Pontiac V8 engine
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===Combustion chamber volume=== | ===Combustion chamber volume=== | ||
| − | One of the most important things that need to be known about the heads is the exact chamber volumes. First, look to see that the secondary identifiers are the same. Even if they are, | + | One of the most important things that need to be known about the heads is the exact chamber volumes. First, look to see that the secondary identifiers are the same. Even if they are, '''''both''''' heads need to be checked- although doing the end chambers of each head (four chambers total) will be enough to tell that the chambers are equal. This is necessary because there's no way to be sure the heads were '''always''' a pair from day one, onward. Even if they ''were'' paired from birth, one head might have had a bad head gasket that required it to be resurfaced. If '''both''' heads weren't cut an equal amount that can cause the chambers to vary. If the heads were from different engines originally, obviously they can vary due to all the above reasons plus manufacturing and casting differences from the factory. |
| − | The combustion chamber volume can be changed by milling the head deck. One cc will be removed per 0.005" removed from the deck. All heads can be safely milled 0.050". The 1971 96 and the 1972 7K3 heads can be safely cut 0.070" and the 1971-'73 round port heads can be cut 0.085". If 0.030" or more is removed, the intake side of the head should be cut equally to keep the port and bolt holes in alignment. The chamber volume should be rechecked after a valve job; most times a valve job on the same size valve (not replacing a smaller valve with a larger valve) will add 2-4cc to the chamber volume. | + | The combustion chamber volume can be changed by milling the head deck. One cc will be removed per 0.005" removed from the deck. All heads can be safely milled 0.050". The 1971 #96 and the 1972 #7K3 heads can be safely cut 0.070" and the 1971-'73 round port heads can be cut 0.085". If 0.030" or more is removed, the intake side of the head should be cut equally to keep the port and bolt holes in alignment. The chamber volume should be rechecked after a valve job; most times a valve job on the same size valve (not replacing a smaller valve with a larger valve) will add 2-4cc to the chamber volume. |
There were three sizes of combustion chambers on 6X heads. For most builds, the small chamber head is what is wanted. The late large chamber 455 heads on a 400 will put the compression ratio in the 7.8:1 range, even with a 0.040" quench. The small chambered 6X and similar head on a 0.030" over 455 with a 0.040" [['''quench''']] will result in a CR of about 9.75:1. On a 400 the CR will be around 9.2:1. | There were three sizes of combustion chambers on 6X heads. For most builds, the small chamber head is what is wanted. The late large chamber 455 heads on a 400 will put the compression ratio in the 7.8:1 range, even with a 0.040" quench. The small chambered 6X and similar head on a 0.030" over 455 with a 0.040" [['''quench''']] will result in a CR of about 9.75:1. On a 400 the CR will be around 9.2:1. | ||
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Be aware that published figures often differ. The only sure way to know what the combustion chamber volume is, is to actually measure it. This will take everything into account: previous valve jobs, milling done to the decks, manufacturing and casting tolerance creep/stack up, etc. | Be aware that published figures often differ. The only sure way to know what the combustion chamber volume is, is to actually measure it. This will take everything into account: previous valve jobs, milling done to the decks, manufacturing and casting tolerance creep/stack up, etc. | ||
| − | <table border="1" cellspacing="1" cellpadding="1" bgcolor="#FFF8DC"><tr><td colspan="5" align="center"><h3>Pontiac Cylinder Head Applications</h3></td></tr><tr><td><strong>Casting</strong></td><td><strong> | + | <table border="1" cellspacing="1" cellpadding="1" bgcolor="#FFF8DC"><tr><td colspan="5" align="center"><h3>Pontiac Cylinder Head Applications</h3></td></tr><tr><td><strong>Casting</strong></td><td><strong>Secondary ID</strong></td><td><strong>Valve diameter (inches)</strong></td><td><strong>Chamber volume</strong></td><td><strong>Original application</strong></td></tr><tr><td>4X</td><td>1/1H</td><td>2.11/1.66</td><td>112 cc</td><td>1973-'74 455ci 4-bbl</td></tr><tr><td>4X</td><td>3/3H</td><td>2.11/1.66</td><td>98-99 cc</td><td>1973-'74 400ci 4-bbl manual trans.</td></tr><tr><td>4X</td><td>4/4H</td><td>1.96/1.66</td><td>98-99 cc</td><td>1973-'74 400ci 2-bbl</td></tr><tr><td>4X</td><td>7/7H</td><td>2.11/1.66</td><td>98-99 cc</td><td>1973-'74 400ci 4-bbl auto trans.</td></tr><tr><td>4C</td><td>5/5H</td><td>1.96/1.66</td><td>96 cc</td><td>1973-'74 350ci 2-bbl w/A.I.R.</td></tr><tr><td>4C</td><td>8/8H</td><td>1.96/1.66</td><td>98 cc</td><td>1973-'74 400ci 2-bbl w/A.I.R.</td></tr><tr><td>46</td><td>6/6H*</td><td>1.96/1.66</td><td>96 cc</td><td>1973-'74 350ci</td></tr><tr><td>5C</td><td>4</td><td>2.11/1.66</td><td>92-94 cc</td><td>1975 350ci</td></tr><tr><td>5C</td><td>7</td><td>2.11/1.66</td><td>99-101 cc</td><td>1975 400ci w/A.I.R.</td></tr><tr><td>5C</td><td>8</td><td>2.11/1.66</td><td>99-101 cc</td><td>1975 400ci</td></tr><tr><td>5C</td><td>9</td><td>2.11/1.66</td><td>92-94 cc</td><td>1975 350ci w/A.I.R.</td></tr><tr><td>6X</td><td>4</td><td>2.11/1.66</td><td>92-94 cc</td><td>Mid-1975-'77 350ci; 1977-'79 W72 400ci</td></tr><tr><td>6X</td><td>8</td><td>2.11/1.66</td><td>99-101 cc</td><td>Mid-1975-'78 400ci</td></tr><tr><td>6S</td><td>7</td><td>2.11/1.66</td><td>99-101 cc</td><td>Mid-1975-'76 400ci w/A.I.R.</td></tr><tr><td>6S</td><td>9</td><td>2.11/1.66</td><td>92-94 cc</td><td>Mid-1975-'76 350ci w/A.I.R.</td></tr><tr><td>51</td><td>6</td><td>2.11/1.66</td><td>124.5 cc</td><td>1975 455ci</td></tr><tr><td>6H</td><td>6</td><td>2.11/1.66</td><td>124.5 cc</td><td>Late-1975-'76 455ci</td></tr><tr><td colspan="5">*Some have been reported to have a secondary stamp of "2"</td></tr></table> |
An alternative to using the larger chamber heads is to install dished pistons and use any of the smaller chambered performance heads. | An alternative to using the larger chamber heads is to install dished pistons and use any of the smaller chambered performance heads. | ||
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