 In
1963, Chrysler increased the size of the 413 to 426 cubic inches. On paper this change
resulted in a 5 horsepower increase (from 420 to 425 on the 13.5:1 engine), although
records show that performance on the drag strip remained basically the same.
Chrysler fired the shot heard around the industry in 1964, as this was the year the mighty
426 Race Hemi emerged. Developed mainly as the result of Ford’s 1-2-3 sweep at
Daytona in 1963, Chrysler engineers adapted the mid-’50s hemi head onto the stout RB
bottom end. This is, of course, an oversimplification, since a great deal of development
work was done to the heads in the interest of performance and reliability. Valve sizes
were a substantial 2.25” intake and 1.94” exhaust, and the ports had very little
curvature for great breathing, an inherent benefit with the hemispherical layout.
The Race Hemi featured a revised bottom end with a forged, shot-peened, magnafluxed and
nitride-treated (a chemical hardening process) crankshaft. The main bearing caps were
cross-bolted, meaning two bolts held each cap in the standard location and two more ran
through the side of the block and tied into the main cap horizontally. This secured the
main caps into the block webbing for increased rigidity. The connecting rods were
strengthened considerably over the wedge-engine pieces, and rod bolt diameters increased
to 7/16”. Compression was set at 12.5:1, and of course very strong forged pistons
were used. The induction system varied upon the intended use of the engine; NASCAR engines
used an aluminum dual-plane high-rise manifold with a single Holley 4-bbl. Engines
intended for drag-strip duty used an aluminum cross ram with dual Holleys. The mechanical
camshaft sported a stout .565” lift intake and exhaust, with duration figures of
328°. By the way, Hemi-powered cars finished 1-2-3-4 at the 1964 Daytona 500...mission
accomplished.
 Strength and rigidity are hallmarks of the Hemi
bottom end. This cutaway drawing shows the deep-skirt block design and the cross-bolted
main caps, two of the reasons the engine can harness all the power made by the
hemispherical heads. |
Developments for 1965 included a new magnesium cross-ram
intake for the drag version of the Race Hemi, and to further lighten the heavy powerplant
aluminum heads were used. NASCAR changed the rules for the 1965 season, specifying that
the manufacturers had to produce at least 500 engines and offer them for sale to the
general public through regular dealer outlets. Chrysler had only planned on building 20-30
Race Hemis per year, and thus boycotted the 1965 NASCAR season. This would explain the
additional development for drag racing and not oval track racing. Chrysler would return to
NASCAR however, and with the Hemi. They simply had to put the engine into production. In
doing so they created the ultimate muscle car powerplant -- the 426 Street Hemi.
The conversion from racing engine to street engine was not as extensive as it would seem.
The very same block, crank and rods were used, as were the heads (cast iron, not
aluminum). The compression ratio was dropped to a more livable 10.25:1, and the camshaft,
although still mechanical, made use of .480” intake and .460” exhaust lift with
276° duration. This allowed lighter, and longer lasting, valve springs to be used. An
aluminum intake manifold with dual inline-mounted Carter carburetors provided the fuel.
Since the Hemi’s heads had no provisions for carburetor heat, a key ingredient for
cold-weather driveability on a street engine, engineers routed steel tubes from the RH
exhaust manifold to the back of the Street Hemi’s intake. It worked beautifully.
Chrysler tagged the Street Hemi’s output at 425 horsepower, a rating it would keep
throughout its production life (which ended in 1971). A few changes were made along the
way, most notably a block strengthening which allowed the engine to be used with
nitro methane, and in 1970 a hydraulic camshaft (specs were virtually identical to the
mechanical version). Designed for the race track, the fabled 426 Hemi ended up being the
ultimate street engine.
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