Rectangle vs Cathedral Port Cam Test

By Richard Holdener

When it comes time to choose an LS cam, does it matter what heads you have?

Okay LS enthusiasts, by now you should have gotten the memo that the first upgrade you should make to your motor is a cam swap. Of course, a bigger cam will also require a spring upgrade, but that is just common sense (or it should be). We all know that the right cam can offer huge power gains on even a stock LS application, with greater gains coming on a modified motor. The question isn’t so much should you change your cam, but rather, which cam should you pick? Ultimately, that choice comes down to the intended usage of the motor, as the cam choice for a turbocharged, drag-race application will certainly be different than a daily driver. One of the deciding factors also seems to be your cylinder heads, as most cam manufacturers, including COMP Cams, offer dedicated cams for cathedral-port heads and rectangular-port heads. Naturally, the LS3, L76 or L92 guys wouldn’t be caught dead running a cam designed for an LS1, LS2 or LS6, and vice versa. The question now is, does it really make a difference?

Before we start testing, we need to understand why there is even a difference between the cams for the two head configurations. The difference in the cams actually has less to do with the respective shapes of the intake ports, and more to do with airflow. A review of the flow numbers generate on the flow bench for each head will paint a clear picture, starting with the cathedral-port, 799 heads. Essentially a 243 head with different casting numbers, the 799/243 heads are considered to be the top of the line, factory, cathedral-port offerings. Tested on a flow bench, the 799 head offered right at 249 cfm at .700 lift on the intake and 191 cfm on the exhaust. It should be pointed out that the 799 head featured a 2.0/1.55 valve combination. By contrast, the 823, rec-port heads featured a 2.165/1.59 valve combination and peak flow numbers of 317 cfm on the intake and 223 cfm on the exhaust.

Despite the fact that the rec-port LS3 heads easily out flowed the cathedral-port heads, we need to look at the relationship between the intake and exhaust flow. Measured as a percentage, the rec-port heads featured a 70.3% (223/317) intake-to-exhaust flow relationship, meaning the exhaust flowed only 70% of what the intake flowed. Looking at the numbers, getting the air into the motor on a rec-port application seems to be easier than getting it out. By contrast, the cathedral-port heads checked in with a 76.7% (191/249) rating. Though both the intake and exhaust flow were down compared to the rec-port heads, the cathedral-port heads offered a better balance between the intake and exhaust flow. If we look at the cams available for the different applications, we see that cam manufacturers have designed cams for the rec-port applications to improve exhaust flow with increased exhaust duration. Relatively speaking, rec-port cams offer more exhaust duration, or a bigger split between the intake and exhaust duration specs. This was done to help correct the intake-to-exhaust imbalance inherent in the different heads.

Now that we better understand why cam manufacturers offer the two different cam profiles for the different head configurations, we can set up a test to see if the different heads do indeed respond to the different cam profiles. Naturally this is a rather involved test, as it required running two different cam profiles on two different head configurations. For this test, we chose one cathedral-port and one rec-port cam from the COMP Cams catalog and ran them on a 6.0L. Both cams were first run on the 6.0L equipped with a set of 799 (cath-port) heads and cathedral-port intake, then again after swapping over to a set of 823 (rec-port) heads and LS3 intake. Running both cams on both heads would illustrate if one cam favored one type of cylinder head. To keep the test as even as possible, we chose cams that featured identical intake lift and duration specs. The 54-459-11 cathedral-port cam offered a .617/.624 lift split, a 231/239-degree duration split and 113-degree lsa. The rec-port counterpart 54-469-11 cam offered the same lift and intake duration, but increased the exhaust duration to 247 degrees (8 more than the cathedral-port 459 cam).

The test motor used for this cam and head comparison was a 2008 LY6 6.0L. Designed to replace the LQ9, the LY6 was originally equipped with 823 rec-port heads, flat-top pistons with valve reliefs and variable valve timing (VVT). Obviously, the VVT operation was replaced by fixed cam timing and the LY6 truck manifold was ditched in favor of a low-profile LS3 intake. Run with the 799 heads, we included a Dorman LS2 intake that has shown to be equal to the factory TBSS manifold. Dialing in the combinations was a FAST XFI ECU and 89-pound injectors. To get things started we swap the two cams on the 6.0L equipped with the 799 heads.

Low and behold, the cathedral-port heads ran well with both cams! In fact, the biggest difference in power anywhere in the curve was a scant five horsepower. The cathedral-port cam offered more torque at the lowest portion of the rev range, but the rec-port cam offered slightly more power there on up. Equipped with the (cath-port) 459 cam, the 799-headed 6.0L produced 522 hp at 6,300 rpm and 481 lb-ft of torque at 5,100 rpm. Run with the (rec-port) 469 cam, the 6.0L produced 525 hp at 6,400 rpm and 485 lb-ft of torque at 4,700 rpm.

The results of the cam test with the rec-port heads mirrored those of the 799 heads. The two cams performed the same, with only minor differences in power at the lowest and highest engine speeds. Through most of the curve, the two cams produced identical power outputs.

Obviously, this was some sort of anomaly, right, as cathedral-port and rec-port cams have to make a difference. Surely things would change once we installed the rec-port 823 heads and ran the test again! Actually, the power curves were even closer on the rec-port heads, as the two produced identical peak torque numbers of 492 lb-ft of torque at 5,200 rpm. The peak power outputs differed by a whopping 3 hp, with the edge once again going to the 469 cam. The 469 cam produced a peak of 544 hp at 6,600 rpm to the 541 hp at 6,400 rpm offered by the 459 cam. Looking at the curves, the cath-port cam once again offered more torque at the lowest engine speed, but the rec-port countered with more power at the very top. Through the remainder of the curve (this is very important), the two cams produced identical power curves! Truth be told, we have run this test a number of times with identical results. The cam with more exhaust duration (the rec-port cam) makes less power down low and more at the top.

COMP 459 vs 469 Cam-Cathedral-Port 799 Heads

Looking strictly at the peak numbers, there is very little to choose from between the cathedral-port 459 cam and the rec-port 469 cam on this 799-headed 6.0L. The 459 cam produced peak numbers of 522 hp at 6,300 rpm and 481 lb-ft of torque at 5,100 rpm. By contrast, the 469 cam produced 525 hp at 6,400 rpm and 485 lb-ft of torque at 4,700 rpm. The reduced exhaust duration offered by the 459 cam improved power at the very bottom of the rpm range, but the 469 cam offered more at the top.

Graph 2

COMP 459 vs 469 Cam-Rec-Port 823 Heads

After topping the 6.0L with factory 823 (LY6) heads, the results of the cam test were the same. Run with the cathedral-port 459 cam, the 6.0L produced 541 hp at 6,400 rpm and 492 lb-ft of torque at 5,200 rpm. The rec-port 469 cam produced almost identical results, with peaks of 544 hp at 6,600 rpm and 492 lb-ft at 5,200 rpm. Once again, the cath-port cam offered more power at the bottom of the rev range and the rec-port cam (with more exhaust duration) offered more at the top. Through the majority of the power curve, the two cams were all but identical.