Differences Between LT-1 and LT-4 Small Block Chevy

The LT-1 and LT-4 small block Chevy have many things in common. Swapping parts with second generation small block Chevy is not like it used to be with the first generation. LT-1 and LT-4 hold differences with in the same generation.

Heads

Cylinder heads have a different design from LT-1 to LT-4. They look alike, but are not.They will line up to each others block, but the intake manifold and exhaust manifolds have different patterns. The older small block Chevy (first generation) heads were all interchangeable with the exception of the 400 small block Chevy.

Both crankshafts are cast iron, the LT-4's fillets are undercut and rolled for more strength. Other than that they are the same but the crankshaft is weaker from the LT-1.

Rocker arms are not the same either.The LT-4 uses roller rockers, while the LT-1 the ball type rockers as found on the first generation small block Chevy. Ratios on the rocker arms are 1.5:1 on the LT-1 and 1.6:1 on the LT-4. The studs for the rocker arms are 3/8" for LT-1 and 7/16" for the LT-4.

The valves are lighter on the LT-4, with lighter springs with and more pressure. The reason for this is to handle higher RPM's.

Compression is 10.5:1 on the LT-1 and 10.8:1 for the LT-4. This is mostly because of the raised reliefs in domed pistons.The differences show definite improvement towards performance from small block Chevy from LT-1 to LT-4.

Because of reverse cooling on the LT-1 and LT-4 engines the cylinder heads from the older, first generation small block Chevy won't work.All heads for the LT engines must be made specifically for that engine.They are not interchangeable like they used to be.

Both the LT-1 and LT-4 use 17 head bolts in three lengths Thread size is 7/16 x 14 and lengths 1.5", 3.0" and 3.75". All the head bolts go through water and require sealant. All the bolts have a 5/8" hex head design

The LT-1 and LT-4 have different bolt patterns for the intake manifold.The earlier (first generation) small block Chevy were the same. Also, you can not swap an early style intake manifold for an LT-1 or LT-4 manifold.

Head gaskets are the same for the LT-1 and LT-4, both for reverse cooling, but slightly different than the earlier(first generation) small block Chevy design.

The Engine Block.

Both are made of cast iron and have water jackets around the cylinders like the older small block 400. The casting number is the same for the blocks designed with two bolt mains and blocks with four bolt mains. The only way to be certain is to remove the oil pan. If you know for sure the LT-1 or LT-4 came from a Corvette chances are it is a four bolt main block, but not in every case. They all (second generation ) have 327 cast on the side of the block. This does not mean it is a 327.

Timing Cover

LT-1 and LT-4 have 3 different timing covers, all require 11 bolts. A 3/8" wrench or socket is needed.There are differences among the 3 designs. Pay extra close attention to detail if swapping timing covers.

Distributor

Second generation small block Chevy featured a front mounted (pancake) style distributor in the place of the previously rear mounted design of the first generation small block Chevy. The proper name for the pancake distributor is Optispark.

Connecting Rods

LT-1 used forged pistons from 1992 - 1994 for more hardness. Powdered metal rods were used after that until 1996 some of the LT-1's used forged rods.

Pistons

The LT-1 and LT-4 both used hyper eutectic pistons. They are made from aluminum and have a slight dome with 4 valve reliefs.

Second generation small block Chevy used a water pump driven off the camshaft rather than a belt as the first generation had been. LT-1 and LT-4 also featured a roller camshaft also different from the first generation small block Chevy.

Lt-4 has approximately 30 more horsepower than the LT-1 used in the Corvettes. GM used the LT-1 name in the past as well in the 1970 1/2 Z38's and Corvettes. Too bad the older LT-1 has nothing in common with the new one.

Source:
HP Books How to Rebuild Small Block Chevy LT1/LT4 Engines