The question of the day seems to involve the difference between the original cathedral-port and modern rectangular-port LS heads. More specifically, which one is better? Were we to compare two sets of production heads, there would be little comparison, as even the very best production (cathedral-port) LS6 heads (243 castings) offer nowhere near the flow and power potential of the current LS3 heads. Based on flow numbers alone, the LS3 head has the LS6 heads covered by 50-60 cfm on the intake side (though the change in chamber volume will drop the static compression ratio). The flow figures alone make the swap over to the LS3 components very desirable, especially considering the fact that the stock LS3 head will support well over 600 horsepower (we recently made 690 hp on a 468 stroker with stock LS3 heads). The downside is that the head swap from LS1/LS6 heads to LS3 heads is more involved than a simple R&R. The new heads require a 4.00-inch bore block, LS3 rockers and intake manifold as well, both increasing the cost and limiting the swap potential to 6.0-6.2L motors (and big-bore stroker variants).
Measured stock to stock, LS3 heads will offer power gains over the cathedral-port heads, but what about the aftermarket stuff? After the introduction of the LS3 heads, many enthusiasts started looking down their noses at the original cathedral-port heads, obviously not remembering how much better they were than the original small-block heads they replaced. Has the advent of the rectangular-port LS3 heads relegated the cathedral-port heads to second-tier status or is this just a case of bad PR? To answer this question, we enlisted the aid of Mast Motorsports. Included in their listing of performance components for the LS engine family was an impressive array of both cathedral and rectangular-port cylinder heads. In fact, they had the ideal set of heads for our comparison. One of our concerns for the head test was addressing the potential change in static compression ratio. Traditionally, cathedral-port heads offer smaller chambers than their rectangular-port cousins. The change in compression ratio can be as much as a full point or more, which can improve power by as much as 3-4 percent (near 25 hp on a 600hp motor).
To cure this issue, we selected a set of cathedral-port heads with 70cc combustion chambers to match the chamber volume typical in rectangular-port heads. Many aftermarket manufacturers are now offering larger chambers on their performance cathedral-port heads, as many find their way onto larger displacement strokers. Equalizing the compression ratio eliminated the variable, so we could concentrate on the power difference associated with the flow numbers, port size, and efficiency. For our test, Mast sent us a set of their cathedral-port, 11-degree, six-bolt, medium-bore CNC heads along with a set of LS3 heads that also featured an 11-degree valve angle and full CNC porting. In the tale of the tape, the cathedral-port heads offered 245cc intake ports, a 2.08/1.60 valve combination and peak flow numbers of 338 cfm on the intake and 237 cfm on the exhaust. The CNC-ported LS3 heads stepped up these numbers with an intake port volume of 256cc, a 2.165/1.60 valve package, and peak flow numbers of 372 cfm on the intake and 261 cfm on the exhaust. On numbers alone (like their production counterparts), the ported Mast LS3 heads seemed to hold a clear advantage over the cathedral-ports.
Another area of concern when it comes to any comparison between the cathedral and rectangular-port heads is cam timing. Optimum cam timing is a function of effective operating range, but another important factor is the relationship between the intake and exhaust flow. Typically, the superior intake flow offered by the LS3 head lowers the intake to exhaust flow relationship. This can be equalized (or optimized) by increasing the exhaust duration on the cam relative to the intake. On our Mast heads, the LS3 head offered both more intake and exhaust flow than the cathedral-port heads, resulting in an intake-to-exhaust flow percentage (372/261 cfm) of 69.89 (meaning the exhaust flowed roughly 70 percent of the intake). By comparison, the Mast cathedral-port head checked in slightly higher at 70.1 percent (338/237 cfm). Typically we'd see more of a difference between the two, but the intake-to-exhaust flow differed by less than 1 percent (actually 3/10 ths of a point). Despite the similarity, we decided to run two different cam profiles with the heads. Before you cry foul, know that we ran both heads with both cams just to see how each would respond.
Running a pair of heads with a pair of cams meant we had to double up on cam swaps, but it was all in the name of science, so we didn't mind putting in the extra work. For cam choices, we went right to the Comp Cams catalog and selected cam profiles designed specifically for the cathedral and rectangular-port heads. On the cathedral-port side, we chose a 289LRHR14 (PN 54-461-11) that offered .624-inch lift, a 239/247 duration split, and a 114-degree lobe separation angle. The 289LRRHR14 rectangular-port cam offered the same .624-inch lift, 239-degree intake duration and 114-degree LSA, but increased the exhaust duration to 255 degrees. LS3 cams typically offer a wider spread between the intake and exhaust than comparable LS1 cams. The idea is to help the (relatively) limited exhaust flow with additional exhaust duration. Not only would we show the difference between the two head configurations, but also the difference with their respective cams. How would the cathedral-port heads work with the rectangular-port cam and vice versa? Questions like these are what keep us up at night and why we do so much testing here at GM High-Tech.
Obviously our high-flow heads required something other than a stock 6.0L or even an LS3 short-block. Knowing the heads will support over 600 hp, we built our test motor accordingly. The 6.0L iron block was bored .030 over then treated to a forged 4.0-inch stroker crank and rods from ProComp Motorosports along with a set of dished pistons from Probe Racing.