[billkinney]

I found a set of 6x-8 heads. The only other casting marks are GM2 and a146. What will these heads add to my Basic stock 67 400 if they are polished and matched to a performer intake ? 8.5 to 1 pistons , rv cam, turbo 400 and 3.90 posi. with 3" pypes. Are these large chamber? Currently I have 4x heads. Not looking for wheel stands, just a little something special.
Thanks for your help.

[ponyakr]

6x-8 heads were rated at 7.6 CR on a 400. Some charts show them to have about 101 cc chambers. Without shaving quite a bit off of 'em, about the best you can do is use the Cometic .027 x 4.160 head gaskets and run one of the small CC XE or Lunati Voodoo cams. But, if the shortblock is not fresh, increasing CR or cylinder pressure could possibly cause excessive blowby.

Not a whole lot of difference in 4x and 6x heads. Some say the 4x are more prone to cracking between the valves. The 6x heads seem to be the most popular iron heads in Pontiac circles.

Most seem to hate domes, in a Pontiac. They talk about flame travel and more detonation. Then you'll read that others have run the small domes with good success, making more power with the same heads. But if you rebuild the shortblock, some small domes, like the cheap SP's will increase your CR.

http://www.summitracing.com/parts/sl...nf30/overview/

You can use the Wallace CR calculator to figure your CR.

http://www.wallaceracing.com/cr_test2.php

[rexs73gto]

Depending on which 4X heads you have using the 6X heads , the only thing your going to do is get some praticce in replacing heads. The heads were used for the same engine usage , to keep the compression down . Both set are going to give you about 7.8-8.2 compression so you won't be gaining anything. If you want better heads look for some # 12's, 13's, 96's or 48's but be aware that when you go up in compression you run the chance of spark knock. Your best bet is to save up & buy a good set of Alum. heads that way you won't have to worry when you go bigger. & faster as most end up doing.

[Overkillphil]

X2. Save the 6x-8's for a possible larger displacement (455>) future build.

[Ollie]

Read the sticky at the top of this page

[Overkillphil]

Originally Posted by Ollie

Read the sticky at the top of this page

Ollie's correct.

This should be moved/posted in the Street section.