Just wondering if there's any sort of mathematical formula or what have you that allows you to figure gross hp from net hp?
Junkyard Dawg
i've kind of wondered how this differed myself. does net lower the HP for losses in the transmission or driveline or just accessories? i suppose not, that would be more a rear wheel HP reading. near as i can tell looking a published HP numbers, it seems like basically the same engine produced about 10-15% less power after going to net in '72.
Certainly 'accessories' like A/C, P/S, etc rob some horsepower but so can altitude, air temp, air density, etc. Different transmissions will drain horsepower as well, a TH400 for example takes more hp to drive than a TH350 or 4-speed. I'm sure someone has a formula based on certain generic factors but I don't know of any that are probably more accurate than another.
Probably a few reasons for 72 hp figures to be down - emissions changes and net as opposed to gross hp. and mfgs. were starting to feel insurance and fuel crunches. The LS3 402 went from 300hp to 240hp or a 20% drop; the LS5 went from 365hp to 270hp or a 26% drop and the L48 from 270hp to 175hp or 35% drop.
Probably a few reasons for 72 hp figures to be down - emissions changes and net as opposed to gross hp. and mfgs. were starting to feel insurance and fuel crunches. The LS3 402 went from 300hp to 240hp or a 20% drop; the LS5 went from 365hp to 270hp or a 26% drop and the L48 from 270hp to 175hp or 35% drop.
There is a mathmatical formula - but believe me you don't want to know it, because it is not a straight % difference.
The rating of horsepower has gone through several notable changes over the years, with the newest methods being the most honest.
Through ~ '71 most U.S. manufacturers used a "gross" horsepower rating. In this method the power was measured on an engine dyno with absolutely no power accessories of any kind - some manufacturers even had the water pump driven by an external source. No air cleaner or mufflers. Non standard ignition advance curves and other than production tuning tricks were allowed. It is easy to see that this resulted in grossly over rated horsepower claims - but it did help to sell a lot of cars. At times the gross hp figures were skewed in one direction or another. But you can forget about any L88 ever making 560 hp, in as delivered setup and tune - of course, like any engine, the story was different after a race shop program.
Starting in '72 nearly everyone went to a net hp rating. This required that all accessories be installed and functional, and that the full air cleaner and complete exhaust system be in place. It also required the same tune up ignition advance etc. as the car would actually be delivered with. This gave much more realistic power ratings, but left many buyers disappointed with the loss of bragging rights. The combination of lowered compression ratios and the trend of ballooning car weights with a lowered hp rating made a lot of people think the day of factory performance was dead.
The latest standards for rating horsepower did away with the "ideal" conditions of refactoring horsepower. Namely that the previous method "correcting" horsepower to sea level at 60 degrees with favorable humidity. Instead, the new standards call for correction to 77 degrees ambient and 1,000 feet elevation with a less favorable humidity. The newest test also requires an independent laboratory test - in other words it takes manufacturer shenanigans out of the equation. The latest standards were meant to give horsepower figures that really were achievable by most engines in a real world situation. The first car to fall under the new method of rating power was the 427 LS7 Corvette Z06. It actually resulted in the final power rating being bumped 5 hp from the GM's testing results. For more than a few Japanese brands it resulted in manufacturers having to restate the horsepower downwards because the independent lab could not duplicate the factory test results.
If you took a new LS7 and tested it under the old gross method, it would probably approach 600 hp.
Most high performance engine and chassis dynos have not kept pace with the recent revisions in calculating horsepower - usually the sea level 60 degree and good humidity correction factor is still used. This is one reason why so many performance shops are showing the new LS7 Z06 Corvette so close in rear wheel hp rating vs. Chevy's flywheel hp rating. It is also why so many magazine engine building articles were showing the high performance factory crate engines as making more power than advertised. It wasn't because the magazines were getting ringer engines, just a simple matter of how the raw horsepower rating was factored. Chevy had actually been using the less favorable newer correction factor for some time with respect to their high performance crate engines.
Thomas
The rating of horsepower has gone through several notable changes over the years, with the newest methods being the most honest.
Through ~ '71 most U.S. manufacturers used a "gross" horsepower rating. In this method the power was measured on an engine dyno with absolutely no power accessories of any kind - some manufacturers even had the water pump driven by an external source. No air cleaner or mufflers. Non standard ignition advance curves and other than production tuning tricks were allowed. It is easy to see that this resulted in grossly over rated horsepower claims - but it did help to sell a lot of cars. At times the gross hp figures were skewed in one direction or another. But you can forget about any L88 ever making 560 hp, in as delivered setup and tune - of course, like any engine, the story was different after a race shop program.
Starting in '72 nearly everyone went to a net hp rating. This required that all accessories be installed and functional, and that the full air cleaner and complete exhaust system be in place. It also required the same tune up ignition advance etc. as the car would actually be delivered with. This gave much more realistic power ratings, but left many buyers disappointed with the loss of bragging rights. The combination of lowered compression ratios and the trend of ballooning car weights with a lowered hp rating made a lot of people think the day of factory performance was dead.
The latest standards for rating horsepower did away with the "ideal" conditions of refactoring horsepower. Namely that the previous method "correcting" horsepower to sea level at 60 degrees with favorable humidity. Instead, the new standards call for correction to 77 degrees ambient and 1,000 feet elevation with a less favorable humidity. The newest test also requires an independent laboratory test - in other words it takes manufacturer shenanigans out of the equation. The latest standards were meant to give horsepower figures that really were achievable by most engines in a real world situation. The first car to fall under the new method of rating power was the 427 LS7 Corvette Z06. It actually resulted in the final power rating being bumped 5 hp from the GM's testing results. For more than a few Japanese brands it resulted in manufacturers having to restate the horsepower downwards because the independent lab could not duplicate the factory test results.
If you took a new LS7 and tested it under the old gross method, it would probably approach 600 hp.
Most high performance engine and chassis dynos have not kept pace with the recent revisions in calculating horsepower - usually the sea level 60 degree and good humidity correction factor is still used. This is one reason why so many performance shops are showing the new LS7 Z06 Corvette so close in rear wheel hp rating vs. Chevy's flywheel hp rating. It is also why so many magazine engine building articles were showing the high performance factory crate engines as making more power than advertised. It wasn't because the magazines were getting ringer engines, just a simple matter of how the raw horsepower rating was factored. Chevy had actually been using the less favorable newer correction factor for some time with respect to their high performance crate engines.
Thomas
Might want to take a look at this article:
http://www-unix.oit.umass.edu/~tcroy/articles/horsepower.htm
http://www-unix.oit.umass.edu/~tcroy/articles/horsepower.htm
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