But because the L92 was destined for production truck engines, says Dennis Gerdeman, the L92 program really only took the racing knowledge early on, after which big-volume production concerns had to kick in. For example, the L92 maintains the 15 degree valve angle typical of most Gen III and IV engines, while the LS7 eventually took more of a page from the C5R book and ended up with a more upright 12 degree spec. "We worked with the racing guys hand in hand and learned as much as we could from them, and we still learned some further things on the LS7 later in its development," says Gerdeman. "But the L92 is a little different animal because of the high-volume manufacturing impact: it had to go down our existing production lines. We took everything we could feasibly apply to the program, but then that's where a lot of the lines got drawn in the sand: we had to maintain certain push rod lengths, valve angles, and so on. There were a lot of features that if we would have tried to incorporate them, it would have had a huge impact on manufacturing and would have driven up the cost of the L92 cylinder head tremendously."
But even with those production-related constraints in place, substantial modifications were made as compared to previous production Gen III and IV cylinder heads-race-inspired changes obvious from even a quick glance at an L92 head. The raised intake port design is said to have been a very big enabler in meeting the stated power goals. "We pushed the roof up just about as far as we could without wiping out the valvetrain, and then we brought the floor up to accommodate the raised roof," says Dennis Gerdeman. "And the other big constraint for us has always been the width of the port: if you compare its width to a Gen III, the L92's is significantly wider, and right near the pushrod has always been our constraint. So we incorporated an offset inlet valve rocker arm, and that allowed us to pull that pushrod off to the side; now it's at more of an angle to the lifter, and when we did that we were able to widen the port substantially."
What It Means To You
Despite its race lineage, the L92 retains one feature that's somewhat unfamiliar to performance enthusiasts: it's as-cast, meaning it is not CNC ported in any way. Going this route had the primary purpose of keeping production costs down-and with how inexpensive this head is, you should be able to appreciate that. But don't let the fact that the L92 head never sees a CNC machine fool you: it's possibly one of the finest head castings ever, and man does this sucker flow (see flow bench numbers in the L92 test elsewhere in this issue). In fact, although GM Performance Parts will begin offering a CNC'd version of the L92 sometime in 2007, it won't deliver the kind of huge improvements the process often garners. According to one man on the project, GM Racing's Jeff Kettman, "The production L92 head is already so good, there's not much room to gain without altering the production casting. And for the CNC'd L92 head, we want to retain that casting so we can keep all the benefits of GM production quality control as well as price point." (As a side note, we've also been assured that there will be more development in the coming years of L92-style heads; in fact, there will reportedly be a full family of rectangle-port GM heads in the future, both CNC'd and not! Stay tuned...). In other words, despite their cost-effective design and manufacture, big gains are in order just by popping L92s right on your ride!
Unlike the LS7, the LS2 head (left, actually a CNC-ported unit) shows some visually striking differences with the L92. Tall and narrow ports have been a part of the new generation small-block engine family since the original LS1-and while the LS2 continues that tradition, the L92 does not. Notice the raised floor of the L92 port, a main reason why standard intake manifolds won't bolt up. Intake port flow is claimed to be improved by a staggering 17 percent over the LS2 (which shares common ports with the LQ4 and LQ9 truck heads). | 
The LS2 and L92 combustion chamber designs are also conspicuously divergent. The LS2 (valves installed, here also a CNC-ported unit) has more of a traditional "heart-shaped" design, whereas the L92's looks more like a butterfly. The combustion chamber of the LS2 is somewhat smaller, at under 65 cc, and it has smaller valves: 2.00 intake / 1.55 exhaust. | 
Thanks predominantly to sanctioning body rules changes, the C5R race head has gone through different iterations since it was first introduced. However, it has always exhibited the rectangular-shaped intake ports more typical of older SBC heads than of a Gen III. An original C5R head is shown at top; a newer "restricted" version below. |