niceputera said:
Amin,
"Readings taken in third gear will definitely yield more power but its not accurate."
a) How is it possible that power from 3rd gear is higher than 4th gear? Is it significant?
b) How come it is not accurate? Not accurate compare to what standard?
Hope you'll share with us. Thanks.
Cheers.
Let me try and answer that, although the R3 technical team may do a better job.
Chassis dyno results may vary as much as 3-4% depending on different gears used. The
de facto standard is actually 4th gear.
As most people know, there are power losses through the drivetrain so wheel hp is always lower than flywheel hp. Front wheel drive cars engines tend to be more efficient than most rear drive configurations due to the layout of components. However most publications overestimate these losses considerably. Most rear drive cars have a 1 to 1, 4th gear which means that the power path goes directly through the mainshaft of the transmission. The only losses here are bearing drag which is less than 0.5% and the viscous drag of the gears running through the oil which is about 1% with hot oil. Indeed, published data indicates a transmission efficiency of 98 to 98.5% for conventional transmissions in 4th gear.
Losses within the driveshaft account for about 0.5% if they are properly aligned, balanced and with fresh U-joints.
Differential losses in the commonly used Hypoid type gearset is in the order of 6 to 10%.
The
worst scenario case for a rear drive setup is on the order of 12.5% in 4th gear, not the 20 -25% often published. If 25% was being lost in the drivetrain, the oil would boil in the differential housing in short order and aluminum transmission cases would fatigue and break from the temperatures generated. On a 200 hp engine, something on the order of 37,000 watts would have to be dissipated out of the transmission and differential housings. Obviously, this is not the case.
Transverse, front drive transaxles usually have no direct lockup gears and no 1 to 1 ratio, however, since the torque path is never turned 90 degrees as in the rear drive setup and efficient helical gears are usually employed for the final drive set, losses are more on the order of 6 to 9 percent in the upper ratios.
So why 4th gear? In general, its what you call "direct drive" or "straight through" becoz of its 1 to 1 ratio, so the most optimal gear due to the least loss through drivetrain and best power reading at the wheels on the dyno!
As an example - R3 quotes about 12% loss to the drivetrain from the flywheel while Links would quote 18%. Just for kicks, let's say we have a 125whp reading from a SR3 from both dynos.
125whp + 12% = 140bhp
125whp + 18% = 147.5bhp
Do you see the discrepancy? But in general, dynos are not fantastically accurate. There are many variables to consider and these variables influence the final result. However, it is proven that dynos run on 4th are more accurate.
Dynos are a good way to benchmark your car, as a stepping stone to tune and mod it, but don't take single, one-off readings as a definitive.
Hope this helps man!
