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:
BASIC LT1 and LT4 SPECIFICATIONS
||Cast iron with undercut and rolled fillets|
||Cast Aluminum, 2 valves/cylinder
||Cast Aluminum, 2 valves/cylinder|
||Fuel Injection, Electronic sequential port
||Fuel Injection, Electronic sequential port|
|Horsepower, factory rating
||300 @ 5000 RPM
||330 @ 5800 RPM|
||4-bolt centers, 2-bolt ends
||4-bolt centers, 2-bolt ends|
||Eutectic aluminum/silicon alloy
||Eutectic aluminum/silicon alloy|
|Redline, factory rating
||Conventional, stamped steel, 1.5:1 ratio
||Roller, forged aluminum, 1.6:1 ratio|
||Link chain, powdered-metal sprockets
||Roller chain, steel sprockets|
|Torque, factory rating
||340 @ 4000 RPM
||340 @ 4500 RPM|
|Valve Diameter, inches
||1.94 intake, 1.50 exhaust
||2.00 intake, 1.55 exhaust|
|Valve Duration, degrees
||279 intake, 276 exhaust
||282 intake, 277 exhaust|
|Valve Lift, inches
||.447 intake, .459 exhaust
||.476 intake, .479 exhaust|
|Valve Overlap, degrees
||Oval section, higher closing force|
LT1 and LT4 COMPONENTS & PART NUMBERS
||Same as LT1 belt
||See specs below
||See specs below
|Cap, (Retainer) valve spring
||Powdered metal butt link
||Single Roller (made for GM by Cloyes) Steel
||Nodular Iron (Additional Machining)
||Overlap leaves EGR redundant
||Composite w/ steel rings
||for higher ports
|Head (L/R Same)
||212 CFM flow
||240 CFM flow Higher ports
||Higher ports 0.10" (2.5mm) than LT1
||Less Sensitive to engine noise
||Shallower valve pockets than LT1
||1.5:1 Ratio Stamped steel
||1.6:1 Ratio Aluminum Roller
|Shim/spacer under spring
||85 lbs. (38.6 kg.) Seated
||100 lbs. (45.4 kg.) Seated -260 lbs. (117.9 kg.)
||Solid (Add Mass)
||Twin port, 48mm
||Twin port, 48mm
||1.50" (38.1mm) Solid stem Weight:95 grams, 3.35 ounces
||1.55" (39.4mm) Sodium filled stem Weight:75 grams, 2.65
||1.94" (49.3mm) Solid stem Weight:110 grams, 3.88
||2.00" (50.8mm) Hollow stem Weight:85 grams, 3.00
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)
||Valve Lift @
||Lobe Lift @
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.
The LT4's new valve springs are made from egg shaped wire for higher seat pressure and improved valve dynamics at higher RPM.
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
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.
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 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.
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 from the factory to differentiate them from the standard LT1 manifold.
Click here for more info about the differences between LT1 and LT4 intake manifolds.
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.
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
These LT4 components are considerably stronger than their LT1 counterparts and resist failure at higher RPM's.
TEFLON REAR CRANK SEAL
Offers improved durability over the standard LT1 crank seal.
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 [email protected].
- 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
Many thanks to "Steve" as the original compiler of much of the above information.