There is a larger question, however, that should be addressed. In the original story, the swap to the rectangle-port heads and big cam sacrificed mucho torque below 4,800 rpm. Add to this that the engine's power band (the rpm between peak torque and peak horsepower) has now migrated to between 5,400 and 6,800 rpm. That means the engine's power curve doesn't start until 5,400 rpm. To take full advantage of all that power, you also must shift the engine at 7,000 rpm or higher. The reason the L92 heads are so attractive is that you can purchase a pair of brand-new heads and the necessary offset intake rocker arms for around $1,100. The heads we tested were from Scoggin-Dickey Parts Center, priced at $899.90 a pair while the rocker arms and valvetrain parts are another $225. While these parts make excellent peak horsepower, the combination sacrifices midrange torque to get there.
An alternative idea is to port the original cathedral heads to improve the airflow while maintaining the higher port velocity of the smaller cross-section port area. A milder cam will put the peak horsepower rpm at 6,000 (rather than almost 7,000), and the peak torque will also drop down to a more useful 4,500 rpm. Why would we want to do this? To better illustrate the idea, I plugged the 551hp power curve into the Quarter Pro dragstrip simulation program using a 3,600-pound street car with a 3.55:1 rear gear and a 2,800-rpm converter. The program estimated the run at an 11.52-at-114.9-mph pass shifting at 7,000 rpm. Next, I increased the torque by 22 lb-ft between 3,000 and 5,000 rpm and reduced the peak horsepower by 30 hp to 520 at 6,000 rpm. While most enthusiasts might think that losing 30 hp would guarantee a slower run, the Quarter program generated a quicker and faster 11.42-at-115.3 mph. This occurred because the added torque in the middle helped accelerate the car more efficiently with the car's street-oriented 3.55:1 gear and mild stall speed. Conversely, if you had a drag-race-only car with a much deeper rear gear and perhaps a four- or five-speed, close-ratio manual trans, then that big head/big cam package would take advantage of the higher power band and run quicker.
The bottom line here is that you could spend roughly the same amount of money to have a stock set of cathedral heads CNC ported and match them to a milder camshaft and you would have an engine that makes great power and is very tractable on the street. This milder package won't make heroic horsepower numbers, but if your goal is quicker acceleration rather than just bragging rights at the next bench racing session, then the more conservative approach just might be the way to go. As an example, West Coast Racing Cylinder Heads offers a Stage 2 CNC porting operation for the stock LQ4/LQ9 317 cathedral-port heads that will flow virtually the same numbers at the midlift up through 0.500-inch lift as the L92 heads while delivering far superior port velocity. Plus, WCRCH will port your supplied castings for $1,375, which is only a few bucks more than the price of the new L92 heads and valvetrain. The WCRCH heads come with new 2.02/1.57-inch stainless valves and Patriot springs. The cathedral heads will tend to run better with a shorter cam similar to a Comp XR269HR (216/220 at 0.050 with 0.525/0.532-inch lift and a 114 degree lobe-separation angle). This will make a bunch of torque in the usable rpm range for a street engine compared with the larger 281 cam. The thundering herd is infatuated with L92/LS3 heads right now because those big-port heads offer sewer pipe flow numbers. But for a mild street engine, I believe the more conservative cathedral-port heads are a smarter choice.
Scoggin-Dickey Parts Center
West Coast Racing Cylinder Heads