Static compression, (C.R.) on a Pontiac is a function of the chamber volume and the engine displacement. If the chamber volume is increased, the C.R. goes down; if the displacement is increased, the C.R. goes up.

Following is a listing of the chamber volume in cubic centimeters (cc) of selected Pontiac heads used from 1966 to 1979.

COMBUSTION CHAMBER VOLUME

YEAR HEAD  I.D. CC –
66 092(389,421) 70 –
66 093(389GTO,421HO) 68 –
67 670(400,428) 72 –
67 97 (400 RA) 72 –
68 62 (400,428) 72 –
68 16 (428) 72 –
69 16 (400,428)- 72 –
69 48 (400 RAIII) 72 –
69 722(400 RAIV) 71 –
70 12 (400 RAIII) 72 –
70 13 (400) 72 –
70 614(400 RAIV) 71 –
70 64 (455) 87 –
71 96 (400) 96 –
71 66 (455) 114 –
71 197 (455 HO round port) 111 –
72 7K3 (400) 96 –
72 7M5 (455) 114 –
72 7F6 (455 HO round port) 111 –
73/74 4X (400, casting 488534) 98 * #
73/74 4X (455, casting 488541) 114 * #
73/74 16 (455 HO round port) 111 *
76/79 6X (400) 101 *

(*) Contains 1.66″ exhaust valves rather than 1.77″.

(#) It is necessary to measure the chamber volume for positive ID of these two different heads.

Following is a chart of compression ratios based on the most popular engine displacements and different sizes of combustion chambers (specified in cc’s).

Note:

The indicated CR’s are for planning only, and complete calculations using all variables are recommended Prior to assembling any engine.

COMPRESSION RATIOS

Cylinder Head Volume

DISP 60 65 70 75 80 85 90 95 100 105 110 115
350 9.97 9.45 8.98 8.57 8.19 7.85 7.54 7.26 7.00 6.77 6.55 6.34
400 11.11 10.52 10.00 9.52 9.10 8.72 8.37 8.06 7.77 7.50 7.25 7.02
406 11.26 10.66 10.13 9.65 9.21 8.84 8.48 8.16 7.87 7.57 7.34 7.10
428 11.04 10.5 10.0 9.57 9.17 8.8 8.47 8.16 7.88 7.21 7.38 7.15
455 12.52 11.85 11.26 10.72 10.24 9.81 9.41 9.05 8.71 8.41 8.13 7.86
462 12.68 12.01 11.40 10.85 10.37 9.91 9.53 9.16 8.82 8.51 8.23 7.96

These C.R.’s are for new heads with stock type pistons, nominal deck height, and .042″ (crushed) head gaskets.

The 428 engines also had pistons with dished tops and value relief’s in order to lower the compression so the 400 heads could be used. That is the reason the actual compression for 428’s is lower than the 400’s in some cases.

Note that Pontiac normally overrated the compression by about .5 point.

The volume of the chamber can be changed by milling the head surface. One cc will be deleted for each .005″ removed from the head. All heads can be safely milled .050″. The 1971 96 and the 1972 7K3 heads can be safely cut .070″ and the 1971/73 round port heads can be cut .085″. If .030″ or more is removed, the intake side of the head should be cut equally to maintain port/bolt alignment. Note that a normal valve job sinks the valves about .025″ which adds 2 to 3 cc to the combustion chamber.

Dome type pistons were not used by Pontiac. They will increase compression, but they also disturb the operation of the machined combustion chamber. Any power gained by the increased compression very likely would be lost by poorer combustion efficiency due to the misshapen piston.

Will unleaded gas destroy the valve seats in your valuable heads? Personally, I have never heard of a single instance of a valve seat problem caused by unleaded gas in a passenger car. I have felt that heavily loaded vehicles running almost continuously, such as trucks and tractors, could have a problem.

In the March, 1992 issue of “Cars And Parts” Magazine, “Chevron”, a West Coast gasoline producer, furnished information on old car performance. “Cars And Parts” stated: “Addressing another issue of concern to hobbyists – valve sink or recession, caused by no-lead gasoline – Chevron said most old-car owners need not worry. The majority of passenger cars and some light duty trucks are safe from the problem because they are not subjected to severe enough driving conditions.”

“Only when a car is operated at high speeds and/or with unusually heavy loads for an extended period of time, is the use of a lead substitute warranted, according to Chevron.”

“Although higher-octane unleaded gasoline’s can be used successfully under normal operating conditions, adding a lead substitute is a good idea for cars operated under extreme conditions, Chevron said.”

Should you change your exhaust valve seats? I suggest you leave the old seats alone until they are worn out from normal use, then replace them with the modern hardened seats.

How important is the exhaust valve size on the 455, Not nearly as important as you might think. In later issues, we will explain how the 455 exhaust system is not as critical as is the 400. Also, the 455 power range is much lower than the 400 and the 1.66″ exhaust valve may work as well as the 1.77″ on the 455 street cars. This means the later 98cc #4X or the ’76/79 6X heads could be a viable substitute for the ’71 #96 or the ’72 #7K3.