C3/ LS5 vs LS6

[Larry Daigle]

I HAVE A 1971 LS5 AND ALWAYS WONDERED WHAT WERE THE DIFFERENCES WITH THE 1970 AND 1971 454's? WAS IT THE PISTONS OR THE HEADS THAT GAVE THEM LOWER COMPRESSION IN 1971? AND WHAT ARE THE DIFFERENCES BETWEEN THE LS5 AND LS6 OTHER THAN THE HEADS, CARB, INTAKE AND 4 BOLT MAINS ON THE LS6? WERE DID THE DIFFERENCES IN HP COME FROM? THANKS SO MUCH!

[Joe L.]

Re: C3/ LS5 vs LS6

Larry------

The LS-5 and LS-6 engines for 1971 were virtually completely different engines. Here are the basics:

block (2 bolt versus 4 bolt [of same casting number])

cylinder heads (cast iron oval port 2.06/1.72 versus aluminum rectagular port 2.19/1.88)

pistons (cast 8.5:1 dome versus forged 9.0:1 dome);

connecting rods (low perf forged versus high perf forged;

crankshaft (forged non-cross drilled, non nitrided versus forged cross-drilled and nitrided journals);

camshaft (medium performance versus special high performance)

lifters (hydraulic versus mechanical)

rocker arms ("short slot" standard versus "long slot" high performance)

balancer (7" standard performance versus 8" high performance)

oil pump (standard performance versus high performance)

intake manifold (oval port cast iron low rise versus rectangualr port aluminum low rise)

carburetor (Rochester Q-Jet versus Holley model 4150)

clutch (11" single plate versus 10" dual disc)

So, as you can see from the above, what's left is what the engines had in commmon, which is very little. They had the same CID, though.

[Norris W.]

Re: C3/ LS5 vs LS6

To answer the first part of your question, the combustion chamber was changed from closed to open chamber design after the end of 70 production and that's where the loss in compression came from.

I don't have part number reference material at my easy reach, but I'm almost sure the 71 LS6 Corvette used the same piston with the open chambered heads that the 70 LS6 Chevelle used with closed chambers to reach it's 11:1 advertised compression ratio.

[Joe L.]

Re: C3/ LS5 vs LS6

Norris----

It is true that the 70 Chevelle LS-6 used a closed chamber type combustion chamber whereas the 71 LS-6 used an open chamber type head. The 70 LS-6 was a cast iron head of 109 cc, nominal, combustion chamber volume whereas the 71 LS-6 used aluminum cylinder heads with 118 cc, nominal, combustion chamber volume. However, the 1970 Chevelle LS-6 and 1971 Corvette LS-6 used totally different pistons. The dome on the 70 LS-6 piston was significantly larger than the 71 Corvette LS-6 piston dome. The difference in the compression ratios between the two engines is a function of BOTH the difference in the chamber volumes as well as difference in the pistons.

[Clem Z.]

Re: C3/ LS5 vs LS6

1970 454 450 hp dome heigth .303,dome volume 31.60,chamber volume 106.9. 1971 454 425 HP dome heigth .095,dome volume 13.10, chamber volume 103.1. NHRA specs.

[Norris W.]

Re: C3/ LS5 vs LS6

Of course it's common knowledge that the 71 LS6 engine changed to alum. head's from the 70's cast iron, and even though there've been some claims of a handful of 70 LS6 Chevelles with alum. closed chamber heads, I've never run across a crediable example or documentation for that matter.

My opinion on the pistons was based on a 71 LS6 Corvette I looked at in Texas at least 10 years ago, that had been partially canibalized. The heads and intake were off and the car had the original block assy and appeared to have 70 style pistons. At this time the car was already 20+ years old, so that's not to say that a previous owner hadn't changed em, because there were other changes on the car, headers, etc.

This makes me curious about any difference in the pistons in the 70/71 LS5's.

[Clem Z.]

Re: C3/ LS5 vs LS6

just adding OC aluminum heads to a 70 454 450 HP will only lower the CR 1.0 point. also the large dome may cause emission problems and that may be why GM did it a different way. my specs came from the NHRA specs supplied to them by GM for checking engines. back in the 60s i was also a tech inspector for a east coast drag racing assoc. and all the auto companies supplied this info.

[Joe L.]

Re: C3/ LS5 vs LS6

Norris-----

The 1970 and 1971 LS-5 pistons are considerably different. 1970 used closed chamber heads whereas 1971 used open chamber heads, so the combustion chamber volumes were considerably different. Also, 1970 LS-5 compression ratio was 10.25:1 and 1971 was 8.5:1. The year-to-year differences do not "offset" and allow the use of the same pistons. Also, although closed chamber pistons can be used with open chamber cylinder heads (but not vice-versa), GM never did so in PRODUCTION.

[Joe L.]

Re: C3/ LS5 vs LS6

clem-----

I think that the 1971 LS-6 chamber volume must be a mis-print or otherwise incorrect. The 1971 LS-6 heads were open chamber and I don't know of any open chamber heads with a combustion chamber volume as small as 103 cc. Most that I'm familiar with are in the 116-119 cc range.

[Clem Z.]

Re: C3/ LS5 vs LS6

joe you are correct it should be 113. the specs supplied to race assoc. are set up to allow the max comp ratio so they use the max/min manufacturing specs to get the best results.

[Clem Z.]

Re: C3/ LS5 vs LS6

the correct piston for the 71 454 425HP engine is a TRW L-2399 which has a .095 high dome with 13.10 cc volume.

[Joe L.]

Re: C3/ LS5 vs LS6

clem-----

As a matter of fact, the 1971 Corvette LS-6 pistons are still available through GM, too. Standard, +.001", and +.030" are still available. The pistons are GM #6262976, 6262977, and 6262978, respectively. These may or may not be the same pistons as sold by TRW. Even if the "slugs" are TRW-sourced, the pistons, themselves, may be machined internally by GM or TRW may machine them to some other specification than the one used for the units they supply under their brand name.

[Clem Z.]

Re: C3/ LS5 vs LS6

i guess i should have used the term "the correct TRW replacement piston",but it has been proven that the 1971 454 425 HP does not use the same pistom as the 1970 454 450 HP engine. a customer of mine has a 1971 454 425 HP corvette convert. with TH-400 that he bought new.

[lyndon k sharpton]

Re: C3/ LS5 vs LS6

And some times those LS5 came with a Four bolt main!

[lyndon k sharpton]

Re: C3/ LS5 vs LS6

some times the LS5 came with a four bolt main!