This is a sidenote to Paul's LT-1 vs L82 thread...

I am wondering what the exact differences between 70, 71 and 72. We all know compression was dropped for 71, and net ratings were adopted in 72. Otherwise, some people say these engines are identical???

Those with other LS5's can you verifiy this data, and provide the differences between the other years?

The ratings were 390 gross, 365 gross and 270 net respectively.

Here are the factory specs for the 72 LS5:
270 hp @ 4000 rpm (NET)
390 lb-ft @ 3200 rpm (NET)
*Note: my console data plate says 455 lb-ft torque. Where does this come from?

Block 3999289 two-bolt mains
cast aluminum flat pistion
forged steel crankshaft

Heads 3999241
113 cc head volume
8.5:1 compression
2.06/1.72 valves

Intake
P/N ????

Camshaft
high-lift hydraulic 3883986 (?)
.271/.282 lobe lift
.461/.480 valve lift
268/274 duration at ~.008 lift off base circle

Carb
77 main jet, 45 main rod, 7036019 spring, CM secondary rod

Ignition
1112051 distributor
8 deg BTDC @ 800 rpm base
1145 start, 14 @ 2000, 22 @ 3000 centrifugal advance
8 inHg start, 20 @ 17 inHg max vacuum advance

 

All LS-5's used the same cam. The '70 390 hp LS-5 had closed chamber heads that yielded higher compression than the later '71 and '72 LS-5 which had open chamber heads. The '70 LS-5 also had a aluminum intake manifold that was otherwise identical to the later iron LS-5 manifolds. Even though the compression was lower with the later heads, they are better flowing heads and will ultimately make more power if the compression is raised through milling or pop-up pistons. Other than the heads and intake manifold, all LS-5's are essentially identical.

 

"The '70 LS-5 also had a aluminum intake manifold that was otherwise identical to the later iron LS-5 manifolds."

That statement is going to require some further research. My original engined 70 LS5 air roadster has a cast iron intake with a casting date in line with the dates on the heads and block. I can't say for certain that this is the intake that came on this car from the factory, because of 6 previous owners, but I've always accepted it as such. I don't recall seeing a 70 LS5 with an aluminum intake, although in all honesty haven't looked around at them a lot. My NCRS spec guide which is over 10 years old does list aluminum only for the 70 LS5, but all of that is opposite from what I've always thought. Not that it means anything here, but I'm sure that the LS5's in the 70 Chevelle/El Camino were cast iron.

 

What were the differences between 71 and 72? I think I've heard the 71 LS5 was rated at 285 net hp. Can someone verify this?

If all engine parts were the same, why would the 71 have 15 hp more? I see 71 had slightly different carb internals: 49 main rod, BG secondary rod, but can this account for 15 hp? Why would power have been lowered again for 72?

BTW, according to Lars' paper, 71 and 72 454 4-speeds have the same WOT metering area, but the 72 has a larger cruise metering area. Why would they need to enrichen the mixture for 72? Seems to contradict the trend to lower emissions.

:confused:

 

It's possible that a very late production '70 LS-5 could have received a iron intake. Every one I've seen had aluminum but there are no absolutes as changes happened during production on many occasions.

 

It is probably a matter of GM playing games with the rated HP more than anything. The '71 and '72 LS-5 are virtually identical with the exception of part throttle jetting and possibly spark curves, neither of which affect peak HP. Other than that, only total timing advance would be a possible reason for the different ratings. GM was experimenting with various TCS settings at that time so timing is the only possible difference between the engines.

 

RR, I didn't think I was losing my mind. I know I did a lot of research on the 70 LS5 10 years or so ago when I was foolin' with mine. Unfortuantely, in my case, memory has a way of foggin' with time, especially when there's only one car of a particular vintage involved. Anyway, here's a direct quote from a noted NCRS member who's factoids I've never found an error in:

"The 1970 Corvette LS-5 intake manifold, GM casting #3955287, was a CAST IRON manifold. The 1968-69 Corvette L-36 manifolds were the ONLY 4 barrel carb, aluminum, oval port manifolds which Chevrolet ever used in PRODUCTION.

After 1969, the only aluminum manifold used on any Corvette big block was the rectangular port GM CASTING #3963569."


This falls EXACTLY inline with what my sometimes questionable memory was spittin' out. Since there were no rectangular port Corvette engines in 70, the statements above would mean no alum. big block intakes in 70 Corvettes. My car had original components when I found it, right down to the exhaust manifolds, heads, transmission & rear. Talking with previous all owners revealed no hints of any components having been changed, but the block itself did appear early and the block appeared to have been stamped over. I've never stated with certainty that the engine was completely authentic, but the authenticity was traced back as far as the second owner, who aquired the car in 72 and wrote down every number and code before he sold it in the mid 80s. Restamping certainly wasn't common or thought necessary in 72 and prior. A subsequent conversation with Noland Adams led me to believe that the block may have been pulled from production on an earlier car, then put back on the line later for whatever reason. Apparently there are documented cases of them snatching one for any defect to keep from slowing the assy. line.

Anyhow, again, my car has an intake, heads and exhaust manifolds, with dates that would be expected with the build date. Assuming that the first owner's mechanic, aparently "ole Bubba" down at the corner grease rack, didn't swap any components around, they're all still intact.



[Modified by Solidlifters, 1:04 PM 9/29/2002]

 

My ´71 LS5 has 4-bolt mains. Rated 365 hp gross, 285 net.

 

I've never seen a documented original engined LS5 of any model year with 4 bolt mains. My 70, which may or may not have the original block is 2 bolt. My 71 Coupe which absolutely has the original block is 2 bolt also. The 512 big blocks that were used in most 69's, all 70's, 71's, & 72's, were available in both 2 & 4 bolt blocks. I've never seen a 4 bolt after 70 dated 512 in anything except a crate motor or a 71 LS6 Corvette.

I wouldn't say that it never happened, but if it did, I'd have to think it was either a production error, or GM using up some surplus parts that had already been machined as replacement parts, since both blocks were cast exactly the same and the only difference was the machining process. (the 4 bolts were drilled for the extra bolts of course, had the bosses above the oil filter drilled and tapped for oil cooler lines, and had valve reliefs machined in the top of the cyl walls on the intake valve side of the cyl)

[Modified by Solidlifters, 2:44 PM 9/29/2002]


[Modified by Solidlifters, 2:50 PM 9/29/2002]

 

I did some research on intakes for use on my LS-4, All the books that I have seen list only the 68-69 L-36 intakes as the only Oval port aluminum intake with a spreadbore bolt pattern. I don't know of any other BB chevy intake that was aluminum for use with a quadrajet. I have seen a lot of 68-69 L-36 intakes on some later LS-5 and LS-4's, deffinitly worth the swap as far as weight savings alone, plus I think it flows a little better then my stock LS-4 intake did.

:cheers:
Pat Kunz

 

From the newest Judging Guide of the National Corvette Restorer's Society (NCRS)

"Small block engines use cast iron intake manifolds, except LT1 which is aluminum. 1970-72 big block intakes are cast iron, except LS6 which is aluminum."

Well at least y'all made me look and realize how outdated some of my literature is. Now if I could just retain it all memory wise.

 

Forum member Allan71 (not been around lately) also discovered 4-bolt main bearings on his original 71 LS5. I think it is surprises like these that create so much confusion today over whether a block is 2 or 4 bolt, as even GM seemed to not know what they were doing. I would say 95% of LS5 and LS4 blocks are 2-bolt as they were suppose to be, but the only way to tell is to pull the pan. (BTW, I have verified mine is 2-bolt)

 

Zwede, what were the rpm's of these ratings? What about tq numbers?

 

 

Net power rating: The company sent me to Europe for the next 3 months so I can't check it. Just happened to remember the hp figure.

As for the 4 bolt block: It has all the signs of being the original block. It has the correct stampings, casting #, VIN and suffix. I guess it could have been faked, but the car was in pretty rough shape when I bought it, so I don't see why someone would go through the trouble to fake all the numbers on a $10k car.

 

Re: HP 1971 vs 1972 in an engine that was unchanged between the two model years, you will notice that in '72 GM rated peak HP at 4000 rpm while in 1971, it was at 4800. :confused:

I do not belive there were ANY real difference, both in composition and performance between the 1971 LS5 and the 1972 LS5

1972 from MT, June 1972 (1971 above):

 

I'm going to say my car has 285 hp from now on :)

Actually, the first real chassis dyno in this city was installed a few weeks ago. I will got get a baseline soon. Can I expect ~240 rwhp with my recurved dist and K&N? Otherwise the motor should be stock.