View Single Post
Old 06-18-2013, 06:25 PM   #62
blehhh
Scooby Specialist
 
Member#: 316517
Join Date: Apr 2012
Chapter/Region: NESIC
Location: Portland, ME
Vehicle:
2012 WRB WRX Hatch
TP Stage III & ELH

Default

Quote:
Originally Posted by Corners View Post
Blehhh. Engineer here. I understand what you're trying to say, it's just that you are incorrect in this case. The WRX and STi actually have very similar power levels no mater which way you want to measure it. They just do, as shown in the real world. Also, note that the 142 mph top speed of the WRX is electronically limited, not drag limited. If the difference between the two was as pronounced as you're claiming, then using the relationship of aerodynamic drag power being proportional to velocity cubed, then if 265 hp can only 142 mph, then that would mean that the STi would have to be putting down 364 hp to hit 158, which we all know is incorrect. If there really was a 40 hp difference between the two cars (which there's not) then at 150 mph, 40 hp equates to an 8 mph higher top speed.
I'm an engineer as well. The top speed was a bad example. I didn't know they were electronically limited. I haven't done the math in years, but a lot of cars are actually drag limited given their power levels. And many cars have to be in lower gears to achieve their top speeds because in their top gear the engine isn't making enough power to overcome the drag at the given speed, at the geared rpm.

And the power to speed relationship is squared, so it does take exponentially more power.


While I'm not sure if the empirical differences are substantial, there is a conceptual difference in whp measured on a dyno under acceleration, and the whp actually put down in a steady-state system, in which drivetrain inertia is not a factor.
* Registered users of the site do not see these ads.
blehhh is offline   Reply With Quote