How are you all doing i have a quick question for someone which heads will be better for my pontiac 400 6x heads or # 13 the guy building my motor was telling me for awhile to get 6 x heads now he is singing a different tune after i got them he is tryin to swap them out for the number 13 heads what should i do i believe he ripped me off on the machine work already so please help me figure out something about the heads :nono:

 

6X are a great flowing head, but will have your compression pretty low on a 400, around 8- 8.5:1 the 13 is a great head look for casting numbers 12,13,18,48,62, those are the best heads for 400's they have the 2.11/1.77 valves and will give you about 10-10.5:1 compression.

 

Well thank you for the reply i appreciate all the help i get from you guys on here this sight is really great .:lala:

 

what year are the #13 heads? make sure your heads are smog happy (come with hardened exhaust seats. 6X will have these)

Lower compression will let you add turbo or supercharger more readily. If not doing these, go with "prepped" heads & bump compression. My Old 77T/A had 6X-4 heads & was rated @ 8-8.5:1 compression. my 76 bonneville, has 6X-? heads, as is rated @ 7.6:1 comp. kicks a$$ now, wait till I do a manifold & cam swap. add supercharger later

 

the 6x-4 head was only used on 75-78 350 pontiac engines mostly in the big cars, all the t/a 400's got the 6x-8 which has slighty bigger chamber to bring the compression down.

 

#s for 400 motors. ref: High Performance Pontiac -Pontiac Tech Mag #1, summer 2000.

yr code valve size Chamber size comments
70 13 2.11/1.77 72cc standard RAIII
76-79 6X 2.11/1.66 101cc standard (-8?)

6X also on 350 w/91cc chamber.

maybe the T/A had 350 heads on them, but the heads are the 6X-4 ones. Heads on bonneville will wait untill the -4 heads are ready for swap.