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GM Gen III V-8 Cylinder Head Comparison - Gen III Tech

Which Cylinder Heads Offer The Most Power Potential?

Photography by Will Handzel

With all the interest in the General Motors Gen III V-8-based LS1 and LS6 engines, there has been a need for a story showing what the best production cylinder heads are for making power with this engine family. This excerpt from the just-released book How to Build Performance Chevy LS1/LS6 V-8s, by Will Handzel, contains key cylinder head information performance enthusiasts need to know when working with the Gen III V-8.

Overview Of The Gen III V-8The General Motors Gen III V-8 engine, what most refer to as the LS1, is an engine family that was initially released as the GM Regular Production Order (RPO) code "LS1" engine in the '97 Corvette. The exact same V-8 architecture went on to be used in other GM products like Camaros/Firebirds ('98-'02), all fullsize trucks and SUVs ('99-present), some midsize SUVs ('04 to present), GTOs ('04), and the special production run of CTS Vs ('04). This means a set of LS6 performance cylinder heads will bolt on to your '00 Suburban, '99 Camaro, or '04 GTO!

Power HeadsThe best heads for most performance applications are by far the LS6 cylinder heads built beginning in 2001. They were designed for higher-rpm applications that require increased airflow and higher compression ratios. These cylinder heads will fit any Gen III V-8 engine, but you'll need to mind the details when switching heads such as combustion chamber volume, valve-to-piston clearance, and valvetrain geometry.

The combustion chamber volume does vary across the Gen III V-8 engine family. We've included a chart listing combustion chamber volumes (including gasket volume) and engine compression ratios. On some engines, like the 10.08:1 compression ratio LQ9 345hp 6.0L Escalade Gen III V-8 engine, installing a set of LS6 cylinder heads without changing the head gasket thickness or swapping in dished pistons would result in almost an 11:1 compression ratio. This would force the use of 100-octane fuel to prevent detonation and yet would not net enough power gain for the hassle-not a winning situation for a street vehicle.

In fact, swapping LS6 heads on the aluminum-head-equipped LQ4 or LQ9 6.0L would only result in higher compression. This is because the LQ heads were derived directly from the LS6 heads. This means the LQ intake and exhaust ports are exact copies of the LS6 heads. The main difference is the LQ cylinder head has a 72cc chamber volume while the LS6 head has a 64cc chamber volume.

The valve-to-piston clearance in swapping LS6 heads on another Gen III engine is also an issue because GM has used multiple piston dome/dish and cylinder head combinations. Going with the small LS6 combustion chamber cylinder heads, a nearly 0.600-inch lift cam, and a flat-top piston will challenge the accepted 0.080-inch clearance between the piston and valve at max lift and piston TDC. This is best accomplished by using modeling clay (not Play-Doh or similar play clay, as it "springs" back giving a false reading) between the valve and piston during the "mock-up" stage of the engine build.

The production valvetrain on the Gen III V-8 is non-adjustable, but geometry issues can be addressed by installing different-length pushrods. In general, most Gen III V-8 builders say a stock valvetrain with the proper geometry will require the rocker holddown bolts be turned another 111/42 turns beyond the point where the pushrod will not spin between your fingers. This is considered the valvetrain "preload" with the components "hot."

These same engine-builders have found power by running less valvetrain preload. The general consensus is that power exists in valvetrains with a preload of about 31/44 turn of the holddown bolts. On a stock head, this would mean you'd run about a 0.040-inch shorter pushrod to achieve this-but always measure your valvetrain for this measurement. Most of the aftermarket valvetrain companies offer Gen III pushrods in multiple-length increments to allow geometry tuning. You could buy adjustable rockers, but they will just add valvetrain weight (not good for high-rpm applications) and cost that can be rectified with the proper pushrod length.

The LS6 heads were first released in '01 on the Z06 Corvette and are available today in assembled form from GM Performance Parts (PN 12564824, casting number 12564243) loaded with hollow stem valves (these were factory pieces from '02 to today) and LS6 valvesprings.

To learn more about the Gen III V-8, check out www.cartechbooks.com to order How to Build Performance Chevy LS1/LS6 V-8s.

Power SecretHere's a secret: If you want the best heads for performance, make sure your heads have these small triangle marks below every spark plug hole. The marks are left by "chills" used during the casting process in GM's semi-permanent mold (SPM) tooling. Of the three casting facilities that make Gen III cylinder heads, two use SPM tooling, the other heads are sand-cast. The SPM tools produce heads with very smooth, consistent ports and chambers which consistently dyno at least 4 to 7 hp better than the sand-cast heads.

Common Gen III V-8 Engine RPO Packages and Power Output
RPO Description Power Output Comp. Chmbr. Int/Exh.
Displacement HP/Torque Ratio Vol. Port Vol.
Liters (ci) (cc) (cc)
LS1 5.7 (346) car
Corvette 345/340 10.19 67 200/70
Camaro/Firebird 305/335 to 325/350
GTO 350/365
LS6 5.7 (346) car
Corvette 385/385 '01 10.46 65 210/75
405/400 ’02-today
LQ9 6.0 (364) truck/SUV
Escalade & 345/380 10.08 71 210/75
Silverado SS
LQ4 6.0 (364) truck/SUV
GMC/Chevrolet 300/360 to 330/370 9.41 71 210/75
Fullsize
LM4 5.3 (325) truck/SUV
'01 and later 290/325 to 300/335 9.49 61 200/70
Fullsize, ’04 SSR
’04 Trailblazer/Envoy/
Bravada
LM7 5.3 (325) truck/SUV
'99 and later 285/325 to 295/330 9.49 61 200/70
Fullsize
LR4 4.8 (293) truck/SUV
'99 and later 270/285 to 280/290 9.47 61 200/70
Fullsize
Gen III Cylinder Head Casting, Machined and Final Part Numbers
RPO Size Part Numbers
Year Casting Machined Final Assembly
LS1
'97-'98 10215339 10215339 10215337
12558806 12558806 12558808
'99-'01 12559853 12559854 12559856
'01-'04 12564241 12559854 12559856
12559853 12564242
LS6
01-04 12564243 12560800 12560802
12569315
12564824
LQ9
'02-'04 12562317 12562318 12565363
LQ4
'99-'00 12581873 12561874 12561876
'01 12562317 12562318 12562320
'02-'04 12562317 12562318 12565363
12565364*
LM4, LM7
LR4, L59
'99-'01 12559862 12559863 12559865
'01-'04 12561706 12561708 12563678
12569862 12559863
*(this PN also good for RPO KL5; the natural gas engine)
Head Bolts
(do not reuse these stretch-to-torque fasteners!)
Description PN Type Torque & Twist
      Specifications
      lb-ft, degrees twist
Long (16/eng) 12560744 M11x2.0-155.5 22, 76 deg, 76 deg
Short (4/eng) 12560745 M11x2.0-101 22, 76 deg, 34 deg
Short (10/eng) 12558840 M8x1.25-46 22, no twist

Important: In January, 2004, the depth of the blind, threaded head bolt holes on the Gen III V-8 engine blocks were changed to use the common head bolt, PN 17800568. There was not a new engine block part number issued for this change, so it is difficult to determine what head bolts you will need until you see the existing head bolts in your engine.

"Lost" Power For the sake of mass production, the production Gen III V-8 cylinder heads were designed to fit the smallest-bore Gen III V-8 engines-the 4.8L and 5.3L, 3.780-inch-diameter cylinder bore SUV and truck engines. The displacement difference between the 4.8L and 5.3L engines is created with the stroke. The 4.8L engine uses a 3.27-inch stroke crank, while the 5.3L engine has the common Gen III V-8 3.622-inch stroke crank. Since the 5.7L and 6.0L Gen III V-8 engines have 3.898- and 4.00-inch bores, respectively, this means there is room for larger valves and better valve placement in the larger bores.

Why did GM do this? For every LS1 or LQ9 engine built, there are about two 4.8Ls or 5.3Ls built. The 5.3L is GM's bread and butter, so the heads had to be common to work with all the designs.

Virtually every GM engineer who worked on the Gen III V-8 knows there is power in moving the valve spacing to install bigger intake valves and achieve improved flow into the engine. They also know this change would require a new intake port design, offset rockers, and other changes to accommodate the design, but the power potential is great enough to warrant this work.

Obviously, a cylinder head with all these changes requires considerable work to create and manufacture, but it's possible it might appear in the near future-the multiple-digit power potential is too great for hot rodders to pass up. For those not capable of creating a cylinder head, just know there is power left on the table there!

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