P51 Cylinder Heads - Precedent Ford

Huge Power For Big-Block 460 Fords Is A Bolt-On Deal With These Breakthrough P51 Cylinder Heads From Jon Kaase Racing.

By Steve Dulcich

Photography by Johnny Hunkins

Of the domestic big-block engines, the 429/460 Ford 385-Series came to the party late in the game. Ford depended on the ancient but effective FE-series of engines for big-inch power through most of the '60s musclecar era, introducing the all-new big-block design in 1969. This new fat-block had commendable features, including a wide 4.900-inch bore spacing for man-sized bores, a healthy 10.320-inch deck height, and a spacious crankcase to swallow a burly long-stroke crank. Up top, the engine featured a canted-valve layout and huge ports, borrowing design themes from the famously successful Cleveland small-block series. It seemed like a design with tremendous power potential, and in Cobra Jet and Super Cobra Jet 429 form, the power output was noticeable (with rated outputs of 370 and 375, respectively). Ford's Total Performance attitude didn't stop there, either. Looking for the ultimate weapon in the NASCAR wars, the Boss 429 was created in 1969, and featured a radical canted-valve semi-Hemi cylinder head layout with the intention of fully exploiting the new big-block's power potential.

Ford's reasoning was well directed with the Boss 429. But for all the positive attributes of the Ford's big-block architecture, and given the promise of power offered by the base canted-valve heads and cavernous ports, the ultimate power available from the new design just didn't cut it. Ford's solution was a radical departure in the cylinder head layout-but why? Though the conventional 385-Series heads seemed like a conceptual winner, there were flawed design elements clear in retrospect. Foremost, the valve placement put the centerline of the valve faces too far toward the plug side of the chamber, crowding the outside of the cylinder bore. Combined with the valve angle, this layout did nothing to take advantage of the naturally spacious bores of the bottom end. The problem is readily evident on the intake side, and is especially acute with the exhaust valve, buried deep within the curvature of the bore's outside corner.

From the standpoint of taking advantage of airflow for maximum power, the conventional 429 and 460 cylinder head was a loser, but this shortcoming became inconsequential as the era of maximum factory performance drew to a close. The big-block Ford 429 and its longer-stroke brother, the 460, came to serve primarily as torquey powerplants in huge Ford, Lincoln, and Mercury luxury cars (and as a truck motor). So much promise seemingly squandered.

The Fix Is In
In the '80s, Ford Motorsport (now Ford Racing) took a step forward in the power evolution of the big-block, introducing the aluminum Cobra Jet head. Borrowing its name from the high-performance big-block designation of the '60s, the Cobra Jet head was a conventional 385-Series in layout, but featured better ports and greater airflow when compared to OEM production heads. Although the Cobra Jet heads were an improvement, the fundamental shortcomings of the original design layout had not been addressed. All of that changed with the introduction of the Ford Motorsports Super Cobra Jet heads in 2001.

Under development at Jon Kaase Racing, this new design finally fixed the basic flaws in the original Ford design. Kaase pulled the valves back toward the intake side of the chamber, where they could open in a more geometrically favorable position in relation to the cylinder bore. Along with the dramatic change in valve position, the valve angle was altered slightly on the intake side and drastically on the exhaust side. This new Kaase layout unlocked the power potential of the big-block Ford, and the modified version of the conventional big-block Ford layout was now capable of exceeding the 350-cfm peak intake flow of the exotic Boss 429. It was really a validation of the original canted-valve wedge concept.

The P51
Not content to remain idle, Kaase recently went to work to revise the big-block Ford head once again. This effort builds on the format originally introduced with the Super Cobra Jet head, as it is more evolutionary than revolutionary. Naturally, the Super Cobra Jet head has benefited from six years of development since its introduction, and its as-cast potential was just the beginning, in terms of power and flow. Fully ported, the Super Cobra Jet head was capable of high 300s on the intake flow scale. With the P51, the idea was to incorporate the gains made in port development on the Super Cobra Jet into an all new casting; essentially, the P51 was to be an "as-cast" piece designed to resemble the ports of a fully ported Super Cobra Jet. Material has been added and improvements made to address issues from the previous series of cylinder heads. Along with the revised ports, the P51 takes the combustion chamber to the latest figure-eight shape, filling in a considerable portion of the "dead" area behind the spark plug.

Kaase's new P51 head features a fully CNC-machined chamber, measuring the same 72 cc as the previous generation cylinder head. Other than the redesigned 310cc intake ports, the intake valve diameter has been increased from the earlier head's 2.200 inches to 2.250 inches. The exhaust ports measure 145 cc and carry 1.76-inch diameter valves, and the port throats below the valves come machined and hand-blended (though the remainder of the port comes strictly as-cast). With intake flow of around 400 cfm and an extraordinarily fat flow curve throughout the range (see flow chart), these heads seem to have what it takes to make outrageous power.