ZL-1 & L-88 head design clarification

[ANDY M]

Closer view.

[markjohnson]

That rectangular port/open chamber #990 cylinder head was not around in 1969 or 70. That and the similar #188 are what came on the newer crate LS-6's and LS-7's. I've seen reference to this so-called cast iron ZL/X head and would like to know what the casting number is. The earliest cast iron rect. port/open chamber head I know about and have owned were 1971 dated #026. They are also listed in Colvin's book as standard issue for the 1971 LS-6 Chevelle. A car that never happened so they must have been over the counter service pieces. The 1971 Corvette LS-6 all used the aluminum rect. port/open chamber head #074, the same as found on the 1969 L-88 and ZL-1 Corvette.

[ANDY M]

According to Colvin, there were 3 #'s starting in '67 that had this "half moon" casting mark, all square port, hi perf applications. (Mechanical lifters).
The first was '67 only, #3904391, and had no temp sending unit.
The 2nd was the #840, and was installed in '67, '68, and '69. It was also cast in '69 as a #291, with a tapered spark plug seat. The #291 was also used in '70, since the 512 block was used in '70 for the 454. The only difference was the crank being stroked for the extra cubes.
The '67 L88/9 aluminum heads (#392) was identical to the #840, except for the casting metal.

[nuch_ss396]

[ QUOTE ]
According to Colvin, there were 3 #'s starting in '67 that had this "half moon" casting mark, all square port, hi perf applications. (Mechanical lifters).
The first was '67 only, #3904391, and had no temp sending unit.

The 2nd was the #840, and was installed in '67, '68, and '69. It was also cast in '69 as a #291, with a tapered spark plug seat. The #291 was also used in '70, since the 512 block was used in '70 for the 454. The only difference was the crank being stroked for the extra cubes.
The '67 L88/9 aluminum heads (#392) was identical to the #840, except for the casting metal.

[/ QUOTE ]

So, the #3904391 is the '67 L-88 head then? Did I read
that correctly, or read something into that?

Steve

[COPO_Anders]

I have read reports about new 1971 LS-6 Corvettes beeing sold with the iron head, for 600$ less than the aluminum head versions. They were the last ones produced.
Is this wrong ?

[ANDY M]

I'm not sure about the '70 heads, but the '67 L88 was a square port head, and the ZL1 or later L88/9 were round port.
I'm just going by what the Corvette guys and Colvin say.

[SuperNovaSS]

Maybe on the exhaust side, the intake sides were all rectangular.


Jason

[nuch_ss396]

Guys,

I had often heard that the '074 ZL-1 heads, with their
round exhaust ports, we an ineffective design concept.
The amount of power required to take advantage of this
port design was impractical. In my research on building
big block engines, the '077 ( over the counter ) heads
seemed to get the thumbs up due largly in part to their
"C" shaped exhaust port designs. Can any of you support
or refute this information? I see the '074 heads on ebay
all the time and often wonder why people will pay big bucks
for them if this it truly the case.

Steve

[71-LS6]

[ QUOTE ]
I have read reports about new 1971 LS-6 Corvettes beeing sold with the iron head, for 600$ less than the aluminum head versions. They were the last ones produced.
Is this wrong ?

[/ QUOTE ]
I think so, the NCRS tech manual / judging guide, makes no mention of an iron head version. 1971 LS6's were only delivered from late February to early June of 1971. I have the earliest known vin # parked in my garage (a GM press car), it, as well as some mid to late May vin #'s, are all aluminum head versions.

[markjohnson]

The #077 over-the-counter aluminum cylinder head is much superior to the factory installed #074 production head. That giant round exhaust port makes for a poor intake-to-exhaust port ratio. The #077 head has a beautiful D-shaped exhaust port with a cast in vane on the port floor. It also has a little bit smaller intake port to try to compensate for the big blocks' infamous 4 good/4 bad intake ports. Both heads do tend to suffer from valve guides that like to move around unless they are pinned at the valve spring seat. People like to pay more money for the #074 head because it is considered more correct for an aluminum head L-88.