Long read but interesting info on the early Chrysler hemis...
The Origin of the “Hemi” Motor
Hemi Motor Development and Description
In 1951, Chrysler introduced their new line of V8 motors, popularly know as the “hemi”, because the combustion chamber was fully machined into the shape of a ½-dome or hemi-sphere; I will refer to this motor and its direct descendants as Generation I, Type I. This was the first use of this design in an American passenger car, although the principle had been in use in other applications for decades.
The intake and exhaust valves were located on opposite sides of the chamber at 90° to the cylinder bank axis, so separate individual rocker shaft assemblies (two on each head: intake and exhaust) were used to control all sixteen valves by the same camshaft.
The hemi motors produced excellent power, but the expense, additional parts and complexity of the valve gear, as well as the weight of the extremely wide cast-iron heads were not a practical choice for all vehicles.
The cylinders were placed in two banks (rows) of four cylinders each, arranged (as is common practice) at 90° to each other; each is 45° from the vertical axis.
Additional Hemi Motor Development
After the original introduction (limited to Chrysler cars only), DeSoto and Dodge developed and introduced their own unique hemi motors in 1952 (Type II) and 1953 (Type III), respectively.
These motors were smaller than the Chrysler hemis by virtue of smaller bores, closer spacing of the bore (the distance between adjacent cylinders in the block), main bearings (shown right; main cap bolts are shaded) & cam bearings, shorter strokes and lower engine block cylinder banks. In 1964, Chrysler introduced yet another hemi motor (the largest: Generation 2) developed from the “RB” and unrelated except in general theory to the earlier types.
Because of these differences, almost no major parts can be interchanged between brand names or “families”.
In the Chrysler literature, hemi motors are also called dual rocker shaft (“DRS”) while poly motors are called single rocker shaft (“SRS”). The lower-priced 1955-58 Chryslers used a polyspheric motor developed from the Chrysler hemi. No Plymouth used a hemi motor until 1964.
Cylinder Bank Height
As development progressed, larger displacement Type I motors were introduced, the largest using an engine block with taller cylinder banks to permit increased stroke.
Chrysler engines have only two stroke lengths; all “low deck” motors have the shorter stroke crankshaft and shorter connecting rods, and all “raised deck” motor have the longer stroke and longer connecting rods.
The Chrysler raised deck motors had their tappet bank angles re-aligned closer to vertical from 61.5° to 59°. This change aligns the tappets with the new (higher) rocker arm positions and maintain proper valve train geometry. The other Types also used different tappet bank angles for their low deck vs. raised deck engines.
Cams should not be interchanged between low deck and raised deck motors of the same family since the new tappet angle will have the effect of “clocking” or rotating the lobes of each cylinder bank in opposite directions, causing each bank to have slightly different opening and closing points of valve operation with the same duration.
This change limits interchange of intake manifolds and heads between low and raised deck blocks to some degree. However, despite the height difference between the 331-354 and 392 blocks, intake manifolds are interchangeable between motors (except for port size) since the 392 intake ports are extended, so that the port-to-port width remains constant. A 392 using early heads will require a wider manifold or spacers. No manifolds were produced for the reverse combination (331/354 using 392 heads), and will require a manifold to be modified, although I’m not sure if this is possible.
Chrysler Motors Grouped by Deck Height
Low Deck, Short Stroke, Short Rod Raised Deck, Long Stroke, Long Rod
Year Size Year Size
1951-56 331, 354 1957-58 392
Component Function
Valve Train
The hemi motor’s tappets and pushrods are accurately “pointed” at the rocker arm ends for best mechanical efficiency. The intake and exhaust rocker arms are different in length, and pivot in opposite directions.
A true hemisphere is a 50% segment of a full sphere (width = diameter, depth = radius), but the Chrysler hemi’s combustion chamber shape is not a “mathematically correct” hemisphere but a partial section, since the chamber depth is less than the radius of the theoretical sphere, and the chamber width is less than the diameter of the theoretical sphere; the theoretical sphere being the shape from which the actual curvature of the chamber wall is taken.
Although aesthetically pleasing, the Chrysler’s deep chamber (where the depth is a large percentage of the radius, although less than 100%) has proven not to be the most efficient shape, and later motors (such as the “RB” based 426 hemi) used a lower percentage of a hemisphere of larger diameter.
Generally, the width of the chamber is equal to or slightly less than the bore diameter; this provides room for the largest valves. However, chambers of smaller diameter (but not larger) can and have been used successfully.
Hemi Advantages
The hemispherical shape has the lowest “surface to volume ratio” of the possible choices. This means that the amount of heat and energy lost by contact with the surface of the chamber is smaller than any other practical shape with the same volume. This factor was given more importance at the time than is held by current opinion, which favors more shallow chambers.
The hemi combustion chamber shape is also conducive to intake flow from the valve seat, as the chamber wall offers less obstruction than most other choices.
However, the large chamber volume requires a large compression dome on the piston to achieve a high compression ratio. This dome adds to piston weight, obstructs port flow when the piston is near TDC, obstructs flame travel, and loses back part of the surface to volume advantage referred to earlier. This is one reason why the hemi is more successful in supercharged and turbocharged applications than normally aspirated: the smaller and lower piston dome for the 8-1 compression ratio (favored for boosted operation) is less of a problem than the tall and irregularly-shaped dome on a 12-1 piston.
Port Characteristics
In a “wedge” motor, the intake valve stem angle is inclined at only 6-23° from the cylinder bore axis, while the intake port is nearly 90° from the bore axis, so the intake port shape must include the remaining 67-84° curve, which reduces flow. Since in a typical wedge motor the intake and exhaust valve stems are parallel, the wedge exhaust valve is inclined against its port flow direction, so the port curve is closer to a 96-113° curve (this is not as critical since the exhaust flow is under pressure).
Hemi valves have an “included angle” (between the two valve stem axes) of 53°. Each valve is inclined favorably from the bore axis by 50% or 26.5°, so the port curves are reduced to only 63.5° (the valve stem more closely follows the direction of the port centerline), and flow is improved.