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Tuesday, December 30, 2008

Nissan GT-R Horsepower: The Truth!!

Yes, we have already dynoed Nissan's GT-R and we made some very controversial claims as to the true power output of said GT-R. The unknown in that test was the elusive "driveline losses." That's the amount of power lost to inertia of spinning parts like gears and driveshafts and friction in the transmission, U-joints, axles, etc., which have traditionally had to be estimated. We used the conservative estimate of 15% that's usually used for simple manuals, but this one uses twin hydraulically actuated clutches, includes all-wheel drive, with two separate prop shafts, so we considered our estimate conservative. Our Dynojet inertia-type machine indicated a mean output of 430.6 hp at 6700 rpm and 425.3 lb-ft at 3900 rpm, which we interpreted as 507 hp and 500 lb-ft after accounting for the 15% loss.

We were recently contacted by a Motor Trend reader who saw our first GT-R dyno story. He said he had a new type of dyno that was able to compute the drivetrain losses directly and accurately, removing the guesswork. How, you might ask, does it manage this? That's what we're here to tell you.

When you arrive at Hyper Power International and first see this new dyno, it looks like any other inertia-type dynomometer -- four big rollers, wires, computer screens, the usual -- and like any other dyno you start the process by strapping your vehicle down. Once secured, the next step is to do a gearing calculation run. This provides the dyno with the engine rpm in reference to the roller speed thus eliminating the need for an rpm pickup, which can provide inaccurate information.

Just before we make our first run the ambient weather conditions are entered into the dyno computer. While we were there the temp ranged from 68 to 72 deg, humidity ranged from 44% to 49%, and the barometric pressure ranged from 29.55 to 29.61 in. of mercury. Again, all this is operating norm for dyno runs. Now it's time for some power pulls.

Our target is Nissan's factory claim of 480-hp @ 6400 rpm and 430-lb-ft of torque @ 3200-5200 rpm.

First pull:
Ending at 6500 rpm, our first run's results were: 485.9 hp @ 6116 rpm and 469 lb-ft @ 3834 rpm (corrected for weather and subtracting the measured losses).

Second pull:
Again, going to almost 6500 rpm the second run gave us 485.5 hp @ 5984 rpm and 475 lb-ft @ 3717 rpm. This is only a 0.4 hp (0.08%) difference from run one with the torque differing by 6 lb-ft (1.2%).

Third pull:
The final run provided us with 483.4 hp @ 6106 rpm and 466 lb-ft @ 3811 rpm, a difference of 0.4% for hp and 1.9% for torque.

Averaging the runs gives us a Hyper Power rating of 485 hp @ 6050 rpm and 470 lb-ft @ 3800 rpm.

So, how did Hyper Power USA measure our driveline drag? At the end of a pull, the computer instructs the operator to put the vehicle in neutral, the dyno then measures the rate at which all the rotating parts slow down, which is directly related to friction and inertia. Our instruments found the loss to be exponential -- as the speed grew so did the loss. We saw a loss of 23 hp at 50 mph and 84 hp at 100 mph. Over the three runs we saw a driveline loss range from 88 hp to 93 hp. Thus, there is no longer the need to guess at a percentage, Hyper Power Dynamometer's DYNOmite measures it and gives you an actual figure generated by the engine and consumed by the driveline

Here is an example of why using a fixed percentage number for drivetrain loss is flawed in calculating measured dyno horsepower:

Take a turbocharged vehicle, dyno runs put its output at 400 hp and 400 lb-ft of torque. Applying 15% for driveline loss yields an actual output of 460 hp and 460 lb-ft of torque. Now, take same vehicle, turn up the boost on the turbo. The dyno pulls now have the same exact vehicle rated at 600 hp and 600 lb-ft torque, apply 15% driveline loss, actual output is 690 hp and 690 lb-ft torque. Why does the loss increase with more horsepower? It's the same exact car, just with the boost now turned up. The loss should be the same; whatever the horsepower number is, it's the same driveline.

So, what does this all mean? Well, it suggests that Nissan is basically telling the truth with their horsepower numbers, but the torque numbers are conservative by about 40 lb-ft.

So what does this tell us about Nissan's mighty Monster's miraculous performance? It suggests that the super-short gearing, the twin-clutch gearbox's uninterrupted torque flow, and the amazing (and also controversial) launch-control system are more responsible for generating the incredible acceleration numbers than hidden unclaimed horses.

Pull one versus two
Pull two versus three