Testing The New GM L92 Cylinder Heads - 550 hp for Under $4,900
| Cam Specs | | Camshaft | Duration @ 0.050 | Lift (inches) | Lobe Separation Angle (LSA) | | GMPP Hot cam, intake | 219 | 0.525 | 112 | | PN 12480033, exhaust | 228 | 0.525 | | | | | Comp Cams XR281HR, intake | 228 | 0.571 | 112 | | PN 54-428-11, exhaust | 230 | 0.573 | |
The new cylinder heads also require a different carbureted intake manifold because of the wider but shorter intake-port design. GM Performance Parts has already filled that void with a brand-new casting. | 
The cam change to the Comp Cams hydraulic roller was again, very easy, and we reused the production GM hydraulic roller lifters. | 
This is the plenum area of the new GMPP intake manifold. Most modern single-plane intakes tend to extend the port-runner dividing walls into the plenum area. This manifold does not. We might experiment with this to see if there is some power hiding in this manifold. |
| Dyno Day | Test 1: is the best power curve from last month's story using the stock LQ4 6.0L iron heads, GMPP Hot cam, GMPP carbureted intake manifold, Holley 750-cfm carb, and Kooks 1-3/4-inch headers.
Test 2: is where we jump directly to the big L92 heads combined with the larger Comp hydraulic roller camshaft. We also increased carburetor size to an 850-cfm Holley to make a little more power at the top while retaining the Kooks 1-3/4-inch headers. | | | Test 1 | Test 2 | DIFF | | RPM | TQ | HP | TQ | HP | TQ | HP | | 2,200 | 399 | 168 | 378 | 157 | -21 | -11 | | 2,400 | 392 | 179 | 378 | 172 | -14 | - 7 | | 2,600 | 393 | 194 | 386 | 191 | -7 | -3 | | 2,800 | 401 | 214 | 396 | 211 | -5 | -3 | | 3,000 | 414 | 237 | 406 | 232 | -8 | -5 | | 3,200 | 427 | 260 | 414 | 253 | -13 | -7 | | 4,300 | 435 | 281 | 419 | 271 | -16 | -10 | | 3,600 | 438 | 299 | 422 | 289 | -16 | -10 | | 3,800 | 437 | 315 | 423 | 305 | -14 | -10 | | 4,000 | 434 | 331 | 424 | 323 | -10 | -8 | | 4,200 | 434 | 347 | 427 | 341 | -7 | -6 | | 4,400 | 436 | 366 | 432 | 362 | -4 | -4 | | 4,600 | 441 | 387 | 438 | 385 | -3 | -2 | | 4,800 | 447 | 408 | 447 | 408 | 0 | 0 | | 5,000 | 450 | 428 | 455 | 434 | +5 | +6 | | 5,200 | 450 | 444 | 462 | 457 | +12 | +13 | | 5,400 | 444 | 456 | 464 | 478 | +20 | +22 | | 5,600 | 436 | 466 | 464 | 494 | +28 | +28 | | 5,800 | 430 | 475 | 459 | 506 | +29 | +31 | | 6,000 | 423 | 483 | 451 | 515 | +28 | +32 | | 6,200 | -- | -- | 443 | 524 | -- | -- | | 6,400 | -- | -- | 438 | 535 | -- | -- | | 6,600 | -- | -- | 435 | 546 | -- | -- | | 6,800 | -- | -- | 425 | 551 | -- | -- | | Avg. | 428 | 336 | 427 | 364 | | | | Power/ci(peak) | 1.23 | 1.32 | 1.27 | 1.51 | | | | Comparing peak numbers, the torque gained only 14 lb-ft, but the horsepower jumped a staggering 68 hp with the combination of the larger cam and the L92 heads. |
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