uklad

August edition of European Car, page 66. These guys dynoed a BMW Z4M with the stock wheels getting 253/215. Slapped on a set of forged aftermarket wheels weighing 4 lbs less each. Re tested it and got 266/225.
8lbs less rotating mass (2 x 4 lbs) got em 13hp and 10 lb/ft.
Huh!? This sound right to you guys?

kodeTT

less rotating mass. better around corners.

mosmoe (OPG)

from the technical write ups I've read, it does make some difference, but not a whole lot. Certainly not 13hp, not unless the OEM wheels were made of lead.

MCPaudiTT - DEFCON 1

the decrease in mass. That would have a dyno on the wheels report higher, but to get the engine HP the correction factor should probably change to keep them the same.

rabbiTT

It's not that the engine is making more horsepower, but rather losing less energy in moving heavier parts. The whole idea of unsprung weight correlates to better handling, I've done that with my car and the instant I got back behind the wheel I felt teh difference. With the lighter weight wheels and other things such as lightened flywheels, mean less rotating mass and therefore less energy wasted spinning them up. A sort of ad hoc increase in power:weight ratio if you will.

Audiboy

since it takes more torque to rotate heavier wheels from 0.

And since roller dynos measure how fast the heavy drum spins from one speed to another, it doesn't help to have another heavy drum (heavy wheels and tires) spin it first. That is energy lost.

Horch it

I like the rotational energy of my fat wheels