The slugs were combined with a ring set from Total Seal and a precision balance job from the boys at L&R Automotive. The combination has already exceeded 1,000 hp in boosted form, so we knew it was plenty stout and more than sufficient for our head test. The short-block was assembled first with the Comp cathedral-port cam (and new hydraulic roller lifters), FAST LSXr intake and matching 102mm throttle body. FAST also supplied the fuel rail and 75-pound injectors for our test along with an XFI engine management system. Additional components used on the motor included a complete Moroso oiling system, Fel Pro MLS head gaskets, and American Racing 1-7/8-inch headers. Equipped as such, the cathedral-port heads and cam combo pumped out 631 hp and 575 lb-ft of torque, with torque production exceeding 550 lb-ft from 4,200 rpm to 5,800 rpm.
Having dialed in the combination, we proceeded to swap out the cam. After installation of the rectangular-port cam, the peak power numbers improved slightly to 639 hp and 578 lb-ft. The increased exhaust duration offered by the rectangular-port cam (255 degrees versus 247 degrees) improved power production slightly from 5,500 rpm on up, but the improvements in top-end power came with a penalty. The increased exhaust duration had a negative effect on low-speed power, as power was down from 3,000 rpm to 4,700 rpm, the greatest difference of 20 lb-ft coming at 3,100 rpm. This should not come as a huge surprise, as increased duration usually follows this trend. Increased duration is done to increase the effective engine speed. The additional exhaust duration also decreased idle vacuum slightly compared to the cathedral-port cam, so low-speed drivability might also suffer (we never loaded the motor below 3,000 rpm).
Though the dyno test was run in reverse order, we will provide the data for the rectangular-port heads first with the cathedral-port cam. Swapping over to the Mast LS3 heads required a change in intake manifold as well. Once again we relied on a FAST LS3 LSXr intake and 102mm throttle body along with the 75-pound injectors, rails and XFI management. Given the difference in airflow, we were naturally excited about replacing the cathedral-port heads with the rectangular-port heads. Run with the Mast CNC LS3 heads and Comp cathedral-port cam, the 408 stroker produced 634 hp and 577 lb-ft of torque, meaning a difference of just 3 hp and 2 lb-ft measured peak to peak. In reality, the cathedral-port heads were within 1-2 hp at the peak, but offered as much as 20 additional lb-ft below 4,000 rpm. We suspected that the rectangular-port heads were not optimized with the cathedral-port cam, but we were still surprised to see such a small difference in power given the extra airflow offered by the LS3 heads.
The final test run was to combine the rectangular-port cam with the Mast LS3 heads. The combination produced the highest peak power numbers of the day (640 hp and 680 lb-ft), but only by 1 hp and 2 lb-ft over the cathedral-port head and rectangular-port cam combo. The LS3 heads did respond better to the installation of the rectangular-port cam. The cam improved power from 5,300 rpm on up, with only a slight loss in power below 3,300 rpm. Obviously there is something to increasing efficiency of the exhaust port with increased duration. Though we were happy with the results of the cam test, we were still surprised that the LS3 heads offered little or no power over their cathedral-port counterparts. Given the disparity in airflow, we expected to see 10-15 hp, but the results of this head-to-head shootout just go to show that airflow isn't everything. It also showed that you can't go wrong with either set of Mast heads on your performance LS combination.
Coefficient of DischargeThough this may be a tad on the technical side, it might well help explain why the cathedral-port heads did so well against the rectangular-port heads despite the difference in flow. The coefficient of discharge is essentially a measurement of the efficiency of the port (actually the curtain area of the valve). The curtain area of the valve is easy to visualize. Imagine dropping a circular shower curtain down from the outside diameter of the intake valve. The length of the shower curtain would be determined by the maximum valve lift, but (more importantly), the curtain area can be calculated at every valve lift where airflow is measured (usually .100-.700-inch lift). Much like average airflow through the head port, we can take the average coefficient of discharge by combining the averages for all the lift values and then dividing by the number of lift points. The formula for coefficient of discharge is as follows: C/D=airflow/curtain area. Curtain area=valve diameter x Pi x Lift. Applying the formula to our heads, we see that though the LS3 heads offer a better coefficient of discharge at the maximum lift of .700, the cathedral-port head offered not only a superior coefficient of discharge from .100-.400 lift, but a better average coefficient of lift from .100-.600 lift. Could it be that the efficiency of the port overcame the absolute flow of LS3 heads?