
60,000 miles, and the OEM plug wires aren’t the best either. So if
you’re doing preventive maintenance on a high-mileage LT1, and the cap
and wires have never been replaced, new ones would be highly

Removing the distributor on these engines requires removing the air
duct and serpentine belt, then the water pump, then the crankshaft
pulley and damper, then the belt tensioner, and finally the three bolts
that attach the Opti-Spark distributor to the front timing cover. When
you pull the distributor out, note the position of the drive and mark
it so you can align the distributor drive with the cam gear correctly
when the distributor goes back in.

Sequential Fuel Injection

In 1992 and 1993, the
Corvette still used a speed-density multiport fuel injection system
that gang-fired the injectors simultaneously. There was no airflow
sensor, so fuel metering was based on engine rpm, temperature, throttle
position and MAP sensor inputs to the PCM. In 1994, GM upgraded to a
mass airflow system with sequential fuel injection. The result was
better performance, fuel economy and emissions.

The only TSBs GM has issued on the LT1 fuel injection system are
those dealing with keeping the fuel injectors clean by using Top Tier
gasolines (which contain adequate levels of detergent), and
recommendations for cleaning dirty fuel injectors.

Some LT1 injectors have had failure problems due to internal
corrosion, which some have blamed on gasoline mixtures that contain 10%
ethanol. The injector coils short out internally and cause a lean fuel
condition and/or misfires in one or more cylinders. On 1996 and later
OBD II models with misfire detection, this should set one or more P030X
cylinder misfire codes depending on how many injectors are misbehaving.

Injector resistance can be tested with an ohmmeter, and should read
11.8 to 12.6 ohms. If the injector reads out of specs, it should be

Mass airflow (MAF) sensor problems on these engines typically cause
lean fuel conditions and possibly hard starting or even no starts. If
the sensor element becomes contaminated with dirt, fuel varnish or oil
(from oiled aftermarket low restriction air filters), the sensor won’t
read accurately. MAF sensors can often be cleaned with aerosol
electronics cleaner.

A good MAF sensor should read about 6 to 9 grams/second when the MAF
PID is viewed on a scan tool. If the sensor is reading out of range,
check for air leaks and/or try cleaning the sensor. If it still reads
out of range, it probably needs to be replaced (which costs about $300
list price!).

Typical fault codes that may be set by a bad MAF sensor include
P0100 (circuit malfunction), P0101 (system performance), P0102
(frequency low) or P0103 (frequency high).

Bad Knock Sensor

Another problem that may be
encountered on these engines is detonation or spark knock when
accelerating under load. Detonation can be caused by a variety of
things, including engine overheating, loss of EGR, low octane fuel or a
buildup of carbon in the combustion chambers. But another possibility
is a fouled rear knock sensor (see GM TSB 02-06-04-023A). The knock
sensor sits in a cavity in the back of the engine. Water can get into
the cavity (often because somebody power washed the engine), causing it
to fail. The fault may set a code P0332. The fix is to replace the
corroded knock sensor with a new one, and to build a dam around the
cavity with RTV to help keep water out.

Cranking Out Power

Gen II crankshafts are all nodular iron with one-piece rear seals.
LT1 cranks have radiused fillets while the LT4 cranks are prepped with
rolled and undercut fillets. 1996-1997 versions have a powdered-metal
crankshaft position sensor. This reluctor ring has four timing teeth
and is aligned on the crank via the keyway.


A two-piece damper and hub assembly is used on Gen II engines. The hub
adapter is installed on the crank snout with an interference fit. The
crank snout key does not engage the hub adapter. It has a notch to
clear the adapter. From 1992 to 1995, the key was notched back flush
with the crank sprocket, but with the advent of OBD II engine
management in 1996-1997, the key protrudes farther in order to locate
the crankshaft position sensor reluctor wheel. The hub adapter is
secured with a 7/16 number 20 bolt and a dedicated flat washer. The
combination damper/crank pulley then bolts to the hub assembly with
three bolts.

— From the book: Small-Block Chevy Performance 1955-1996 by John Baechtel.

Originally published as two separate volumes, Small Block Chevy Performance 1955-1996
covers the latest information on all Gen I and Gen II Chevy small
blocks in one volume. This book continues to be the best power source
book for small-block Chevys. The detailed text and photos deliver the
solutions for making your engine perform. Extensive chapters explain
proven techniques for preparing blocks, crankshafts, connecting rods,
pistons, cylinder heads and much more. Other chapters include popular
ignition, carburetor, camshaft, and valvetrain tips and tricks. One
chapter focuses exclusively on service and building tips for the LT1
and LT4 engines.

To order, contact CarTech Inc., at 1-800-551-4754 or visit A $4.95 shipping and handling fee is added to each order.

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