Horsepower, Torque, And Current Restorer Issue

[Joe C.]

In the current issue of the Restorer, there is an excellent article on page 9, by Everett Ogilvie which is part 2 of 2 of a detailed discussion of the legendary "450 horsepower L72 engine". It has been a long time since I mothballed my engineering textbooks, and the article piqued my interest in relearning the relationships between horsepower and torque. For those that may be interested, I found an excellent discussion at the following website ------------- http://home.inu.net/davidstua/horsepower_and_torque.htm

Everett: If you see this post, does your source know if that 460+ horsepower MK IV tested at the Chevrolet Engineering Center (note, page 10, Restorer), was a specially built drone, designed to yield extraordinarily high peak horsepower numbers. I understand that you are using published numbers, but they can be manipulated in the same manner as the air cleaner decals. This is not meant to to criticize anything that you have written, but if that 460 HP was an honest figure, wouldn't it mean that if the tripower on all the '67 L71's were replaced with 4bbl Holleys like those used on the L78 and L72, then these would have technically produced 460 HP.

Joe

[William C.]

Re: Horsepower, Torque, And Current Restorer Issue

Keep in mind that Tripower was "in Vogue" in the mid-late 60's, as seen on the GTO and the W-30 Olds packages. In fact for high performance applications, the flow charactistics of the tri-power manifold are quite inefficient, and the '66 (or '67 pass car) Hi-gh rise singe carb configuration arguably would produce greater HP. As to your question regarding the 435hp engine producing 460 if outfitted with a single carb-high rise intake it is fair to say the Intake and carbs were the only differences in the engine specifications.

[William C.]

Re: Horsepower, Torque, And Current Restorer Issue

Also remember that "advertised" horsepower in the '60's was calculated using adjustment to HP relative to a standardized air temperature and I believe adjusted to correct altitude to sea level, open exhaust, and no accessory loads were a part of the test. I rember reading some of them when I was in school, I believe it was SAE test #20. SAE probably still has the test standards available. I'm not sure if the results were published within SAE, or if we had access to them thru a class.

[Duke W.]

Re: Horsepower, Torque, And Current Restorer Issue

"SAE gross" also allows the fuel and spark advance calibrations to be optimized for all test points, which rarely corresponded with actual production calibration. As you stated - no accessories, air cleaner, if any, was completely optional, the exhaust system consisted of generously sized pipes from the manifold exit, and a pump was used to maintain negative exhaust system pressure to assure that exhaust gases were evacuated from the dyno cell. Also, observed data was corrected to standard sea level conditions - 29.92" Hg. and 59F.

SAE net is "as installed" in the vehicle with the production calibration production inlet and exhaust systems, and the observed results are corrected to 29.335" and 77F dry air.

A lot of games could be played with SAE gross. One trick was to run the engine up when cold to achieve high flash readings before the inlet manifold heated up and reduced inlet air density.

Duke

[Everett Ogilvie]

Re: Horsepower, Torque, And Current Restorer Issue

My source for the 460+ HP spec is the Chevrolet Engineering Center graph, with the corrections for 60 F, and gross/net power curves. The test is from an early L72 engine, June 1965. The "marketing" of the L72 was not done by engineers - they rounded off to 425 HP to meet a management requirement that no HP figure above 425 would be published (as opposed to the actual gross HP at 5600 rpm, which is about 440 according to the Eng. Center graph, tests #20 and #1).

As mentioned above, the intake on the tripowers is the limiter - it does not flow as well as the L72 intake, which is almost identical to the L88 intake with the exception of the plenum divider. The L71 should perform the same as the L72 if the intake setups were swapped.

A key takeaway from the article is not that the L72 really made more power than later 427 engines (although the L72 1/4 mile time of 12.8s at 112 mph tested by Car and Driver was the fastest time of any big block Corvette from '65 to '74), but rather that the 450 rating of the L72 is still the highest "rated" Corvette engine even to this day. It is only the paperwork of these engines that makes them rare or different, but it is an interesting part of Corvette history.

[Clem Z.]

interestin 3X2 carb story

the CFM of the 3X2 system is 894 CFM if you do it the way 4 barrel carbs are rated. you need to multiply the 2 barrel rating by .71 to get the equal 4 barrel rating. several years ago SCCA rules said that big block corvettes had to run a certain size restrictor plate under the carb to reduce the HP. ed lowther wanted to race a big block corvette so we use the 3X2 carb setup with the specified size restrictor plate under each carb. this of course gave the engine more HP because the engine now with 3 restrictor plates vs 2 restrictor plates for the 4 barrel. SCCA could not change the rules in the middle of the stream so to speak and ed won every race that year because of the greater air flow of 6 barrels vs 4 barrels even though it was restricted.

[Joe C.]

I Agree That The L72 Was .................

........the highest RATED horsepower big block, for that short period in the beginning of 1966 production. We have had the carburation discussion here, many times before. The 4BBL setup unquestionably flows better than three deuces. If tripower yielded more HP, then NASCAR would probably have been using it on Winston Cup racecars. The L88 was, after all, equipped with a 4BBL. So was the ZL1.

We all know that the horsepower RATING was strictly marketing. Considering the 460+ dyno result for the L72, I suppose that the dyno would yield around 440 for the L71. Does anybody have the L71 results, assuming that the Engineering Center put the L71 on a test stand?

In those days, 3 deuces was the "boss" setup, and Ronnie And The Daytonas immortalized it in their song "Little GTO", but if Chevy had carried the L72 over to 67-68-69, then they would have had a hard time explaining why the L71s were always getting beat by L72s (if the horsepower RATINGS were also carried over).

Joe

[Harry Koba]

Re: L72 Horsepower.................

I'm completing the rebuild of an early L72 to as original as possible; correct block, heads , cylinders, cam, compression, etc. I'll have it dyno'd with the correct manifold and correct Holley 4BBl. The only performance enhancement will be a torrington roller thrust bearing between the timing chain sprocket and the block. I'll pass on the results.