70 ss 390 hp?

[erich]

I purchased my 70 ss 15 years ago and the previous owner had claimed it was 390 hp. I do not have the build sheet because the car was re upholstered ,I do have the protecto plate with the crn code. Over the years I have seen 2 other cars in classified ads claiming the same thing .I did some research and found that there was a 390 hp 454 offered in the corvette and impala is it possible some of these made it into the mcss and how could you tell it apart from the 360 hp even though they share the same crn code.

[ejw71]

First Gen '70 SS454 was rated at 360hp and '71 at 365hp. Sellers tend to overstate whatever they're selling. Could such be ordered via COPO? If so, then documentation is the issue. Of course, some

believe that anything could be ordered via COPO. Ed

[gprimm]

A point of clarication on the 1970 390hp 454, vs. the 360hp 454. They were the same engine, LS-5.

 

The big blocks were built at the Tonawanda, NY engine plant but the Vettes had to have more advertised horsepower so they were labeled 390. It even got to a point where the Vette owners were saying their engines were assembled separately from our "regular" engines, but they weren't. The Chevelles had the L-79, 327, it was rated at 350hp in '66 and then 325hp in '67; same engine. Also, in 1970 they sold SS396 Chevelles but they were 402 cubic inches, but Chevrolet knew the 396 held a better ring to it so they were labeled 396.

 

The Monte came with the LS-5, rated at 360hp in '70 and 365hp in '71. If you found a buildsheet it would say LS-5.

[ejw71]

Thanks for the enlightenment, gprimm. You are OH! so correct. I'd forgotten about GMs policy of always assigning Corvettes the highest hp rating. Nevertheless, the LS5 in our dear 1st Gens isn't 390hp. One could correctly say the Corvette LS5 of 390hp is the same engine used in '70 & '71 SS454 Monte Carlos. Ed

[gprimm]

Ed, you are right; our LS-5's are the same as Vettes, and that is what drives some of those guys crazy. They thought their engines were assembled on a different line because they were in a Vette. The engines were assembled in Tonowanda and shipped to the assembly plants, regardless of the model. Oh well. George

[erich]

Thank you for the reply. what you are saying is that corvette 454 is also 360 hp it was just advertised at 390.Is it possible then that my car along with a few others were simply mislabeled and if so does that hold any significance as far as value.

[gprimm]

Yes, the engine is the same. By mislabeled do you mean the air cleaner decal? The radiator support emissions decal? Those could be changed. You mention that you don't have the buildsheet, which if you did would have an engine RPO of LS-5, which for 1970 was rated at 360hp. The value of the car is not impacted by this. I think the only way it would be impacted is if the engine was not numbers matching and that would decrease the value. Check on the front of the passenger side of the block for the engine stamping. There will be a 3 letter code followed by some numbers and letters.

[Guest]

The LS5 engines were the same but Corvettes did run a different exhaust manifold than the rest. It was suposedly a better design and will not fit A bodies . I don't know if it was worth 30hp though.

[gprimm]

I contacted Terry McManmon, Editor of the Corvette Restorer Magazine. He judges Vettes at major NCRS shows and makes or breaks guys dreams cuz he has all the numbers memorized (alternator # for a manual SB vs. BB AT, etc). He is a friend and longtime member of the Northern Illinois Chevelle Club and has a 1967 Chevelle 327 and a 1971 Vette LT-1. He is a NCRS judge. This is what he had to say, which goes to the "they are the same, Vette had to have more advertised power" thought. Their factory exhaust manifolds were cast iron just as ours were, they were just configured differently for the frame.

 

 

"I am sure the exhaust manifolds and perhaps more importantly the exhaust system are different. 30hp difference?? I doubt it. Corvette had 2.5-inch exhaust pipes, and relatively short ones at that. Remember in those days (and still) Corvette almost always got the highest horsepower engine -- 1970 LS6 being the notable exception. I am sure the Chevrolet dyno operators were not above fudging the numbers to ensure Corvette stayed on top. Also back then they were measuring gross horsepower. My understanding is that was without water pump, alternator, and exhaust system and with cold air induction. So how did they account for the kinds of differences you are asking about? Beats me, but the fact that Corvette always was on top gives us a clue, I think. The fact that all of them are called LS5 would lead one to think that they are all the same, however."

[Landshark]

The reason Chevrolet gave for the difference is the exhaust. The bigger the engine, the better it will respond to exhaust upgrades. The corvette engine, with the better exhaust, made power further up the rpm range.

Look at the engine specs in the factory manual. The 360 hp engine is rated at 4400 rpm, and the 390 is rated at 4800 rpm. This explains the 30 hp difference, as much as the exhaust.

 

An interesting note on horsepower ratings. They were basically arbitrary, and were "dialed" in to fit the application.

In 66, the first 427 hi perf vettes were rated at 450 hp. GM Corporate had a fit, so they were rerated to 425 hp. Take a look at 65 and 66 specs; The 396/425, and the 427/425 vettes had the same cams, heads and intakes, and the same power band, (6500 redline) but the 396 was rated at 6400 rpm, and the 427 at 5600 rpm. What they did was run the 427 up until it made 425 hp, and stopped there, thus reducing the hp rating.

Oh, and does anyone really believe in a 290 hp 302 Z28, or maybe a 430 hp L88?

The L88 was deliberately rated below the L72/435 tri-power to discourage Joe Average. They did not want these things out on the street. You could not get a heater, or a radio with an L88, and it cost a lot more than the 435 hp, which is why they are so rare.

 

The seemingly low hp ratings for the Monte and Chevelle had much to do with insurance classification. Insurance companies were getting nervous. Anything that weighed more than 10 lbs per hp was considered low performance. A Chevelle weighing 3800 lbs would pay more if it had a 390 hp engine, as would a Monte.

And then there were the NHRA Stock and Super Stock Classes, (i.e. 335 hp 428 Cobra Jets), but enough. Rich