Small Block Cylinder Heads

From Mopedia

Rebuilding the Chrysler LA engine offers some interesting choices in modern cylinder heads and performance manifolds. Probably the biggest incentive to consider using modern heads is the presence of hardened exhaust valve seats, enabling the use of unleaded fuel without appreciable erosion of the exhaust valve seat. Induction hardening of exhaust valve seats was begun in the early 70s. In addition, the latest heads were designed to optimize combustion to enhance fuel economy. These so-called "swirl port" heads were introduced sometime during the 1985 model year, and were probably offered in versions for 318-2bbl, 318-4bbl, and 360 (truck) applications. Donor cars for these are becoming plentiful in the salvage yards, and with the information contained in this report, the reader can make some informed choices.

Condition

A few words on condition are waranted. Sometimes, heads found on junkyard cars are cracked. Cracks typically form in the area between the valve seats as a result of engine overheating. Therefore, fleet vehicles which have been abused in city driving (like taxis) may not make the best donor vehicles for cylinder heads. Often valves in used heads are reusable. Check the flatness of the end of valve stems as a crude indication of the amount of wear on the valve. Heavy carbon deposits in the exhaust ports are an indication that the engine was running rich, so the likelihood of burned valve seats due to an excessively lean air- fuel mixture might be lower. My observation is that considerable variation in chamber volume can be expected, especially in later castings, which tend to be of lower quality than earlier ones. Differences of one or even two cubic centimeters (ccs) between chamber volumes in the same head can be seen in later heads. Since it is recommended that chambers vary no more than 0.3 cc, it is advisable to check chamber volumes and consider equalizing them before planned machine work is started. Often it is a single chamber that is unequal to the rest. Another indication of casting quality is the surface texture in the chamber, which is much rougher in later castings. Finally, casting or machining defects are sometimes seen. If the holes for valve cover bolts are not jigged accurately, the drill can penetrate into the intake port, resulting in a potential vacuum leak that should be welded shut.

The Heads

Chrysler Small Block Cylinder Head Volumes (cc)

Casting Number	Part Number	Year	Applications	Chamber(cc)	Exhaust Port	Intake Port	Image
2465315 2658920	?	65-66	273	64.5	60	127	None
3418915	?	71-72	340/360 "J"	71	69	149	None
4027596	?	80-84	360	71	65	149	None
4323345	?	85-91	?	74	62	150	None
4323302	?	85-91	?	62	54	118	None
?	P4452758	?	Mopar Performance	62	54	118	None

Production heads used on 1964-66 LA engines have closed chambers 57-65 cubic centimeters (cc) in volume. The pair I examined had chamber volumes averaging 64.5 cc. Exhaust ports average 60cc in volume and intake ports average 127cc in volume. The valves are 1.78 inches in diameter (intake) and 1.50 inches in diameter (exhaust).

For 1971 and 1972, 340 and 360 engines were equipped with 3418915 heads, so-called "J" heads, because of the cast-in J in three locations (backwards in one place) on each head. These were either equipped with 1.88"/1.60" valves, or 2.02"/1.60" valves, and have "open" chambers with volumes of 65-73 cc. Open chambers have a circular margin. The heads I examined had the larger valves and an average chamber volume of 71 cc. The ports are large, averaging 69 cc (exhaust) and 149 cc (intake).

Police sedans were equipped with a high performance 318 engine, which was equipped with 360 heads and a 4 bbl carburetor (Thermoquad through 1984, Quadrajet from 1985). The earlier ('80- '84) heads have a 4027596 casting number and a cast-in "360" on the top of an intake runner. It has an open combustion chamber with a volume of between 66 and 72.5 cc. The pair of heads I measured averaged 71 cc. The valves are 1.88"/1.60" and the ports are large, averaging 65cc on the exhaust side and 149cc on the intake side. Note how similar the volumes of these heads are compared to the performance heads of the early 1970s. Police engines are equipped with flat top pistons with no valve reliefs. Calculated compression ratio is 8.4:1.

Beginning in 1985, police sedans were equipped with a different 360 head, 4323345, with larger pushrod holes, 11/16" in diameter, instead of 1/2" as found on all earlier heads. The larger pushrod holes are to accommodate hydraulic roller lifters, which were introduced in 1985. The chamber is open and its volume is slightly larger than the earlier 360 heads, 69 to 77 cc. The two heads I measured averaged 74 cc. Port sizes are very similar to earlier heads, and the larger pushrod hole does not narrow the intake port relative to the earlier heads. These heads also have 1.88"/1.60" valves. Because the piston pin height is 0.020" greater, the calculated compression ratio is similar to earlier engines, even though the chambers are larger. It has been suggested that the 345 heads are a swirl port design, but the port and chamber shape is indistinguishable from earlier heads.

It may be problematic to use the 345 head on certain early applications because of the large combustion chamber. In order to preserve the compression ratio, excessive amounts may need to be planed off the deck surface of the head or off the block deck. For any open chamber head, the chamber volume is reduced about 0.2 cc for each 0.001" planed off the deck surface of the head. If more than 0.010" is removed from the deck surface, the intake surface will need to be milled to allow the intake manifold to fit. Mill 0.0095" from the intake surface for each 0.010" milled off the deck surface of the head.

The 318-2bbl heads (4323302) used from 1985 on are a swirl port design with a closed (heart-shaped) combustion chamber design with a chamber volume of between 56 and 65 cc. The 4 heads I examined averaged 62cc in volume. The 302 head has 1.78"/1.50" valves and small ports averaging 54cc on the exhaust side and 118cc on the intake side. The intake ports have a more severe dogleg than earlier heads because the holes for the pushrods are larger - 11/16". Cars equipped with the 302 head have a dished piston to keep the compression ratio from being too high. Some cars left the factory with nail head exhaust valves in 302 heads, others with semi-tulip exhaust valves, which add 0.6-0.75 cc to the chamber volume. There is an interesting excerpt in "Mopar Engines", page 72, describing how such a head was ported and made to flow as well or better than other small block cylinder heads. Apparently, this experimentation resulted in the master for today's Mopar Performance P4452758 cylinder head.

References

Originally compiled by Paul M. Pitcher

• Shepard, Larry (1989). How to Hot Rod Small Block Mopar Engines , HPBooks
• Shepard, Larry. Mopar Engines 8th edition, Mopar Performance
• Schreib, Larry (1991). How to Build Dodge and Plymouth Performance, SA Design

The work of Bob Mullen, one of the principal designers of the W2 cylinder head is highlighted.