Pontiac Cylinder Head ID

        

A.   Cylinder Head Date Code:    The first letter D is the month  ie  A=January   B=February  etc. 

      The next two digits 12 are the day of the month   ie  05=5th    12=12th  etc.

      The last number 8 is the year.    7=1977     8=1978    etc.

B.   Cylinder Head Casting Code Number:  6X         (see below)

C.   Cylinder Head ID Number:   This number 4 (W72 400) reveals the chamber size of the head. An 8 will be found in this position on the Base 400 engine heads. The 403 Oldsmobile cylinder heads use a completely different marking configuration. See Engine ID - Olds 403 section(  Engine Identifiers Section ).

Pontiac Cylinder Head identification on a 1978 Trans Am is expressed as a combination of the casting number and the ID number  ie   6X4, 6X(4) or 6X#8, 6x(8)  etc.

The #4 denotes a cylinder head with an advertised chamber size of about 92cc to 94cc. An 8 indicates a chamber size somewhere between 98cc to 101cc. Installing a #8 head on an engine which had #4 heads will result in a reduced static compression ratio (so long as no other changes were made to engine such as deck milling or a piston change etc).

It is believed, in some circles, that the 6X#4 heads actually have a chamber size of about 89cc. The #4 heads were the same as those used on the Pontiac 350ci engine a couple of years earlier. They were placed on the 1978 W72 400 engine which produced a compression ratio of an advertised 8.1:1. This was an increase from the 7.7:1 compression ratio of the base 180hp 400 engine. Since the #4 heads from the 350ci engine were believed to be closer to 89cc, the actual compression ratio of a 1978 W72 400 is suspected to be in the neighborhood of 8.5:1 instead of the advertised 8.1:1. This point of chamber size is often debated with respect to the 6X#4 heads. The only way to ascertain the true size of the chambers for any particular head is to measure the volume of the chamber. There was an article (2005) from a trusted source which reported that unaltered 6X#4 head chambers measured about 93cc. The magazine conducted the volume testing and reported actual results. 

It is likely that the date code for each head is different. This, however, does not have to be the case. There is no reason that they cannot match.

Visit this site for a listing of Pontiac Head ID info:    Angelfire on Pontiac Heads

Visit this site for a compression ratio chart :             Wallace Racing compression ratio chart

Visit this site for a listing of Intake Manifold #'s:       Wallace Racing intake manifold listing       

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