almx33
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Pinched from a Mopar Muscle article:
While the 906 and its open-chamber configuration presented a small flow improvement over the previous year's 915, it was a step in the wrong direction in terms of performance. Racers in the know have long coveted the 915's closed-chamber heads, not for better flow, or even the higher compression offered by the closed chamber's smaller volume. The racers value the heads because of the superior combustion characteristics associated with the closed chambers. The 906, and all later heads that shared the same open-chamber configuration, did have an advantage in one area--lower emissions. The less intense burn and less effective scavenging resulted in reduced levels of nitrogen oxides (NOx). The 906 casting was retained as the B/RB head for 1969 to 1970 with no notable changes.
In 1971, a new casting was introduced--the 346, again used across the board on all big-blocks, B, and RB (although some early 1971s were reputed to have slipped through with the 906s). Although the 915 of 1967, and 906 of 1968 to 1970, had the same ports, the new 346 was a significant departure in terms of intake port configuration. Exhaust ports were essentially unchanged. Factory information on the 346 casting, as published in the old Direct Connection racing manuals, and carried over into today’s Mopar Performance engine book, was limited to one line: "In 1972 (and late 1971), the new emissions head with the flatter intake port was introduced on all ’B’ engines, and has a casting number of 3462345." Other than printing the wrong year and casting number, it was clear that this was a smog head to be avoided. No supporting information was provided, but in the minds of Mopar enthu-siasts, the written words have long been accepted as gospel. Interestingly, these same smog heads were the recommended heads for Grand National racing when the Wedge returned to NASCAR, and in modified form were used quite a bit by Richard Petty in his domination of the circuit in the early 1970s. The 346 head was retained as the standard big-block head through 1973.
From 1974 to the end of big-block production in 1978, several different castings were used--the 902 in 1974, the 975 in 1975, and the better-known 452 from 1976 to 1978. All these heads were minor variations on the 346 casting from 1971, with differences associated with durability details such as hardened valve seats, improved crack resistance, and an enlarged guide boss in the valve seal area. The intake port remained unchanged from the 1971 346 casting. The exhaust port was never significantly changed and remained true to the form introduced back in 1967 on the 915 heads. To sum up, two intake port configurations were used beginning in 1967 on big-block heads. The first was the 915 closed-chamber head and its successor, the 906 open chamber. The second intake port type was introduced in 1971 on the 346 open-chamber head. This port had a substantially lowered roof and greatly reduced short side, with a "Humber Hump" configuration on the cylinder wall-side floor. This port was carried through on the 902, 975, and the last of the production big-block heads--the 452, all of which were open-chamber designs. On the exhaust side, the 915, 906, 346, 902, 975, and 452 all featured the same basic port. As a result of the need to run unleaded fuel, retrofitting to the late 452 heads, with their induction-hardened valve seats, became a popular swap on earlier engines. Interestingly, the 452 gained a reputation as a desirable performance head casting; through experience, Mopar enthusiasts discovered the performance was on par to that of the esteemed 906 castings of the musclecar era. This regard did not return to the earlier 346 heads, even though the port configuration was virtually identical.
While the 906 and its open-chamber configuration presented a small flow improvement over the previous year's 915, it was a step in the wrong direction in terms of performance. Racers in the know have long coveted the 915's closed-chamber heads, not for better flow, or even the higher compression offered by the closed chamber's smaller volume. The racers value the heads because of the superior combustion characteristics associated with the closed chambers. The 906, and all later heads that shared the same open-chamber configuration, did have an advantage in one area--lower emissions. The less intense burn and less effective scavenging resulted in reduced levels of nitrogen oxides (NOx). The 906 casting was retained as the B/RB head for 1969 to 1970 with no notable changes.
In 1971, a new casting was introduced--the 346, again used across the board on all big-blocks, B, and RB (although some early 1971s were reputed to have slipped through with the 906s). Although the 915 of 1967, and 906 of 1968 to 1970, had the same ports, the new 346 was a significant departure in terms of intake port configuration. Exhaust ports were essentially unchanged. Factory information on the 346 casting, as published in the old Direct Connection racing manuals, and carried over into today’s Mopar Performance engine book, was limited to one line: "In 1972 (and late 1971), the new emissions head with the flatter intake port was introduced on all ’B’ engines, and has a casting number of 3462345." Other than printing the wrong year and casting number, it was clear that this was a smog head to be avoided. No supporting information was provided, but in the minds of Mopar enthu-siasts, the written words have long been accepted as gospel. Interestingly, these same smog heads were the recommended heads for Grand National racing when the Wedge returned to NASCAR, and in modified form were used quite a bit by Richard Petty in his domination of the circuit in the early 1970s. The 346 head was retained as the standard big-block head through 1973.
From 1974 to the end of big-block production in 1978, several different castings were used--the 902 in 1974, the 975 in 1975, and the better-known 452 from 1976 to 1978. All these heads were minor variations on the 346 casting from 1971, with differences associated with durability details such as hardened valve seats, improved crack resistance, and an enlarged guide boss in the valve seal area. The intake port remained unchanged from the 1971 346 casting. The exhaust port was never significantly changed and remained true to the form introduced back in 1967 on the 915 heads. To sum up, two intake port configurations were used beginning in 1967 on big-block heads. The first was the 915 closed-chamber head and its successor, the 906 open chamber. The second intake port type was introduced in 1971 on the 346 open-chamber head. This port had a substantially lowered roof and greatly reduced short side, with a "Humber Hump" configuration on the cylinder wall-side floor. This port was carried through on the 902, 975, and the last of the production big-block heads--the 452, all of which were open-chamber designs. On the exhaust side, the 915, 906, 346, 902, 975, and 452 all featured the same basic port. As a result of the need to run unleaded fuel, retrofitting to the late 452 heads, with their induction-hardened valve seats, became a popular swap on earlier engines. Interestingly, the 452 gained a reputation as a desirable performance head casting; through experience, Mopar enthusiasts discovered the performance was on par to that of the esteemed 906 castings of the musclecar era. This regard did not return to the earlier 346 heads, even though the port configuration was virtually identical.
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