Maybe I can be a little clearer. Crank HP and wheel HP will not lower just by changing wheel size. The ability to transfer that HP to a dyno graph will. A larger wheel will accelerate the dyno drum more slowly. The dyno software will interperate this as less HP. This is assuming that the tire diameter is larger than stock as well as the 19" wheel. If it because of a significant weight increase then the result will be the same. For example, remember as a kid when you'd turn your bike upside down and spin the wheel? Now, mount a solid wheel made of steel on the bike. Even though YOU have the exact same amount of power to apply, the wheel will be much harder to spin. If you them mounted a 6" aluminum wheel it would be very easy to spin with the same power.

Your car will read lower HP and feel slower with larger, heavier rear wheels and tires even though it still makes the same HP. If there was a way to offset the wheel difference in the dyno program then the readings would be the same. There are dynos that bolt on in place of the wheels. I've never used one but would guess that they'd be more accurate.

A dyno is just a tuning tool. Different machines, operators, weather conditions, tire pressure, how tight the car is strapped down, etc can all effect the results. You can get too wrapped up in "dyno numbers". I build Harley drag racing motors. A dyno is used as a tool to help us tune the bike not as a success or failure guage.

Mark

2003 Sedan AT. Ivory Pearl/ Willow
20% tint. Z-tube. Clear Corners w/ PIAA.
Aluminum Pedals

 

...or ...

the power to the ground doesn't change, but the effects of increased Inertia eat up more of the power (causing reduced acceleration), which causes slower times.
Torque = Inertia * Acceleration.
The torque supplied by the engine is constant at a given rpm(regardless of the type of wheel you have on your vehicle)... If the Inertia increased by a factor of 1.15 because you add bigger and or heavier wheels, then the acceleration is reduced by the same factor,1.15.

Once you get the car rolling, the effect of the increased inertia is very slight. Getting the car going can make a noticible difference in real life situations. Add a set of larger Inertia wheels to a GEO Metro and you'll see what I mean...

<font color=blue>The above statements are only my take on the issue. If you disagree with anything I typed then you are 100% right!</font color=blue>