Chris Meagher, Assistant Chief Engineer for Small-Block Truck Engines, picks up the story. "We came to realize that with the high-flow heads that had been in development for the LS7, we had the prototype for a head that could be put on the truck small-block and pick up the airflow we needed to get to our 400hp design goal." With this preliminary head design in hand, the team was on its way toward what would eventually be the L92. Although the engineers ended up meeting their stated power goals in conjunction with increasing the L92's bore, bringing final displacement to 6.2L, happily, they made sure that the cylinder head configuration would also fit on the original 4-inch bore-and therefore be applicable to the existing 6.0L truck engine as well. "So the high-flow head on the L92 actually will work on both the 6.0 and 6.2L engine variants," continues Meagher. "The direct LS7 head continued to evolve and has a somewhat different configuration: the valve angles were changed, and you have a bigger intake valve in there-these features will not play with the truck engine. So the two head designs are not identical by any means, but the L92 is in essence what we were starting to develop along the way for the LS7. It has bigger intake and exhaust valves and a much higher flow capacity than the 'low flow' head currently on the 4.8/5.3L engines in production today."
Race Bred
As alluded to earlier, the LS7 program took significant cues from lessons learned in GM Racing programs, most notably the C5R. Says GM's Chris Meagher, "We had the C5R guys in to talk about what they did to get the flow they got out of a Gen III/IV head, because that was our obstacle to get to the 500hp LS7 design target. They showed us how they did it and gave our guys some ideas; we just had to match all that up with something we could do in high volume production." Fortunately for the L92, it also significantly benefited from some of these advances: features like a raised port floor and increased port width can be closely linked to racing influences.
The L92 and its not-too-distant cousin, the LS7. While the two heads have an outwardly similar appearance, a close look reveals that the LS7 (top) has intake ports that are shorter and fatter than those of the L92. Yet both port styles are substantially wider than traditional Gen III and IV heads, and much of this has to do with the offset intake pushrod setup utilized by both. | 
Set side by side, the combustion chambers of the LS7 (left) and L92 show shared features in shape. Valves in the LS7 head, at 2.20 and 1.61 inches respectively, are somewhat larger than in the L92 and won't work on its smaller 4.065 bore (not to mention a 4-inch bore). | 
The exhaust ports of the LS7 and L92 are also quite similar, both using a D-shaped opening. In comparison to other Gen III and IV castings, says GM's Dennis Gerdeman, the L92 port opening is larger and the roof sits higher-so you'll need to watch whether your exhaust manifolds line up. Can you tell which head is which? Here's a hint: the LS7 isn't the one on the right. |