Oval Port Big-Block Chevy Cylinder Heads - The Big O

The best way to illustrate the power potential of performance cylinder heads is on the dyno.

Richard Holdener May 18, 2012 0 Comment(s)
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Ask a big-block enthusiast what they need to make 600 hp and chances are they will point to some sort of rectangular-port head. No one in their right mind would select an oval-port head to make that kind of power, let alone one with a diminutive 265cc intake port, right? Well, big-block fans, times have changed and so has that well-defined line between the pedestrian oval-port and rectangular-port performance heads.

Back in the day, all the factory high-performance Rat motors were sporting rectangular-port heads and the street/strip crowd followed suit. But that was then and this is now. The aftermarket has stepped up in a big way and produced oval-port heads that not only out-flow any factory rectangular-port version, but do so while dramatically reducing port volume. The combination of more flow and less port volume makes these oval-port heads a much better choice for your street/strip motor than their rectangular-port counterparts. For motors up to and even beyond 600 hp, especially small-displacement big-blocks (under 500 inches), the hot setup today is more than likely an aftermarket oval-port.

With oval-port heads taking the street/strip market by storm, we decided it was a good idea to take a long hard look at them and thoroughly check out the performance potential. This meant gathering a number of different oval-port cylinder heads and putting them through their paces, both on the flow bench and the engine dyno.

First up, we needed a suitable test motor. We decided on a 0.060-over 454 equipped with a forged crank and forged rods from Procomp Electronics matched to a set of forged pistons from Probe Racing. The pistons feature an 18cc dome to help provide a streetable static compression over 10.0:1 with the 110cc chambers employed on a couple of the oval-port heads.

Many of these heads offered combustion chamber volumes below the 120cc range used on most rectangular-port heads. The smaller chamber (relative to factory rec-port heads) improved power by increasing the static compression--always a plus. The test motor also featured a healthy hydraulic roller cam in the form of an XR300HR from Comp Cams. It a 0.560/0.580 lift split, a 248/254 duration split, and a 110-degree LSA. All testing was run with an Edelbrock Victor Jr. intake fed by a Holley 950 HP carb.

Before being run on the engine dyno, each of the heads had to be mocked up to determine not only the appropriate pushrod length, but also the correct head bolt/stud combination. Our 2.25-inch dyno headers presented a problem for the heads equipped with stock port locations and required use of head bolts in the lower holes to provide adequate clearance. A few of the heads required different middle stud lengths, but ARP helped cure the situation. All we had to do was juggle the necessary bolts and studs to properly secure the heads. Thanks also goes out to Fel Pro for supplying the necessary head and intake gaskets for our test.

The test Rat was finished using a set of hydraulic roller lifters, timing chain, and a couple of different sets of pushrods from Comp Cams. Comp also supplied the aluminum roller rockers (1.7 ratio) for our head shootout.

The finishing touches included a Milodon oiling system, an electric water pump, and balancer from Procomp Electronics. Prior to their time on the test mill, each head was given the once over on the flow bench, then set up to measure port and chamber volumes.

The heads supplied for the test shared the oval-port configuration, but that doesn't mean they were all the same. We started with a factory oval-port head to establish a baseline, in this case the peanut-port variety. We hoped to include a traditional oval-port head as well, but it fell through at the last minute.

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