This has more than likely been hashed out multiple times in the past but for us newer folks to the forum I submit the following info on the specification differences between the three LS-5 years. It is commonly stated that '72 is when the engine was detuned, lowering horsepower and torque. This is not an accurate statement (I guess I'm a little sensitive since I own a '72 and my manhood is being challenged! )

It was from '70 to '71 when the compression dropped. From my study of the engine specs info from *GM, I believe the '71 and '72 LS-5 engines are identical. The '72 HP rating reflected a change in measurement method. I've not discovered why the '72 also shows 15 net hp lower than '71; the "tune" seems identical between the 2 years... Anyone?




*https://www.gmheritagecenter.com/doc...t-Corvette.pdf
*https://www.gmheritagecenter.com/doc...t-Corvette.pdf
*https://www.gmheritagecenter.com/doc...t-Corvette.pdf

 

This is well documented and there is no mystery to this, and I think you've answered your own question. In '72, car makers went from using Gross HP ratings to SAE/NET, as mandated by the federal govt. Some manufacturers (including Chevrolet) chose to publish both ratings in '72. Other than ratings, 71 and 72 are largely the same, as you have already pointed out.

Most publications say there was a bit of additional detuning that happened in '72, but you won't see anything pop out at you in the published numbers. Urban legend has it that it was largely a numbers game the manufacturers would play to step around regulations a bit. Take a look at the max RPM where the HP numbers were published between 71 and 72 LS5s. They aren't the same. In fact, the difference is screaming at you in the little yellow boxes you made.

For the same reason that the '69 L88 SUPPOSEDLY had less HP than the '69 L71 which measured 435HP @5800 RPM. The '69 L88 measured 430@5200 RPM. You think the L88 was limited to 5200 RPM? Gimme a break. It probably had at least another 600-700 RPM to go. Granted, the L88 had better heads, higher compression, and a solid lifter cam, but they also weren't measuring apples to apples, although many probably thought that they were.

 

I'm afraid I don't see the screaming info in the boxes... The '71 lists both gross and net whereas the '72 only lists net. For the net amounts they run up to the same rpm for both years: 4000 rpm for HP and 3200 rpm for torque. Weird that the torque ratings are the same figure with HP showing a -15 difference. I'm sure this could be attributed to reporting error too.

 

Could be,,,,, There may have been some detuning (more restrictive exhaust, milder cam, etc etc), as I already mentioned. Who cares. You have a BB 72. That's darned cool.

 

I'm not really sensitive to '72 vs. '71 (that was a joke), my interest is academic with this thread. And to correct an oft read comment that the major change to the LS-5 was going from '71 to '72 rather than when it really happened from '70-'71.

I'm not seeing any difference in '71 to '72 camshaft specs or exhaust. Do you have more specific information on these?

If anyone wants to delve deeper... In glancing through the NCRS judging guide it appears the head and intake castings have different part numbers - but the specs in the GM info shows the same combustion chamber volume and other identical measurements. What are the differences between heads 3993820 & 39999241? Intakes 3955287 & 6263753?

 

The Internet's at your fingertips, pal.

 

Thanks for the advice - never thought of that... how does this Internet's thing work? Sounds like you aren't really interested in delving deeper into this, so feel free to ignore the thread, bud.

None of the searching I did revealed anything to explain the differences in the castings. As an aside, the service replacement head for both '71/'72 is 3999240, while the '70 is 3924686. For the intakes, I suppose it could be as mundane as a change in the boss locations for sensors, solenoids, etc. Service replacement intake for all ('70-'72) is 6262901.

 

For 71 vs 72, I think you have to start looking at the distributor curve specifications and the jet and metering rod usage. The curve may have become less aggressive and the jets and rods may have been leaned out.

 

the performance boat guys say 2 degrees of total timing can be as much as 40 hp.

 

Smar@ss'ery aside....You nailed it. Again, as I alluded to previously, those stats aren't going to point out minute changes in tuning for less output. You'd also need to see power curves from factory-provided dyno charts, but that won't tell you specifically what Chevrolet did to detune these vehicles. There were a plethora of (de)tuning options available to the manufacturer, and you made me pull out my books to look into this further - I have FOUR Corvette publications, and all say basically the same (quite ambiguous) thing, and I will directly quote from page 180 of "The Complete Book of Corvette (By the Auto Editors of Consumer Guide)" authored by Richard Langworth.

"...For 1972, Corvette mainly marked time. But engines now bore the full brunt of emissions tuning,and both horsepower and performance were further deemphazied..."

And from page 202 of Corvette 50th Anniversary Book by "Publications International Limited"

"....Amber Turn signal lenses were the most noticeable exterior change for 1972, when convertibles like this (showing a Sunflower Yellow Corvette) started at $5296. GM now quoted horsepower in more realistic SAE net terms instead of gross numbers, so small blocks were down to 200 bhp standard, 270 for the LS5 option. But the power losses werent just from moving to net figures, as engines were further detuned to meet new federal emissions standards..."

Unless someone on the forum worked for Chevrolet (Corvette) Engineering team under Zora Arkus-Duntov, or otherwise has some specific knowledge handed down to them on EXACTLY every little detail as to what emissions tuning was actually done in '72, then this is largely going to be conjecture.

So in summary - Everyone knows that in 71 and 72 (and beyond), horsepower took a decent hit due to federal regulations, and mandates for manufacturers to use NET/SAE horsepower ratings instead of GROSS HP. But microscopic-level details on tuning may never be fully known, but I will say this - If anyone has this level of information and/or has access to Corvette factory service manuals from the era, they'll very likely be on this forum.

Pull out your '72 owners manual. Does anyone here have a '71 manual so we can compare the point gap. dwell angle, and timing? That would rule out some of the obvious stuff. Here's from my '69 owners manual, as an example:





I'm sure someone has factory Corvette workshop/service manuals laying around that should go into considerably more detail in order to try and pinpoint all the emissions tuning changes.

 

Actually it's the initial timing combined with the mechanical advance and rate of advance in the table below. This if from the 1969 FSM and is found in the back of the FSM. As far as the carburetor jetting and rod selection, Lars Grimsrud has a document where he provides specifications on many of the Quadrajets of that era and he might provide an updated one, this is old from 2007. The secondary Rod changed from 71 to 72 and perhaps the primary rod on the 72 MT cars. .

7041204 Chev 71 454 AT Vette Main jet, 77, Main Rod 49 Secondary Rod BG
7041205 Chev 71 454 MT Vette Main jet 77, Main Rod 49 Secondary Rod BG

7042216 Chev 72 454 AT Vette Main Jet 77, Main Rod 49, Secondary Rod CM
7042217 Chev 72 454 MT Vette Main Jet 77 Main Rod 45, Secondary Rod CM

 

let’s just face it . The last of the big blocks were 1970 !! 71 and 72 it was over !! Base 200hp, LT-1 240, LS 270hp.. that’s not a BB to me .. yes it was measured and tweaked for the government but the ERA of the big block was GONE..

 

Did you totally forget about the ‘71 LS6? 425 hp with 9:1 compression. The big block was still a beast in 1971.

 

True, but the '71 LS5's and LS6's were still pretty nice at the time, 365 and 425 hp.

 

you are correct 62 corvette however that motor is so rare not many were sold at all

 

back in the day I had a 72 LS 454 and I raced my friends 70 350/300 . Let me tell you he blew the doors of me

 

Whether they sold a lot or little (188 in all) it was available to the general public and belies your observation that big block performance was gone after 1970.

 

correct they technically did, however as you can tell by the numbers no one had them because General Motors made it so hard for the public to buy them.. it was a year wait for that motor

 

No, the epa made it hard for GM, who began rolling them off the line on March 1, 1971. It was not a year wait unless you ordered one many months before GM mentioned anything about building them. Many who ordered in late January got them in early April.
Would you argue that L88’s really weren’t powerful motors because GM downrated the advertised horsepower on them so those that weren’t truly in the know wouldn’t buy them?

 

You couldn’t get that big block in 3 months like you mentioned.. my cousin worked at a Chevy dealership back in the 60’s to late 70’s .and was told to discourage customers from ordering any lingering big block ..he personally told customers it was a very long wait He said Chevrolet higher ups were pushing all the small blocks saying why would you buy this BB when the gas is so expensive , you can still own a corvette and save on gas ..