HP: SAE Gross or Net ?

[Don]

At some point in time, the production car manufacturers converted from SAE-gross HP to SAE-net HP. From the web site:

"My 2003 Z06 has 405 hp and is advertised as one of the most powerful production Corvettes ever produced...yet my brother had a 69 that was rated at 425 horsepower...some models had 450 hp....what's the story? What is the conversion factor from today's horsepower ratings to those of the 60's?"

" The short answer, is that the '69's horsepower was listed in SAE-gross numbers and your ZO6 power is SAE-net, a big difference. This forms the tip of a big, confusing issue that deserves a major explanation. To do that we need to define "horsepower." Unfortunately, how it is defined depends upon who is doing the defining..."

http://neptune.spacebears.com/opine/horsepwr.html

http://www.secondchancegarage.com/public/201.cfm

Questions:

* Year the change was made from Gross to Net ? 1972 is correct ?
* Production car manufacturers, Ford, GM, etc., use the Gross or Net method for their factory crate engines, or ?
* The individual after market engine assemblers, use Gross or Net for their HP ratings, or another method ? Could vary with no consistent approach ?

[Keithc8]

I feel that most of HP numbers that you see listed today by the car builder is at the rear wheels. With that said I see evry few that willoduce what they say they will at the rear wheels. The new Z06 I believe is rated at 505 HP but is more like 450HP at the rear wheel. I would have to do some research on this myself. I do believe that the new cars get the power to the rear wheels better than the old cars and the Cobras for sure. It take a 550HP engine to get around 450HP at the rear tires on most Cobras and that is with a good exhaust and drive line. It would be interesting to have some of the engine of new design and old on the same engine dyno to compare them.


Good luck, Keith

[Dominik]

The old "conversion" from rwhp to engine looked sort of like this:
496hp at the rear wheels (80%) due to 20% losses in the drivetrain (that's a lot of heat in your drivetrain, mate! About 85 hair dryers) from "620hp" at the flywheel.

The real losses are more like 10% in direct (4th) gear. Hot rodders now like to add another 80hp for poor exhaust flow and accessories.

Now you have 700hp!

Why would one add losses from exhaust or accessories? What you have at the flywheel is what you have. And some exhausts are very efficient.

It's more like this:
496hp rwhp equals to 555hp at the flywheel. Not 700hp (like 555hp isn't enough!)

There is another flaw, quite minor in this overall picture, DIN and SAE readings. DIN is done at 70F, SAE at 60F.

(Above numbers were actual readings from my car's coast down/acceleration test in 1995 with a GM computer attached on a raod with known profile)

[undy]

The accepted drivetrain loss for the new Z06 is 11.5%, +/-. That would support what Keith said. My headers/intake/tune C6 Z06 puts down 494 rwhp, SAE. Factoring in the 11.5% that would equate to 558 hp. I recently redyno'd it and at a 74 deg day it put down 509 rwhp, standard not corrected.

BTW ... It not just the difference between Gross and SAE corrected ratings. There's a new accepted rating on the block which tends to under-rate "conventional" HP ratings. It's the new SAE-J rating. Most OEMs have now switched to it now.

I'd hazard a guess that a totally stock 505hp LS7 crate motor would achieve 550+ corrected engine dyno HP on most of our board vendors' engine dynos.

[Dominik]

Undy,
that goes along with my finding too (pretty much). We just need a dyno'd Z06 engine to confirm. It's "cleaner" to talk in rear wheel horsepower. But then again, you accelerate with torque!

[Bob In Ct]

Don't get too hung-up on torque. When it comes to going fast or going from 0 to 100 in a hurry, it's all about HP. If it were all about torque the NHRA would be running lots of tractors and bulldozers. I've never heard Robert Yates or Jack Roush say he's looking for another 20 FT-Lbs.

Bob

[Don]

* The individual after market engine assemblers/builders, use SAE Gross or Net for their HP ratings, or another method ? Could vary with no consistent approach ?

How then does a hobbyist compare engine A to B to C etc., etc., ?

Any thoughts ?

[Dominik]

Originally Posted by Bob In Ct

I've never heard Robert Yates or Jack Roush say he's looking for another 20 FT-Lbs.

Bob

I learned that torque accelerates you and Hp gives you top speed, which would explain the Nascar search for more Hp?

[DAVID GAGNARD]

Originally Posted by Dominik

I learned that torque accelerates you and Hp gives you top speed, which would explain the Nascar search for more Hp?

Or as my old cousin/mechanic always told me, "torque gets you there, HP keeps you there".......

BTW: he liked big cube/low rpm engines with tons of torque for his trucks...

David

[Bob In Ct]

David:
Engines with tons of torque at the low end are also producing more HP at that same RPM. Which car do you think would accelerate faster, one with 500 FT-LBS at 2,000 RPM or one with 300 FT-LBS at 6,000 RPM? That's right, the 300 FT-LB engine.

Torque is only a measure of twisting force. I can produce 200 FT-LBS on my bicycle! That doesn't mean I can do a quarter mile in 15 seconds when I ride it (because I can only spin the pedals to 100 RPM). Torque has no time component because it is not a measure of power, and power is what does the work. The reason most of us love big low RPM torque values is because those same values produce more HP at those same low RPMs, and that's why we accelerate faster.

Remember, HP = (Torque X RPM) / 5252

Power increases with either an increase in Torque or RPM.

Bob

[Don]

*** Question on the Road & Track Forum:

For the manufacturers advertised HP, is it SAE Gross or Net, or some other method ?
Measured at the flywheel, with or without accessories, exhaust, etc., or at the rear wheels ?

When compared to after market engine builders/assemblers, same procedure or each builder/assembler may use a different method making comparisons to each other and manufacturers engines inconsistent?

*** Reply from Road & Track Associate Engineering Editor :

Over the years the measurement of horsepower has changed. It also differs in other countries.

Currently in the USA manufacturers are required to test their engines to an SAE certification J1349(http://www.sae.org/certifiedpower/brochure.pdf ). The specifications have changed over the years, but they've always measured horsepower at the crank, not at the rear wheels. And thus are listed as bhp, indicating that the measurement was taken with a brake-dynometer.

The current SAE test takes into account all the accessories on an engine, as well as ambient air conditions. The idea being that this is a very close approximation of the power being delivered from the crank in the production car at the dealership.

Hot rod shops use a chassis dyno and measure hp at the wheels, sometimes referred to as rwhp (rear-wheel hp). This will be less than the SAE measure because of the energy lost in driving the transmission, differential and wheels.

Typically chassis dyno results will be corrected to standard conditions, but you are right, it is difficult to compare results taken at different times, locations and dynometers. For best comparison results, the cars would have to be tested at the same time, place and machine.

-Shaun

Shaun Bailey
Associate Engineering Editor

*** Comment:

Correct or ? The 2011 ZR1 6.2L Corvette is rated @ 638HP @ the " crank " , RWHP for the engine would then be in the range of approx. 575 + or - , for comparison to high performance after market engines, that do not have a supercharger ( four-lobe Eaton® Twin Vortices Series™ supercharger with intercooler )