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LT1 vs LT4 ... What are the Differences?

While sharing the same bore and stroke, there is a LOT more to the 1996 LT4 engine than just a red intake manifold! What follows is a detailed listing explaining the technical differences between these two Corvette powerplants. What cannot be shown here is the "seat-of-the-pants" difference only a drive in an LT4 equipped Corvette can provide.

The LT4's horsepower increase and reliability at higher RPM's (compared to a standard LT1) is largely achieved through the following differences:

New camshaft design
Larger valves
Roller rocker arms
Revised Head design
Head Gaskets
Revised Piston Design
Positive-twist top piston rings
Fuel System
Intake Manifold
Roller timing chain
Specially machined crankshaft
Dual-mass front torsional damper
Nodular iron main bearing caps
Teflon rear crank seal

LT1 Engine

LT4 Engine

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BASIC LT1 and LT4 SPECIFICATIONS

Specification LT1 LT4
Block, construction Cast Iron Cast Iron
Bore, inches 4.00 4.00
Compression ratio 10.4:1 10.8:1
Connecting Rods Powdered metal Powdered metal
Crankshaft Cast iron Cast iron with undercut and rolled fillets
Cylinder Heads Cast Aluminum, 2 valves/cylinder Cast Aluminum, 2 valves/cylinder
Displacement 350ci (5733cc) 350ci (5733cc)
Fuel system Fuel Injection, Electronic sequential port Fuel Injection, Electronic sequential port
Horsepower, factory rating 300 @ 5000 RPM 330 @ 5800 RPM
Main Bearings 4-bolt centers, 2-bolt ends 4-bolt centers, 2-bolt ends
Pistons Eutectic aluminum/silicon alloy Eutectic aluminum/silicon alloy
Redline, factory rating 5700 RPM 6300 RPM
Rocker Arms Conventional, stamped steel, 1.5:1 ratio Roller, forged aluminum, 1.6:1 ratio
Stroke, inches 3.48 3.48
Timing Chain Link chain, powdered-metal sprockets Roller chain, steel sprockets
Torque, factory rating 340 @ 4000 RPM 340 @ 4500 RPM
Torsional Damper Single mass Dual mass
Valve Diameter, inches 1.94 intake, 1.50 exhaust 2.00 intake, 1.55 exhaust
Valve Duration 279 intake, 276 exhaust 282 intake, 277 exhaust
Valve Lift, degrees .447 intake, .459 exhaust .476 intake, .479 exhaust
Valve Overlap, degrees 41 46
Valve Springs Round Section Oval section, higher closing force

LT1 and LT4 COMPONENTS & PART NUMBERS

Parts: LT1 Description Part # LT4 Description Part #
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
EGR ? Overlap leaves EGR redundant ?
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
Knock Module 16177700 Less Sensitive to engine noise 16214681
Knock Sensor 10456287 10456287
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
Seal, Crankshaft 10206040 Teflon 10206040
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
Sprocket, Crank 10128346 12555886
Throttle Body Twin port, 48mm ? Twin port, 48mm ?
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

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NEW CAMSHAFT DESIGN...

The LT4's more aggressive camshaft profile compliments all the valvetrain upgrades. The cam has more lift, duration and valve overlap. 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 the intake is .476" (12.09mm) (net is less .004 (0.10mm) inch lash), up from the LT1's .447" (11.35mm). Duration is also increased slightly on both exhaust and intake... 203 deg. Intake and 210 deg. Exhaust with 115 deg. lobe separation and 1 degree of retard. By increasing overlap significantly, it eliminated the need for an external exhaust gas recirculation (EGR) device.

CAMSHAFT SPECIFICATIONS ('96 model year)
(LT1 w/1.5:1 Ratio rockers and LT4 w/1.6:1 ratio rockers)

Engine Valve Lift @ .050"(1.3mm) Specification Engine Lobe Lift @ .050"(1.3mm) Specification
LT1 Intake .447"(11.35mm) LT1 Intake .298"(7.57mm)
Exhaust .459"(11.66mm) Exhaust .306"(7.77mm)
LT4 Intake .476"(12.09mm) LT4 Intake .298"(7.57mm)
Exhaust .479"(12.17mm) Exhaust .299"(7.60mm)
Engine Duration @ .050"(1.27mm) Specification Engine Lobe Separation Specification
LT1 Intake 205 deg. LT1 117 deg.
Exhaust 207 deg. LT4 115 deg.
LT4 Intake 203 deg.
Exhaust 210 deg.

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LARGER VALVES...

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.

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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.

The LT4's new valve springs are made from egg shaped wire for higher seat pressure and improved valve dynamics at higher RPM.

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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 roof was slightly lowered and the walls were moved back slightly from the valves for better breathing, undercutting next to the exhaust valve thereby "unshrouding the valves." Revised port contours enhance air flow into and out of the engine resulting in increased fuel burning efficiency.

Combustion chambers are almost identical with a negligible .4cc increase in volume for the LT4 Combustion chambers at 54.4cc's.

Both the LT1 and the LT4 Heads accomodate 7/16" studs but the LT4 has larger 7/16 rocker studs. There are some reports of early LT4's with 10mm top threads. The LT1 studs are 3/8" on top and 7/16" bottom. GM part numbers are #3921912 for 7/16" LT4 and #1255216 for 3/8" LT1.

Shown below 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. Intake valve sizes are 1.94" on the LT1 and 2.00" on the LT4.

In the photo below, 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. Exhaust valve sizes are 1.50" (38.1mm) on the LT1 and 1.55" (39.4mm) on the LT4. Also notice the LT4's 7/16" top thread on studs vs. the 3/8" on the LT1.

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HEAD GASKETS

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.

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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.

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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."

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FUEL SYSTEM

The engine's higher rpm also exceeded the LT1's fuel injector's ability to keep up, so the LT4 got larger fuel injectors rated at 3.5 grams per second (28lbs/hr), replacing the 3.0 gram (24lbs/hr) injectors of the LT1. The larger injectors were designed to keep pace with the better breathing, higher revving engine.

The 48mm throttle body remained unchanged from the LT1, however all LT4 equipped Corvettes (not just Z16 Grand Sports) featured a throttle body top cover plate with the words "Grand Sport" in red letters.

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INTAKE MANIFOLD

A specific intake manifold was cast for use with LT4 heads. Although the manifold and head intake ports do not precisely match, the manifold was cast with additional material to allow safe machining of the ports to more closely match the LT4's higher head ports. LT4 intakes were powder coated red to differentiate them from the standard LT1 manifold.

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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. Increased inertia in the system reduces the tendency for cam torsion.

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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.

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DUAL-MASS FRONT TORSIONAL DAMPER

Tuned for high engine speed, also helps reduce stress on the crankshaft. LT1 pt.# 10128489 and LT4 pt.# 12551486

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NODULAR IRON MAIN BEARING CAPS

These LT4 components are considerably stronger than their LT1 counterparts and resist failure at higher RPM's.

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TEFLON REAR CRANK SEAL

Offers improved durability over the standard LT1 crank seal.

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References:
GM Assembly Manual (LT4)
Chevy High Performance magazine March '99
Chevy High Performance magazine October '95
Vette magazine October '95
Corvette Fever magazine November '95
Corvette Fever magazine October '96
Corvette Fever magazine March '99
The information presented here is believed to be accurate but the authors and the Grand Sport Registry assume no responsibility for errors. If you have evidence to the contrary on any of the above, or have worthwhile additions to increase the value of this information, please submit to info@grandsportregistry.com.

Many thanks to "Steve" as the original compiler of much of the above information.

Contents ©2003
Grand Sport Registry
All Rights Reserved
Copyright/Trademark Info

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Phil

[This message has been edited by GTDude (edited 11-24-2004).]