Differences between LT1 and LT4 ('96 Corvette)

Parts: LT1 Description LT1 Pt. # LT4 Description LT4 Pt. #
Camshaft See specs below 12551705 See specs below 12551142
Caps, Main bearing     Nodular Iron  
Cap, (Retainer) valve spring       10212808
Connecting rods     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
Fuel rail L/R   17095149 Larger? ?
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
Seal, Crankshaft     Teflon  
Spring, Valve 85 lbs. Seated 10206040 100 lbs. Seated -260 lbs. open 12551483
Sprocket, Cam Webbed 10206039 Solid (Add Mass) 12555885
Sprocket, Crank   10128346   12555886
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
Knock Sensor ?   ? ?
Resultant Changes:        
Elimination of EGR     Overlap leaves EGR redundant  
Compression ratio 10.4:1   10.8:1  
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
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 Intake 203 deg. LT4   115 deg.
  Exhaust 210 deg.  
Exhaust valve lift is .479" inch (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 recalculation. The overlap area amounts to internal EGR.
-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 & potassium 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, elliptical 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. 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. (Enshrouding valve). The Intake port volume has increased by 25cc. Increased from 170cc from 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. "enshrouding 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 accommodate 7/16" studs but the LT4 has larger 7/16 rocker studs. (top threads) The LT1 studs are 3/8" on top and 7/16" bottom. pt#3921912 for 7/16" and pt#1255216 for 3/8" LT1
-Head Gasket
* 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."
-Fuel System
* 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.

manifold1.jpg (16677 bytes)

-Intake Manifold
* 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 comer 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 comer 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 4 bolt
* Stronger
-Teflon rear crank seal
* Improves durability.
-Resultant changes
* Higher redline. The LT4's is at 6300 rpm compared to the LT1's 5700 rpm

IntakeA.jpg (11220 bytes)

Intake2.jpg (11151 bytes)

Here is a cross section comparison of the LT1 (photo A) and the LT4 (photo B) heads. The LT4 Intake port is 15cc 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)

exhaustA.jpg (10745 bytes)

ExhaustB.jpg (10588 bytes)

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 7/16" top thread on studs vs. the 3/8" on the LT1.

lt4asm.jpg (13315 bytes)

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 valve train upgrades. The cam has more lift, duration and valve overlap.
Periodical references -Chevy High performance March '99 pg.114, Chevy High Performance October '95 Pg. 20, Vette October '95 pg. 40, Corvette Fever November '95 pg. 20, Corvette Fever October '96 pg. 20, Corvette Fever March '99 pg. 48.