Differences between LT1 and LT4 ('96 Corvette GS & CE)
|Parts:||LT1 Description||LT1 Pt. #||LT4 Description||LT4 Pt. #|
|Camshaft||See specs below||12551705||See specs below||12551142|
|Cap, (Retainer) valve spring||10212802||10212808|
|Connecting rods||Powdered Metal||10108688||Powdered Metal||10108688|
|Chain, Timing||Powdered metal butt link||10128485||Single Roller (made for GM by Cloyes) Steel||12555887|
|Crankshaft||12556307||Nodular Iron (Additional Machining)||12551485|
|Gasket, Head||10168457||Composite w/ steel rings||12551488|
|Gasket, Intake||12524653||for higher ports||12528884|
|Head (L/R Same)||212 CFM flow||10168448||240 CFM flow Higher ports||12555332|
|Injectors||3.0gps 24lbs/hr||17124248||3.5gps 28lbs/hr||17124251|
|Intake Manifold||12552137||Higher ports 0.10" (2.5mm) than LT1||12550630|
|Pistons||10159437||Shallower valve pockets than LT1||12551526|
|Rings, Piston||Positive twist||12528817|
|Rocker arms||1.5:1 Ratio Stamped steel||10089648||1.6:1 Ratio Aluminum Roller||12557779|
|Shim/spacer under spring||10212809|
|Spring, Valve||85 lbs. (38.6 kg.) Seated||10206040||100 lbs. (45.4 kg.) Seated -260 lbs. (117.9 kg.) open||12551483|
|Sprocket, Cam||Webbed||10206039||Solid (Add Mass)||12555885|
|Valve, Exhaust||1.50" (38.1mm) Solid stem Weight:95 grams, 3.35 ounces||12550909||1.55" (39.4mm) Sodium filled stem Weight:75 grams, 2.65 ounces||12551313|
|Valve, Intake||1.94" (49.3mm) Solid stem Weight:110 grams, 3.88 ounces||10241743||2.00" (50.8mm) Hollow stem Weight:85 grams, 3.00 ounces||12555331|
|Knock Module||16177700||Less Sensitive to engine noise||16214681|
|Elimination of EGR||Overlap leaves EGR redundant|
Horsepower increase and reliability at higher RPM's (compared to a standard LT1) is achieved through
-New camshaft design. '96 model year specs.
-LT1 and LT4 camshaft specifications. Note: LT1 with 1.5:1 Ratio rockers and LT4 with 1.6:1 ratio rockers.
|Engine||Valve Lift @ .050"(1.3mm)||Specification||Engine||Lobe Lift @ .050"(1.3mm)||Specification|
|Engine||Duration @ .050"(1.27mm)||Specification||Engine||Lobe Seperation||Specification|
|LT1||Intake||205 deg.||LT1||117 deg.|
|Exhaust||207 deg.||LT4||115 deg.|
Exhaust valve lift is .479" (12.17mm) (net is less .006" inch (0.15mm) lash), @ .050 (1.27mm) Lift, up from .459" (11.66mm) for the LT1, and for intake is .476" (12.09mm) (net is less .004 (0.10mm) inch lash), up from the LT1's .447" (11.35mm). Duration is increased slightly on both exhaust and intake. 203 deg. Intake, and 210 deg. Exhaust. 115 deg. lobe centers. 1 deg. Retard. Increasing overlap significantly, allowing elimination of external exhaust gas recirculation. The overlap area amounts to internal EGR.
LT4 valves are 2.00-inch (50.8mm) Intake and 1.55-inch (39.4mm) Exhaust valves (as opposed to the 1.94 (49.3mm)/1.50-inch (38.1mm) combination used in the LT1) were made lighter to reduce inertial stress, both intake and exhaust valves are hollow and the latter sodium & potasium filled to improve heat transfer. Valve springs are stronger, with a higher installed seat pressure (100 lbs.in. for the LT4 vs. 85 lbs. for the LT1) and wide open pressure (260 lbs. on the LT4) to keep the valves following the camshaft's contours. Special valve springs, eliptical shaped instead of circular in cross-section (Larger horizontal cross section than vertical), allow a spring with a higher free height to be compressed more for higher operating pressures. LT4 Intake valves weight 85 grams. LT1 Intake valves weight 110 grams. LT4 Exhaust valves weight 75 grams. LT1 Exhaust valves weight 95 grams.
-Roller rocker arms
The LT1's stamped steel rocker arms exhibited ball galling at 6400 rpm, so modified Crane roller rockers were substituted in the LT4 with lock nut and set screws to replace the conventional ball and friction nuts. Shims were added to maintain the valve stem accurately and the stud slot widened. The roller axle reduces friction measurably, the 2 lbs.-ft reduction increasing economy and responsiveness. The rocker arms also have a roller tip. The LT4 has 1.6:1 and LT1 has 1.5:1 Rocker Ratio.
-Revised Head design
The big difference in the heads is the intake and exhaust ports. Both are larger with bigger radius bends. The "short side" radius of the LT4's exhaust port in particular has been increased to reduce back pressure at higher RPM,s. A throat cut just past the exhaust valve seat also opens up the exhaust port. (Unshrouding valve). The Intake port volume has increasd by 25cc. Increased from 170cc on the LT1 to 195cc for the LT4. The flow through the heads have been increased from 212CFM (Cubic feet per minute) @ 28" (711mm) (LT1) to 240CFM. (LT4).
The walls were moved back slightly from the valves for better breathing, undercutting next to the exhaust valve. "unshrouding the valves" "the roof was lowered a tad" Revised port contours enhance air flow into and out of the engine and the revised combustion chambers increase fuel burning efficiency.
Combustion chambers are almost identical (there was a negligible .4cc difference) Volume for LT4 Combustion chambers are 54.4cc's.
Both the LT1 and the LT4 Heads accomidate 7/16" studs but the LT4 has larger 10mm rocker studs. (top threads) The LT1 studs are 3/8" on top and 7/16" bottom
The LT4 also features new composite head gaskets necessitated by higher compression ratio. Rather than the "impregnated surface" of the LT1's gaskets, the LT4's are "graphoil" with stainless steel fire rings.
-Revised Piston Design
A gain in compression ratio, from 10.4:1 to 10.8:1, comes from machining the valve pockets in the pistons shallower than the LT1's.
-Positive-twist top piston rings
A new top compression ring. High-speed durability tests of the engine revealed that ring flutter became a serious problem above 5,500 rpm, and the result was excessive blow-by. To cure this, something called a positive twist compression ring was specified. A chamfer cut into this type of ring's inside edge causes it to flex down in response to the introduction of cylinder gas pressure, leading to better sealing at higher rpm. "If you look closely at the inside edge of the ring, you can see that there's a chamfer cut on it, and by changing the cross-sectional area on that inside-and the properties associated with it was the end gap of the ring comes together, the ring actually takes on a slight positive twist. It is no longer perpendicular to the bore." But when cylinder gas pressure is introduced, the ring flexes down and seals better against the bore. "If you start with a ring that's flat, when you flex, you end up with a line seal, This way you end up sealing across a broader area."
The engine's higher rpm also exceeded the LT1's fuel injector's ability to keep up, so the LT4 got a new fuel rail with 3.5 gram per second (28lbs/hr) injectors, replacing the 3.0 gram (24lbs/hr) injectors of the LT1. Revised fuel-injection system featuring a new fuel rail designed to keep pace with the better breathing, higher revving engine.
A specific intake manifold was created to match the LT4's higher port heads. They were powder coated red to differentiate from the LT1.
Components designed to improve durability.
-Roller timing chain
Stock LT1 has powdered metal butt link chain drive for the camshaft and water Pump. LT4 has smaller but stronger steel roller chain. Although it seems that the new chain and sprockets would reduce mass, The cam sprocket was left solid, rather than webbed, specifically for additional mass. Increase inertia in the system reduces the tendency for cam torsion.
-Specially machined crankshaft
The crank itself was improved via something called undercutting and rolling. Undercutting is the machining of a groove into the corner of a journal. Though this would seem to weaken the crank at that point, it actually makes it stronger where tensile stress is most likely to cause sudden failure. Rolling the undercut introduces compressive stress to the area. This makes it more durable, because potentially catastrophic tensile stress generated during high RPM operation must first overcome the compressive stress before that area of the crank is subjected to tension. Undercutting, a groove cut into the corner of a journal, looks counterintuitive, but makes a crankshaft stronger where tensile stress is most likely to cause failure. By undercutting and then rolling it, compressive stress is introduced. Tensile stress during high-speed operation first has to overcome the compressive stress before there's any tension on the joint. And as tensile stress is what breaks crankshafts, undercutting and rolling make the crankshaft stronger.
-Dual-mass front torsional damper.
Tuned for high engine speed, also helps reduce stress on the crankshaft. LT1 pt.# 10128489 and LT4 pt.# 12551486
-Nodular iron main bearing caps
-Teflon rear crank seal
Higher redline. The LT4's is at 6300 rpm compared to the LT1's 5700 rpm
Here is a cross section comparison of the LT1 (photo A) and the LT4 (photo B) heads. The LT4 Intake port is 25cc larger than the LT1. The revised rocker cover rail is due to the taller intake port opening. Valve sizes: (1.94" LT1) (2.00" LT4)
Note the differences in the exhaust ports on the LT1 (photo A) and the LT4 (photo B). The short side-radius offer much smoother flow and the throat cut is slightly different. Valve sizes: (1.50" (38.1mm) LT1) (1.55" (39.4mm) LT4) Also notice the LT4's 10mm top thread on studs vs. the 3/8" on the LT1.
The LT4 rockers are slightly modified Crane rollers. New valve springs are made from egg shaped wire for higher seat pressure and improved valve dynamics at higher RPM. A more aggressive camshaft profile compliments all the valvetrain upgrades. The cam has more lift, duration and valve overlap.
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For more info on the LT4 and registery check out LT4.com
Information on modifying LT4s
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