Gross Horsepower LT-5 - ZR-1 Net Registry Forums

Paul Workman · 11-25-2012

The trouble with standards, are there's so many to chose from:

J1349 circa 1972, June 1990, 2004 (crank hp)
J1995 (gross hp)
J607
J2723 certification spec
VCA (UK certification spec)

And, NONE of these address AVERAGE torque or hp at the wheel!

Bottom line: You can measure at the crank, or you can measure at the wheels, or somewhere in between. But, trying to predict one from the perspective of the other is full of variables as to make an absolute prediction either way almost folly.

I believe the trick is to pick the one that best fits the application, make the measurements as a base case for comparison to future improvements or modifications.

Examples: Engine builders with clients paying for results aren't interested in driveline losses. Drag racers aren't interested in peak hp at the crank, they're interested in trap speed (and ET to a lesser degree).

To get J1995 (crank net hp) I have to pull the motor. But, considering inertia of drive line hardware (including rotating mass), the hp number derived is significantly removed from how the car is going to actually perform, at least where differences are subtle.

For seat of the pants fun and a more accurate indicator of drag race result prediction, the inertia dyno measuring output at the wheels is a much better fit. However, quarter mile trap speed is a very good indicator for performance comparision! It isn't perfect either, but it does take into consideration "power under the curve" which is largely overlooked when in a pissin match over gross vs. net vs. shaft vs. wheel horsepower ratings.

Where the rubber meets the road is where real performance is demonstrated...I think is the main point ... at least it is for me. I'm not saying either isn't important in their own right - engine builders have their point of view and race drivers have a different perspective on output. But, trying to predict either from opposite ends of the drive line (read: is it SAE 15% standard loss or some other 18% standard) is where things get fuzzy really fast.

Hib...You said Barney measured 413 at the wheels. You used 18% drive line loss to predict you have about 504 net hp at the crank. My 90 Z measured 432 hp at the wheels, using SAE/Marc's 15% drive line loss, my net crank calcs to be 508 hp. Now, going on net crank hp and nothing else, I imagine it would be a near toss-up in the quarter mile, all things being equal. Care to race and compare??

P.

11-25-2012	#11
Paul Workman Join Date: Nov 2007 Location: Squires (near Ava MO in the Mark Twain N'tl Forest) - Missouri Posts: 6,466	Re: Gross Horsepower LT-5 The trouble with standards, are there's so many to chose from: J1349 circa 1972, June 1990, 2004 (crank hp) J1995 (gross hp) J607 J2723 certification spec VCA (UK certification spec) And, NONE of these address AVERAGE torque or hp at the wheel! Bottom line: You can measure at the crank, or you can measure at the wheels, or somewhere in between. But, trying to predict one from the perspective of the other is full of variables as to make an absolute prediction either way almost folly. I believe the trick is to pick the one that best fits the application, make the measurements as a base case for comparison to future improvements or modifications. Examples: Engine builders with clients paying for results aren't interested in driveline losses. Drag racers aren't interested in peak hp at the crank, they're interested in trap speed (and ET to a lesser degree). To get J1995 (crank net hp) I have to pull the motor. But, considering inertia of drive line hardware (including rotating mass), the hp number derived is significantly removed from how the car is going to actually perform, at least where differences are subtle. For seat of the pants fun and a more accurate indicator of drag race result prediction, the inertia dyno measuring output at the wheels is a much better fit. However, quarter mile trap speed is a very good indicator for performance comparision! It isn't perfect either, but it does take into consideration "power under the curve" which is largely overlooked when in a pissin match over gross vs. net vs. shaft vs. wheel horsepower ratings. Where the rubber meets the road is where real performance is demonstrated...I think is the main point ... at least it is for me. I'm not saying either isn't important in their own right - engine builders have their point of view and race drivers have a different perspective on output. But, trying to predict either from opposite ends of the drive line (read: is it SAE 15% standard loss or some other 18% standard) is where things get fuzzy really fast. Hib...You said Barney measured 413 at the wheels. You used 18% drive line loss to predict you have about 504 net hp at the crank. My 90 Z measured 432 hp at the wheels, using SAE/Marc's 15% drive line loss, my net crank calcs to be 508 hp. Now, going on net crank hp and nothing else, I imagine it would be a near toss-up in the quarter mile, all things being equal. Care to race and compare?? P.