1971 454/425hp LS6 Heads - Uncertain Usage?

[David D.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

I have been following along with this very interesting conversation but now I am still wondering where the #3994026 cast iron heads that I have, which are also shown to be an LS6 454/425hp come into play in relation to the #3994026 heads you have been speaking about? I'm still confused.

Dave

[Joe L.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

I have been following along with this very interesting conversation but now I am still wondering where the #3994026 cast iron heads that I have, which are also shown to be an LS6 454/425hp come into play in relation to the #3994026 heads you have been speaking about? I'm still confused.

Dave

Dave------



The CASTING NUMBER 3994026 was used for the PART NUMBER 3994025 cylinder head. In other words, the CASTING NUMBER actually found on the cylinder head is one digit different from the PART NUMBER for the finished bare cylinder head. The PART NUMBER appears nowhere on the cylinder head----only on the box that a SERVICE cylinder head is supplied in.

[Ray K.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Dave,

My thought is that the cast iron heads that you have #3994025 (3994026) could have been installed on a 1971 LS6 454. I do not think it is an error in the parts catalog - I believe there is some validity there. Joe has good documentation about the aluminum heads, and I agree with that, but not 100 per cent. I think Chevrolet might have produced a very " few " engines with the heads such as yours.

Other than that, they were available over the parts counter for 2-3 years.

Ray

[Clem Z.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Dave,

My thought is that the cast iron heads that you have #3994025 (3994026) could have been installed on a 1971 LS6 454. I do not think it is an error in the parts catalog - I believe there is some validity there. Joe has good documentation about the aluminum heads, and I agree with that, but not 100 per cent. I think Chevrolet might have produced a very " few " engines with the heads such as yours.

Other than that, they were available over the parts counter for 2-3 years.

Ray

if they were used on a engine i would think the cast iron heads would have been used on a 71 chevelle not a corvette because we know for sure that 71 corvettes got aluminum heads.

[Ray K.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Clem

I agree the most likely candidate would be a Chevelle. I show part #3964287 for '70 Chevelle LS6 450HP 454 for a cast iron head. Aluminum was probably #3946074 but they left that out, an error. Engine codes are 'CRV' for manual trans 450HP and 'CRS' with TH400 and aluminum heads. I do not have a catalog for '71 Chevelle applications. I think the head should be the same but maybe Joe has the later data.

Ray

[Norris W.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

if they were used on a engine i would think the cast iron heads would have been used on a 71 chevelle not a corvette because we know for sure that 71 corvettes got aluminum heads.

The problem with that theory is that there were NO SHP engines in '71 Chevelles. LS5 was as big as it went, and of course they had oval port heads, also with open combustion chamber design. I think the heads in question here were available on the '70 LS7 crate motor and the LS7's from there foward. I remember somebody saying several years ago on one of the Chevelle sites that he had an open chamber LS6 crate motor with cast iron heads that was bought over the counter. Presumably if there HAD BEEN an LS6 Chevelle or Monte Carlo in '71 that would have been the engine in it.

I've never heard an explanation of WHY there was NO LS6 Corvette in '70 and NO LS6 Chevelle in '71. Multiple sources over the years have speculated that they didn't offer the LS6 in the Corvette in '70 because the LS7 was planned, but given the L88 option on top of the SHP engines in '67, 68, and '69, that supposition doesn't make much sense either.

[Clem Z.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

The problem with that theory is that there were NO SHP engines in '71 Chevelles. LS5 was as big as it went, and of course they had oval port heads, also with open combustion chamber design. I think the heads in question here were available on the '70 LS7 crate motor and the LS7's from there foward. I remember somebody saying several years ago on one of the Chevelle sites that he had an open chamber LS6 crate motor with cast iron heads that was bought over the counter. Presumably if there HAD BEEN an LS6 Chevelle or Monte Carlo in '71 that would have been the engine in it.

I've never heard an explanation of WHY there was NO LS6 Corvette in '70 and NO LS6 Chevelle in '71. Multiple sources over the years have speculated that they didn't offer the LS6 in the Corvette in '70 because the LS7 was planned, but given the L88 option on top of the SHP engines in '67, 68, and '69, that supposition doesn't make much sense either.

GM lowered the CR in 71 so open chamber heads would have done the trick on the 70 454 450 HP chevelle engine so maybe they could not get the engine to meet the emissions after they did the tests with those heads and the project was scrapped and the heads in inventory were sold over the counter where emissions did not apply. a interesting test would be to air box those heads vs the 482 heads to see if there was a difference. you could also "pour" the ports to check the volumes.

[Norris W.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

GM lowered the CR in 71 so open chamber heads would have done the trick on the 70 454 450 HP chevelle engine so maybe they could not get the engine to meet the emissions after they did the tests with those heads and the project was scrapped and the heads in inventory were sold over the counter where emissions did not apply. a interesting test would be to air box those heads vs the 482 heads to see if there was a difference. you could also "pour" the ports to check the volumes.

Wouldn't they have faced the same emissions hurdles then with the 71 LS6 Corvette which was basically the 70 LS6 from the Chevelle with open chamber aluminum heads to lower the compression?

[Warren F.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Several engine options were in the works for the upcoming 1970 model year for the Corvette line up. Of the three new 454 offerings, the LT-2 and LJ-2 engines did not materialize in the Corvette offerings, leaving only the LS-7 option for the '70 model year. Duntov and his engineering group were pushing for this engine option, so that the Corvette would still maintain its king of the hill status as having the highest horsepower engine available in the Chevrolet line up as 460 hp. When this option got de-proliferated at the last minute for the '70 Corvette line up, it was too late for the 1970 LS-6 engine option to be emissions certified to be included in the Corvette engine line up, so for the first time another model, the Chevelle became king of the hill in the Chevrolet pecking order.

Duntov did not give up and was still determined to include the LS7 engine in the 1971 Corvette model line up. This option again shows up in early 1971 Corvette Order Forms. First as a 450 hp incarnation and then changed to RPO LS6 with 425 hp in the next Corvette Dealer Order form.

The 1971 LS6 Corvette option became an outgrowth of the still born LS-7 engine and is NOT similar to the 1970 LS6 Chevelle engine option.

The complete engines as installed in their respective models are day and night in differences.

[Clem Z.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Several engine options were in the works for the upcoming 1970 model year for the Corvette line up. Of the three new 454 offerings, the LT-2 and LJ-2 engines did not materialize in the Corvette offerings, leaving only the LS-7 option for the '70 model year. Duntov and his engineering group were pushing for this engine option, so that the Corvette would still maintain its king of the hill status as having the highest horsepower engine available in the Chevrolet line up as 460 hp. When this option got de-proliferated at the last minute for the '70 Corvette line up, it was too late for the 1970 LS-6 engine option to be emissions certified to be included in the Corvette engine line up, so for the first time another model, the Chevelle became king of the hill in the Chevrolet pecking order.

Duntov did not give up and was still determined to include the LS7 engine in the 1971 Corvette model line up. This option again shows up in early 1971 Corvette Order Forms. First as a 450 hp incarnation and then changed to RPO LS6 with 425 hp in the next Corvette Dealer Order form.

The 1971 LS6 Corvette option became an outgrowth of the still born LS-7 engine and is NOT similar to the 1970 LS6 Chevelle engine option.

The complete engines as installed in their respective models are day and night in differences.

this is why the 70 454 450HP chevelle had the "sunken" intake manifold so it would fit under the hood of the corvette,correct ?

[Ray K.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

The hood had a lower profile so the 70 450HP had a lower profile intake manifold as well.

[Joe L.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Wouldn't they have faced the same emissions hurdles then with the 71 LS6 Corvette which was basically the 70 LS6 from the Chevelle with open chamber aluminum heads to lower the compression?

Norris-----


The compression was lowered for 1971 big blocks mainly by revised piston configuration. For example, the 3946074 cylinder heads used for the 1971 LS-6 was the same as that used for 1969 L-88.

[Warren F.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

The hood had a lower profile so the 70 450HP had a lower profile intake manifold as well.

Not so much a lower profile hood, as it was the styling department did not like the cold air induction style hood as used on the L-88 & ZL-1 optioned Corvettes in '69. The original big block hood design blended well with the body styling of the recently revised lines of the car much better; and this hood necessitated that the intake manifold runners drop down into the intake valley for sufficient hood clearance.

[Ronald L.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Interesting thread, great history Warren, I distinctly remember a high school pal got one of those 70 Chevelle's and instantly had the fastest car in the school, unfortunately he'd flip it on Waterford Hill drag racing.

There were a lot of problems with these early aluminum blocks and heads, not withstanding the power was sufficient to cork screw drive shafts. How many of you did that and was that not the 'real' reason the 70 was slightly de tuned???

Chevrolet had a special performance parts catalog, I have not yet found mine from the mid 70's but I did find a '76 book that talks about how to build a HP big block that lists 'some' part numbers, this is what it says about the heads:
Due to changes in port and combustion chamber design, late model open chamber aluminum heads offer a significant power increase over earlier aluminum and steel heads #3935458.

Joe do you show that number?

[Norris W.]

Re: 1971 454/425hp LS6 Heads - Uncertain Usage?

Norris-----


The compression was lowered for 1971 big blocks mainly by revised piston configuration. For example, the 3946074 cylinder heads used for the 1971 LS-6 was the same as that used for 1969 L-88.

Joe, I'm not exactly sure I'm understanding what you're saying. I agree that the 71 LS6 used the same head as the SECOND DESIGN '69 L88, which was an open chamber motor as was the '69 ZL1. Obviously throwing the open chamber heads on an engine that was previously closed chamber would lower the compression a couple of points and I thought that's what they did. In other words, all of the '69 big blocks (except the LATE or 2nd design L88 and the ZL1) were closed chamber and of course these two had significantly different dome volumes than the early '69 as well as 68 & 67 L88's. In addition to the '69 stuff, ALL '70 big blocks were closed chamber including the LS6 in the Chevelle............. the point being, I THOUGHT (not sure) that the '70 LS6 engine in the Chevelle and the '71 LS6 in the Corvette carried the same piston part number and the compression was lowered by changing to the open chamber heads on basically the same short block as before.

Admitedly I don't have any experience whatsoever with the '71 LS6 Corvette, BUT do have closed chamber (early) '69 L88 and a couple of '70 LS6's as well as LS7 crate and have been in all of 'em. In addition, I did a good bit of work on the Ontario Orange '69 L88 that was the first L88 ever awarded Survivor at Bloomington and was in that one too. It was the 2nd design or open chamber L88 and since it was showing 8K miles at the time I believe the pistons were original to it and the domes were massive compared to my early closed chamber L88. Also, although I no longer own the car, I bought a '71 LS5 once that was maybe 15 years old and noticed that it had the same piston as the '70 LS5, but of course had the open chamber oval port heads. I stuck a set of closed chamber oval ports I had on the shelf on it and woke it up. I'm not saying that the pistons were original to the engine because the car was multi owner before I bought it so I don't know the history, but this too made me think at the time that all Chevy did in '71 to drop the compression was go to the open chamber heads on both the oval and rectangular port stuff and leave the shortblock alone, at least for the '71 model year.

I'd like to know for sure if the 70 & 71 LS6 pistons were the same part number, AND ALSO if the 70 & 71 LS5's shared pistons also as I've always thought.

What do you think?