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CHAPTER 4

LS7, LS3, AND LS9 CYLINDER HEADS

Earlier Gen III LS heads, such as those found on LS1, LS6, and various Q Series truck engines, feature tall intake ports referred to as “cathedral” ports. The cathedral-roof shape at the top accommodates the fuel-injector path. The LS2 engine also features cathedral-port heads but is technically considered a Gen IV engine due to the change in cam sensor location. We’ll limit our discussion to Gen IV heads with rectangular intake ports.

In the Gen IV versions of this later generation of LS engines, we’re dealing with two separate cylinder head castings. The LS3, LS9, and L92 all use essentially the same cylinder head foundation, featuring a four-digit identification number, which is at the top of the head just outside the valve cover rail. The casting number for the LS3 head is 0821. The LS9 head features no such casting number; rather, the designation “LS9” is lightly engraved on the lower left of the exhaust side of the head. The LS7 cylinder head is unique, with identification number 8452.

Gen IV “rectangular”-port cylinder heads include the LS7, LS3, L92, and LS9 variants. Basic differences include intake port dimensions, rocker arm mounting pedestals, and combustion chamber volumes.

All Gen III heads feature tall and narrow cathedral intake ports. This includes LS1, LS6, and LQ truck variants. The LS2 also features cathedral-port heads, although it’s considered a Gen IV engine primarily because the cam timing sensor was moved from the rear of the block to the front timing cover. The cathedral-port design is generally noted for better low-end torque, while the Gen IV rectangular ports are generally known for superior midrange and top-end performance. Cathedral ports measure about 1 inch in width and about 3.125 inches in overall height.

The LS3, LS9, and L92 heads are “as-cast” heads, with intake and exhaust ports and combustion chambers shaped during the precision casting process. The LS7 cylinder head features intake and exhaust ports and combustion chambers with a more refined dimensional and surface finish that is CNC-machined.

Intake port dimensions also vary between the LS3, LS9, and L92 and the LS7 head versions. The intake ports on the head used for the LS3, LS9, and L92 are 1.250 inches wide × 2.550 inches tall, while the intake ports on the LS7 head are 1.350 inches wide × 2.40 inches tall.

The LS7 cylinder head was offered in 2007–2009 Corvette Z06 models. Identifying an LS7 head versus the LS3, LS9, and L92 heads is fairly easy. The LS3, LS9, and L92 heads feature flat rocker pedestals (to accept separate rocker arm rails), and the ports and chambers feature a cast finish. The LS7 head features individual rocker arm radiused stands and all ports and chambers display a machined surface.

From left to right: LS7, LS3, and LS9 cylinder heads. Notice the taller intake ports on the LS3 and LS9 heads.

Shown here is a cutaway view of an L92 head. The intake port runner design differs slightly among the various head versions, but all are very similar in shape.

The LS3 cylinder head has become very popular, in part because of the improved flow of the Gen IV rectangular-port design and its relatively attractive street price. General Motors made a lot of LS3 engines along with its truck counterpart, the L92, and as a result these heads are available in great numbers in salvage yards. Even brand-new head prices have become very reasonable; hence their popularity.

Shown left to right: LS7, LS3, and LS9 heads. Combustion chamber volumes differ among the three variants. The LS7, L92, and LS9 feature 70-cc chambers, while the LS3 has 68.4-cc chambers.

Note that while stock cylinder heads feature a four-bolt-per-cylinder-head bolt layout, GM LSX versions are offered with a six-bolt-per-cylinder layout. Aftermarket heads are also available in four- or six-bolt versions.

LS7 heads are casting number 8452 and feature slightly raised ports, altered valve angle, and larger titanium intake valves. Exhaust valves are sodium filled and have been known to cause failures. A popular remedy is a change to stainless-steel exhaust valves. Approximate flow bench results at .600-inch lift are about 370-cfm intake and about 240-cfm exhaust.

LS7

LS7 cylinder heads feature a larger combustion chamber and wider valve layout. Because of this, LS7 heads cannot be installed onto blocks that feature a cylinder bore size of under 4.100 inches, as valves may contact the top edges of the cylinder bores. LS7 heads may only be installed onto blocks that feature a 4.100-inch or larger cylinder bore diameter. Unlike other factory LS heads, LS7 factory heads are fully CNC-machined at the combustion chambers and ports. Beneficial porting work may involve only port matching the intake ports and/or the intake manifold ports to remove any potential flow obstructions. Due to the LS7 unique intake manifold deck, only intake manifolds designed for the LS7 heads may be utilized due to port match and manifold bolt pattern.

LS7 intake port deck view. Note the shorter intake port height compared to LS3, LS9, and L92.

LS7 Head Specs

PN 12578449 (complete)

PN 12578450 (bare)

Casting number 8452

356-T6 aluminum

Fully CNC-machined chambers and ports

12-degree valve angle

70-cc combustion chambers

270-cc intake volume

85-cc exhaust volume

11:1 compression ratio with stock pistons

Offset intake rockers

1.8:1 rocker arm ratio

Unique intake manifold bolt pattern

2.200-inch titanium intake valves

1.610-inch sodium-filled exhaust valves

Fits 4.100-inch bores or larger

(LS7 heads are also available in various combustion chamber volumes)

Number of 11-mm head bolts: 4 per cylinder/10 total

The LS3 cylinder head has 260-cc intake runners and 92-cc exhaust ports. In addition, it has a combustion chamber volume of about 68.4 cc. These heads fit any LS engine with cylinder bores of 4.000 inches or larger. It also features 2.165-inch hollow-stem intake valves and 1.59-inch solid-stem exhaust valves.

LS3 Head Specs

PN 12629063

Casting number 0821

319-T5 aluminum

As-cast ports and chambers

D-shaped exhaust ports

15-degree valve angle

260-cc intake volume

90-cc exhaust volume

68.4-cc combustion chambers

10.7:1 compression ratio with stock pistons

2.165-inch hollow-stem stainless-steel intake valves

1.59-inch sodium-filled exhaust valves

Fits 4.000-inch bores or larger

Number of 11-mm head bolts: 4 per cylinder/10 total

Shown here is an LS3 combustion chamber with 68.4 cc of volume. All factory production LS heads feature an as-cast combustion chamber except for the LS7, which has CNC-machined chambers.

The LS7 heads feature fully CNC-machined combustion chambers with 70 cc of volume.

LS3

The LS3 factory head features lightweight hollow steel intake and solid-stem exhaust valves. In stock form, flow bench results have shown flow rates of 296 cfm intake and 208 cfm exhaust, at full valve lift of about .600-plus. This head is intended for blocks that feature a minimum cylinder bore diameter of 4.00 inches.

L92

L92 heads are essentially the same casting as the LS3 heads. The difference is the weight of the intake valves: L92 heads feature solid valves and the LS3 has lighter, hollow-stem valves. L92 and LS3 heads flow pretty well in stock form, but they can benefit from porting by opening up the throats a bit and reducing the big rocker bolt bosses in the runners and massaging the exhaust ports. This is best done on CNC for more precise and repeatable results. CNC porting also requires much less time as opposed to hand porting. Performing a good valve job is more critical, making sure that the seats are concentric and that the valves are sunk into the seat at the same depth.

The L92 combustion chamber features 70 cc of volume.

LS9

The factory LS9 cylinder head was designed for higher dynamic compression to suit a supercharger application. This head features a stronger, more-dense alloy along with beefier webbing reinforcements, again to better handle forced induction pressures. Like the LS7 head, the LS9 features a lightweight titanium intake and hollow sodium-filled exhaust valves. To promote air/fuel mixture efficiency, a “swirl wing” is designed into the intake valve boss at the base of each intake runner. The valve guide boss casting features a pronounced vertical riser, or “wing,” that promotes a swirl effect before the air/fuel charge reaches the valve. In stock form, full-lift flow has been documented at 270 cfm at the intake and 201 cfm at the exhaust ports. CNC port machining that removes the swirl wing has been reported to increase flow to 322 cfm intake and 218 cfm exhaust.

L92 Head Specs

PN 12629064

319-T5 aluminum

As-cast ports and chambers

D-shaped exhaust ports

15-degree valve angle

70-cc combustion chambers

260-cc intake volume

90-cc exhaust volume

2.165-inch solid-stem intake valves

1.590-inch solid-stem exhaust valves

Fits 4.000-inch bores or larger

Number of 11-mm head bolts: 4 per cylinder/10 total

The LS9 CNC-ported cylinder heads have enhanced webbing and a stronger deck for greater rigidity and minimal distortion. The A356T6 alloy also withstands higher-performance service, and in stock trim it is supercharged.