The Chevrolet LT1 5.7L V8 engine that was produced from 1992 to 1997has some significant differences compared to the previous small blockChevy it replaced, and the third generation LS1 small block that laterreplaced it. The most obvious difference that distinguishes the LT1from these other engines is the front-mounted Opti-Spark ignitionsystem. Other differences from the earlier small block Chevy include areverse-flow cooling system and mass airflow sequential fuel injectionsystem (though early LT1s were equipped with a speed-density multiportfuel 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), BuickRoadmaster and Cadillac Fleetwood (D-body).
The LT1 has a cast iron cylinder block with aluminum heads in theCorvette, Camaro and Firebird applications, but cast iron heads on theBuick, Caprice, Cadillac and Impala models. The Corvette block also hasstronger four-bolt main caps while the rest have two-bolt mains. Bothblocks 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 torquecurve. It also got better fuel economy, making it a worthy successor tothe long-lived small block Chevy. Horsepower ratings for the variousLT1 engines range from 260 hp to 300 hp depending on the modelapplication and year.
Though the LT1 has a different block and heads than the earliersmall block Chevy V8, the engine mounts and bellhousing configurationwere kept the same, allowing the newer Gen II engine to be swapped intoolder vehicles.
The crankshaft in the LT1 is similar to a small block 350 crank, andhas the same casting number: 14088526. But the LT1 crank is balanceddifferently to accommodate lighter pistons. Consequently, if you arerebuilding one of these engines with an exchange crankshaft, make surethe 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 smallerdisplacement 4.3L L99 (offered in the 1994-’96 Caprice for better fueleconomy. The block casting number on the smaller engine is 10168588 andit also has “4.3” cast on the side. The block has smaller 3.74”cylinder bores and a shorter 3” stroke crankshaft with longerconnecting rods.
There was also a more powerful 330 hp version dubbed “LT4” in the1996 Corvette that featured roller rockers and redesigned cylinderheads with smaller, higher compression combustion chambers, raisedports and better flow.
During its production run, the LT1 used several different camshafts.On 1992-’95 Corvette and Camaro engines with aluminum heads, the enginehad 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 splinesfor the distributor shaft. These cams have “241” stamped on them by thefirst lobe.
On 1994-’96 LT1 engines with cast iron heads, the pilot hole in thefront of the cam is larger and deeper, with a longer (0.685”) dowel pinto drive the revised Opti-Spark distributor. The cam also was a mildergrind with less lift and duration to improve low-end torque and fueleconomy. These cams have “600” or “779” stamped near the first lobe.
In 1995, the aluminum head LT1s also got the revised Opti-Sparkdistributor 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 pinfor the distributor drive, and used the milder “600” or “779” camshaft.
We’ve heard of some instances of LT1 valvetrain noise caused byroller lifters pitting and developing flat spots on some of theseengines. The underlying cause is usually too infrequent oil changes oran oil contamination problem.
If you’re replacing the timing chain and gear set on an LT1, beaware that two different cam gears were used depending on the versionof Opti-Spark distributor on the engine. The 1992-’95 aluminum headLT1s with the short dowel pin setup used a cam gear with a splinedcenter hole (P/N 10128349). Later versions used a different cam gear(P/N 10206039) with a larger hole and no splines for the revisedpin-drive distributor.
In addition to the camshaft and distributor changes, the frontengine covers were also different depending on which distributor wasused. In 1996, the front cover was again revised to accept a crankshaftposition sensor for OBD II misfire detection.
Reverse-Flow Cooling System
One reason why the newengine performed better was the reverse-flow cooling system. By routingthe coolant to the cylinder heads first and then the block, the enginecould handle a higher compression ratio and maintain more consistentcylinder head temperatures.
The water pump on these engines is driven by a small shaft off thecamshaft gear. This improved cooling reliability (no belt to slip orfail), but it also creates a potential leak path for oil. The waterpump driveshaft seal typically leaks once the engine has about 60,000miles on it. The fix is to replace the seal, which takes about twohours and requires a special tool (J39087) to install the seal. Thetool prevents the lips on the seal from deforming when the seal isinstalled. If you don’t use the tool, chances are the new seal willleak — and you’ll have to do the job over again!
The cooling system on the LT1 tends to trap air when you areattempting to refill it, so there’s a small bleeder screw on thethermostat housing to help vent air. But even this may not be enough,especially on the Camaro. If air gets trapped in the system, thetemperature sensor for the cooling fan may not be in contact with thecoolant, preventing the fan(s) from coming on, causing the engine tooverheat. To get the air out, you may have to raise the front of thevehicle up so the radiator becomes the highest point in the system. Addas much coolant as the system will take, then lower the vehicle, startthe engine, let it warm up, then shut it off. After the engine hascooled, recheck the coolant level and add more coolant as needed to topoff the system.
The front-mounted Opti-Sparkdistributor on the LT1 is also driven by the cam gear. Inside are twooptical sensors that read slits in a rotating disk to inform the PCMabout camshaft position and ignition timing. The PCM controls sparkadvance, while high voltage is provided by a single coil and ignitionmodule mounted by the front of the right cylinder head.
Early Opti-Spark systems on the 1992-’94 Corvette and 1993-’94 used anunvented cap that tended to trap moisture, resulting in internalcorrosion, arcing and misfiring. Later Opti-Spark systems used a ventedcap with two hose connections (vacuum and air vent) to keep moistureout of the cap.
GM technical service bulletin 87-65-27 offers a fix for Opti-Sparkmisfiring in the form of a distributor upgrade kit (P/N 10457293 forthe earlier applications, and P/N 10457735 for the later ones).
The distributor caps on these engines typically go bad after about60,000 miles, and the OEM plug wires aren’t the best either. So ifyou’re doing preventive maintenance on a high-mileage LT1, and the capand wires have never been replaced, new ones would be highlyrecommended.
Removing the distributor on these engines requires removing the airduct and serpentine belt, then the water pump, then the crankshaftpulley and damper, then the belt tensioner, and finally the three boltsthat attach the Opti-Spark distributor to the front timing cover. Whenyou pull the distributor out, note the position of the drive and markit so you can align the distributor drive with the cam gear correctlywhen the distributor goes back in.
Sequential Fuel Injection
In 1992 and 1993, theCorvette still used a speed-density multiport fuel injection systemthat gang-fired the injectors simultaneously. There was no airflowsensor, so fuel metering was based on engine rpm, temperature, throttleposition and MAP sensor inputs to the PCM. In 1994, GM upgraded to amass airflow system with sequential fuel injection. The result wasbetter performance, fuel economy and emissions.
The only TSBs GM has issued on the LT1 fuel injection system arethose dealing with keeping the fuel injectors clean by using Top Tiergasolines (which contain adequate levels of detergent), andrecommendations for cleaning dirty fuel injectors.
Some LT1 injectors have had failure problems due to internalcorrosion, which some have blamed on gasoline mixtures that contain 10%ethanol. The injector coils short out internally and cause a lean fuelcondition and/or misfires in one or more cylinders. On 1996 and laterOBD II models with misfire detection, this should set one or more P030Xcylinder misfire codes depending on how many injectors are misbehaving.
Injector resistance can be tested with an ohmmeter, and should read11.8 to 12.6 ohms. If the injector reads out of specs, it should bereplaced.
Mass airflow (MAF) sensor problems on these engines typically causelean fuel conditions and possibly hard starting or even no starts. Ifthe sensor element becomes contaminated with dirt, fuel varnish or oil(from oiled aftermarket low restriction air filters), the sensor won’tread accurately. MAF sensors can often be cleaned with aerosolelectronics cleaner.
A good MAF sensor should read about 6 to 9 grams/second when the MAFPID 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 readsout of range, it probably needs to be replaced (which costs about $300list price!).
Typical fault codes that may be set by a bad MAF sensor includeP0100 (circuit malfunction), P0101 (system performance), P0102(frequency low) or P0103 (frequency high).
Bad Knock Sensor
Another problem that may beencountered on these engines is detonation or spark knock whenaccelerating under load. Detonation can be caused by a variety ofthings, including engine overheating, loss of EGR, low octane fuel or abuildup of carbon in the combustion chambers. But another possibilityis a fouled rear knock sensor (see GM TSB 02-06-04-023A). The knocksensor sits in a cavity in the back of the engine. Water can get intothe cavity (often because somebody power washed the engine), causing itto fail. The fault may set a code P0332. The fix is to replace thecorroded knock sensor with a new one, and to build a dam around thecavity 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 withrolled and undercut fillets. 1996-1997 versions have a powdered-metalcrankshaft position sensor. This reluctor ring has four timing teethand is aligned on the crank via the keyway.
A two-piece damper and hub assembly is used on Gen II engines. The hubadapter is installed on the crank snout with an interference fit. Thecrank snout key does not engage the hub adapter. It has a notch toclear the adapter. From 1992 to 1995, the key was notched back flushwith the crank sprocket, but with the advent of OBD II enginemanagement in 1996-1997, the key protrudes farther in order to locatethe crankshaft position sensor reluctor wheel. The hub adapter issecured with a 7/16 number 20 bolt and a dedicated flat washer. Thecombination damper/crank pulley then bolts to the hub assembly withthree 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 smallblocks in one volume. This book continues to be the best power sourcebook for small-block Chevys. The detailed text and photos deliver thesolutions for making your engine perform. Extensive chapters explainproven techniques for preparing blocks, crankshafts, connecting rods,pistons, cylinder heads and much more. Other chapters include popularignition, carburetor, camshaft, and valvetrain tips and tricks. Onechapter focuses exclusively on service and building tips for the LT1and LT4 engines.
To order, contact CarTech Inc., at 1-800-551-4754 or visit www.cartechbooks.com. A $4.95 shipping and handling fee is added to each order.
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