Impressive right? Absolutely. But then you look at the blue line on the graph and realize it is a 5th gear dyno. The red line is a 4th gear dyno and the numbers drop to to 638 wheel horsepower. Torque also takes a tumble from 636 lb-ft to 602 lb-ft.
What causes this? The fact that the Dynojet is an inertia based dyno. This means a fixed weight is spun and how quickly it is spun is measured. The computer plugs the values into an equation and spits out a graph with numbers. Now did the car suddenly gain horsepower because it changed gears? No.
What happened is that the drum was spun faster in 5th gear than in 4th gear. This graph illustrates the effect perfectly as it is set to read in MPH. Notice the higher MPH has the higher numbers because it is spinning more quickly. If this graph was reading RPM and not MPH someone without morals could claim a horsepower gain saying they made some tuning or modification changes when really all that changed was the gear that the dyno run took place in.
This does not make the Dynojet an invalid tool. Due to the amount of runs out there and its popularity it is an incredibly important and useful tool. However, it is important to understand context and that numbers are not set in stone. There are many variables that affect them. Why we like to see timeslips in addition to dyno graphs to get a complete picture is because you can not do a 1/4 mile pass in this car solely in one gear. Well, you can, but you will not like the result. You have to go through the gears in progression to get an accurate representation on the drag strip you can not simply just say I'm doing this 1/4 mile sprint in 6th gear to get the quickest and fastest possible time.
If a tuner will not provide a run file for download to analyze or only does runs in a higher gear, question it. Tuners who have nothing to hide will show their runs. Also, if you get a baseline dyno and then get some modifications, dyno afterwards to see the difference in that same gear to minimize variables.
No automotive performance numbers are set in stone. No single measurement is perfect. The more data from various sources for analysis, the more light is shed on the big picture.