Anyone?? I had the stockers and switched to 19" Blitz Z1's, i know this will affect real world power/acceleration due to the added weight, but would the bigger size be harder to spin on a dyno resulting in less power?
-Rob

 

I don't think wheel size should matter. Wheel size will affect acceleration, but not dyno results.

 

I have read claims from pretty legitimate sources that decreasing wheel weight (unsprung weight) can actually result in HP gains on the dyno. I will look for some links to back this up....

 

Depends on what kind of dyno it is. Most dynos (ie dynojet) are the inertial type and in this case yes, bigger and heavier wheels (and tires) will decrease reported horsepower. On a brake dyno you will not see a difference. Inertial dynos work by loading up the wheels a specified amount and seeing how quickly it can accelerate to redline. The more power the car puts out, the faster this is acheived. However, because the wheels are accelerating, rotating mass will have an affect on this acceleration, with a higher moment of inertia making it more difficult to accelerate, thus causing a lower reported wheel horsepower even if nothing else has changed. Bigger wheels increase your moment of inertia in two ways: 1. the weight is located farther from the center of rotation and 2. they are (usually) heavier. A lightweight flywheel will also increase your HP readings, as well as, to a miniscule degree, lighter pulleys, driveshaft, and brake rotors, as these are all rotating masses.

Now, a brake dyno is a different story. These work by placing a load on the engine while the wheels are rotating and increasing that load until the car cannot accelerate anymore, even at full throttle. Since in this case a reading is taken when no acceleration is occuring, there is no change in inertia and thus rotating weight has no effect. This type of dyno is a more accurate measure of engine power, although imo an inertial dyno is a more accurate measurement of practical power, since you usually need a lot of power when accelerating and rarely encounter brake dyno-style situations (such as towing a small house up a large hill).