78 Grand Prix project

[papabearxl]

Nice car.......keep us updated. Sounds like you are well on your way!

 

[Bonnewagon]

A 70' 400 will have high compression heads. A nice set of later 70's 4X's or 6X's would allow you to use pump gas. They flow almost as good as the 71'-72' low comp big valve heads. Those early heads are the ticket but lack the end bolt holes for the exhaust manifolds. The matching manifolds for those years are bevelled to press tight on the ends negating the need for the end bolts. Another GM brilliant move.

 

[G-Body_Vet]

A 70' 400 will have high compression heads. A nice set of later 70's 4X's or 6X's would allow you to use pump gas. They flow almost as good as the 71'-72' low comp big valve heads. Those early heads are the ticket but lack the end bolt holes for the exhaust manifolds. The matching manifolds for those years are bevelled to press tight on the ends negating the need for the end bolts. Another GM brilliant move.

I guess that's one more reason to find 6X heads. I was more concerned about the heads on the '70 not having hardened valve seats. There's another guy with a 400/2bbl but he doesn't know what it's out of. I'm waiting to see if he sends me the casting numbers.

 

[Bonnewagon]

Watch for the 3rd digit following the 4X or 6X, it has meaning but I would have to look up a number breakdown to know what is what. I have a set of 6X-8's that are on a 70' 400 GTO motor. Ran like a champ in my Malibu wagon and I could burn rat piss gas with no detonation. The early non-EGR intake manifolds (pre-73') are the best flowing but there is an exhaust crossover port mismatch with the later heads. I know the 72' (non-EGR) intakes will fit the later heads without the adapter. Here is an article by Jim Hand explaining it. His other articles are loaded with great info too. http://www.bafb.org/1968firebird.org/jharticles/jharticles9.htm

 

[tdvick]

This is a good site for more engine related parts. Very much knowledge for both street and race applications too.

http://forums.performanceyears.com/forums/

 

[G-Body_Vet]

Watch for the 3rd digit following the 4X or 6X, it has meaning but I would have to look up a number breakdown to know what is what. I have a set of 6X-8's that are on a 70' 400 GTO motor. Ran like a champ in my Malibu wagon and I could burn rat piss gas with no detonation. The early non-EGR intake manifolds (pre-73') are the best flowing but there is an exhaust crossover port mismatch with the later heads. I know the 72' (non-EGR) intakes will fit the later heads without the adapter. Here is an article by Jim Hand explaining it. His other articles are loaded with great info too. http://www.bafb.org/1968firebird.org/jharticles/jharticles9.htm

The casting list I made reference to shows the 6X-4 and 6X-8 were on engines with different hp levels. The blocks with the 6X-4 had 8.0 to 1 comp, wheras the 6X-8 had 7.6 to 1 comp. Not sure what the difference was though. I haven't checked to see if they had different chamber sizes or not....I doubt it knowing GM.

 

[tdvick]

6X-4 is a smaller chamber. Those are the preferred heads to get. They were on 350s and the HP 400s to add the slight increase in compression.

 

[G-Body_Vet]

6X-4 is a smaller chamber. Those are the preferred heads to get. They were on 350s and the HP 400s to add the slight increase in compression.

Thanks. Now I know what I'll be looking for after I find a good block.

 

[tdvick]

As Bonnewagon said, the '72 intake is also the the best one to get. It's the only non-EGR intake that matches up to the exhaust crossovers in the heads. I picked up both the 6X-4s and a '72 intake this summer for my Lemans.

 

[G-Body_Vet]

As Bonnewagon said, the '72 intake is also the the best one to get. It's the only non-EGR intake that matches up to the exhaust crossovers in the heads. I picked up both the 6X-4s and a '72 intake this summer for my Lemans.

What about the Edelbrock or Crosswind intakes? Any fitment issues there? I'm all about saving weight so I don't plan on running a cast iron intake manifold.