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Oval Port Heads vs Rectangle

By Richard Holdener, Photography by Richard Holdener

Life is full of constants. The sun will rise each morning, Newton’s apple will fall, and if you are going to make power with a big-block Chevy, you have to use rectangle-port cylinder heads. So honored is this tradition among Chevy enthusiasts that many would rather face death and taxes than install a set of oval port heads. The reasoning is sound and not without precedence, as every factory high-performance big-block came equipped with rectangular-port heads. The oval-port variety was left for more pedestrian people movers and (of all things) high-torque truck applications. No self-respecting L72, L88, or LS6 owner would be caught dead sporting a set of oval port heads on his high-performance machinery. That Chevy further downgraded the oval port heads to peanut-port status is a clear indication that maximum performance was never high on the list of design criteria. Now that we all agree on the superiority of rectangular-port heads, the only thing left to do is convince you that oval port heads are actually the hot setup for the vast majority of street/strip big-blocks.

Big-block enthusiasts have always looked down their noses at oval-ports, but we’re here to tell you that round is the new square. As a general rule, round (or oval) ports will outflow their rectangular counterpart given equal dimensional area. The reason is that the corners of a rectangle are actually wasted space in terms of flow. The flow rate in the corners is very limited, so the wasted space does little more than increase port volume. This is obviously not a desirable situation, as power production is a product of both airflow and volume. The ideal situation (for most street/strip motors) is to maximize flow while minimizing port volume. There’s an argument to be made for increased port volume on high-rpm (dedicated) race motors, but for most, small port and big flow is the best combination. Looking strictly at flow, we see a direct correlation between port volume and flow on the factory BBC heads. The smallest of the bunch (peanut port) flows the least at around 250 cfm, while the conventional oval-ports flow around 275 cfm. The performance-oriented, rectangular-port heads step these numbers up above 300 cfm, but flow (as we know) is the equivalent of go.

When it comes to the amount of power a cylinder head will support, a good rule of thumb is to double the maximum airflow. Most (mild-cammed) street engines never achieve this level, while dedicated race motors can exceed this, but it’s accurate nonetheless and makes for easy math. Using this simple formula, we see that the factory heads can support 500 hp (peanut-port), 550 hp (oval-port), and over 600 hp (rectangular port). Ported versions obviously have the potential to exceed these numbers, but it shows a well-established trend in head selection. If you’re looking to exceed 600 hp, the obvious choice is the rectangular port, but what about all the other big-block buildups that range from 400–550 hp? These would likely be better served with the smaller oval-port heads, but our comparison doesn’t stop there. So far we have examined only the factory offerings, but how many buildups actually rely on stock castings? Given the number of aftermarket head choices available for the BBC, more often than not the factory stuff gets ditched in favor of something superior.

The introduction of performance castings by the aftermarket has greatly blurred the distinction between rectangular- and oval-port heads. Current aftermarket oval-port heads (like the AFR 265s tested here) not only match or exceed the flow offered by a set of stock rectangular-port heads, they do so with considerably less port volume (265 cc versus 310–320 cc for stock heads). More flow is always welcome, but even more so when it comes with a reduction in port volume. The combination equates to increased cylinder filling combined with improved low-speed and part-throttle response. Fuel mileage will likely increase as well, though the number of times you stomp on the loud pedal will have more of an effect on your actual fuel economy. The peak airflow offered by the AFR 265 heads (332 cfm) suggests they would support nearly 700 hp on the right application. That is a far cry from the factory ratings of the rectangular-port headed L72, LS6, or even L88s of yesteryear.

Since man does not live by flow numbers alone, we decided to take the cylinder head comparison to the next level. In addition to flow-bench numbers, we decided to compare a set of factory rectangular-port heads to the oval-port AFRs on the engine dyno. Sure, street and strip testing would be the ultimate follow up, but the dyno should illustrate the correlation between head flow, port volume, and power. Since the airflow/horsepower formula only hints at potential power (the whole combination must be optimized), we decided a real-world test on a streetable big-block was the hot setup. Though the airflow offered by the AFR 265s will certainly support a 496 stroker, we decided to test the heads on a 454 (actually, 0.060-over 468). The 468 featured machining and balancing from L&R Automotive, forged crank and rods from Procomp Electronics, and forged (18cc dome) pistons from Probe Racing. Combined with the 119cc chambers on the rec-port heads, the pistons produced a static compression of 9.70:1. This was increased to 10.47:1 with the 112cc chambers employed on the AFR heads. Given that every point of compression is worth 3 to 4 percent, the change in chamber volume was worth about 2.5 percent.

Cam timing is important for a couple of reasons, not the least of which is that it determines the effective operating range of the motor. It is also a limiting factor in terms of potential power since it determines the actual airflow employed by the motor. If your heads flow 350 cfm at 0.700 lift but your cam is only 0.600 lift, then you’re not taking full advantage of all the airflow available from your cylinder heads. Naturally, this limits the power output of your motor. For our test motor, we chose a streetable, but powerful, hydraulic roller cam. The XR294HR offered a 0.540/0.560 lift split, a 242/248 duration split and a 110-degree lobe-separation angle. This was a healthy cam to be sure for a 468, but we wanted to ensure our test motor was powerful enough to properly test the merits of the AFR oval-port heads. Comp Cams also supplied a set of roller rockers and hardened pushrods, while the various heads required dedicated intakes. We chose single-plane intakes for testing, but also ran the AFR heads with a dual-plane RPM Air Gap to illustrate the benefits offered for a dedicated street application. The stock rec-port heads received a Weiand Team G, while the AFRs were run with an Edelbrock Victor Jr. For many street/strip motors, the dual plane will offer improved torque production through most of the rpm range (see dyno results), but the cam was a better match for the single plane (at least for peak power production).

Though we have covered the major components employed on the test motor, the 468 also featured Fel Pro head gaskets, ARP head studs, an MSD billet distributor, and Holley 950 HP carburetor. All testing was run with a Meziere electric water pump and a set of 2.125-inch dyno headers.

By Richard Holdener
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