Oval Port Big-Block Engine Build - The Ovalnator

We try to coax 800 hp out of a big-block street engine using oval-port heads and hydraulic roller cam.

Richard Holdener Mar 5, 2013 0 Comment(s)
Oval Port Big Block Engine Build Engine Dyno Prep 2/23

Enthusiasts seem to be divided on just what constitutes a real street motor. On one side, we have the daily driver camp, where “street” is defined as the motor used to power your daily driver, as in the car you could jump in to take a transcontinental trip to visit grandma. Positioned to the far side of the daily drivers are those who take the term “street motor” more literally, as in, if it motors down any street, well then by God, it is a street motor. Never mind the cantankerous behavior, the tendency to overheat and the abysmal fuel economy—if it rolls, it tolls.

For the rest of us, the definition lands somewhere between these two extremes. Being performance enthusiasts, a mild, stock powerplant just isn’t going to cut it, even if it was reliable enough for that coast-to-coast adventure. Then again, we all crave power and performance, but who wants to drive a car that bucks and stumbles at anything less than wide open throttle? No, an honest-to-goodness performance street engine must combine the proper measures of both performance and reliability.

To illustrate the far end of a real street engine, we decided to set our sights high in terms of power, while simultaneously limiting those variables that otherwise detract from reliability. For the most part, the performance of any combination is dictated by the big three—namely heads, cam and intake. For any given short-block, changes to the big three will determine where the motor makes peak power and torque. Low-speed power requires only minimal head flow, mild cam timing, and a simple dual-plane intake.

In addition to making modest power, these components also tend to enhance reliability, as engine speed and reliability are often inversely proportional. The recipe for making big power is hardly proprietary. Start by simmering a set of cylinder heads equipped with cavernous port volumes, then stir in an aggressive, solid-roller cam profile, and just a pinch of high-rise, single-plane intake. Combined to perfection, the combination can yield impressive high-rpm power, but power alone doesn’t make for a successful street mill.

In truth, a successful combo requires balancing the fine line between performance and reliability. For this build up, we decided to go big on the performance side. What is big for a street engine? How does 800 hp sound? Eight hundred horsepower constitutes serious power in anyone’s book, but making power is rarely a problem. Properly chosen, big heads, big cam and big intake all combine to produce big power. The problem is said combination rarely includes big reliability or driveability. Making our goal of 800 hp considerably more difficult was the fact that we decided our street-oriented BBC must include two components not usually associated with big power, namely a hydraulic roller cam and (of all things) oval-port heads. From the beginning of time, performance Rats have come with rectangular-port cylinder heads, leaving the more pedestrian pushers with oval ports. The same can be said of hydraulic valvetrain. Even just one of these variables should greatly limit power production, but the combination should make the goal all but impossible, right?

As always, we had a few tricks up our sleeve. Naturally, a set of stock oval-port heads were out of the question, but the aftermarket has produced oval-port heads that offer amazing flow and power potential. One need only to check out the results of our recent big-block head shoot out to realize that oval-port heads no longer take a back seat to their rectangular-port brethren. In fact, the results of the head shootout combined with this particular build up indicate that oval-port heads might just be the hot set up for the majority of street/strip build ups.

For our oval-port big-block build, we chose a set of AFR 300 Magnum heads. As indicated by the name, the oval-port offerings from Airflow Research featured 300cc intake ports combined with 123cc exhaust ports and 110cc combustion chambers. The 300 Magnum combined full CNC porting, a 2.30/1.88-inch valve package, and five-angle competition valve job to produce serious flow numbers of 387 cfm on the intake and 296 cfm on the exhaust. We have come to expect big flow numbers from aftermarket aluminum heads, but these 300 Magnum heads offered tremendous flow through reasonably sized ports. As indicated by our previous BBC head testing, big flow and small ports combine to make serious power.

The next two components used in the build up enhanced both power and reliability. The use of a hydraulic roller cam provided a set-it and forget-it approach to valve lash, to say nothing of the milder valve springs compared to a typical solid roller. Though we chose a hydraulic roller profile, we chose one capable of making serious power. Straight from the Crane catalog (PN 139851), the Crane hydraulic roller profile offered .680 lift (both intake and exhaust), a 270/282 duration split (@ 0.050) and a wide 115-degree LSA. The wide LSA improved idle quality while the 0.680 lift ensured we took full advantage of all the flow available through the AFR heads. The long duration shifted the torque curve high enough in the rev range to help the Rat produce the desired peak power number. Cam selection is a critical element in any build up, as the cam helps determine the effective operating range and overall character of the motor. Excessive cam timing limits drivability, but insufficient cam timing limits power. The key is to find the proper balance.

The second trick up our sleeve helped not only improve power, but also effectively tamed the cam timing. Knowing that there is no replacement for displacement, we decided what this build up needed was additional cubic inches. Reaching 800 horsepower with a 396 would be extremely difficult and result in a dedicated race motor that would only be happy above 8,000 rpm. Stepping up to a 427 or 454 would help reduce the engine speed where peak power occurs, but even these displacements would result in wild combinations ill suited to any street use. No, to be successful, we had to step up significantly in displacement from any production big block. Stepping right past the usual 496 (4.28 x 4.25) or the 540 (4.50 x 4.25), we upped the inches to a full 555 cubes. The key was to use an aftermarket block from Procomp Electronics that provided the ability to swallow not only a 4.25-inch stroker crank from Scat, but also a set of 4.560-inch (small dome) forged slugs from JE Pistons. The Scat forged steel crank was precision ground, heat treated and shot peened and also featured chamfered oil holes, a large radius on all journals and nitride hardening for superior wear resistance. Scat also provided the necessary H-beam connecting rods that featured doweled caps and 7/16 ARP cap screw bolts. Combining the forged rotating assembly with the beefy, four-bolt block ensured not only plenty of power-producing displacement, but also a bullet-proof bottom end.




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