Dyno question

[Nader]

On a chassis dyno, do larger, heavier wheels adversely affect torque/horspower readings?

[piedogger]

The wheels wouldn't make a diff on the dyno. The dyno I use has 500lb rollers you have to get rolling. the heavy wheels would hinder susp.(unsprung wieght) possibly launching off the line. Factory porsche wheels are some of the lightest wheels made. I notice your in bellvue one of the best dyno places around our
area is in kirkland: Carburetor Connection (425)820-1871
www.carbconn.com one of the guys there is a porsche freak just got rid of his 930 to start on a 944 project (?)

[ChrisBennet]

I belived rotating weight (flywheel, wheels, etc) will make a difference on an inertial dyno like a Dyno jet. Sport Compact Car magazine did an article/test on it once but I can't remember how much of a difference it made.
-Chris

[Peter Kelly]

On any dyno, the faster you accelerate, the lower the indicated torque and HP will be, unless the Dyno's instrumentation inlcudes inertia correction. This is why if you sweep a motor from low to high rpm, then hi to low rpm, it will always show higher than actual HP on the down sweep and lower than actual on the up sweep.

I am pretty sure that the DJ does not do inertial correction, since you generally get higher numbers for HP the higher the gear you use (do runs in 3rd and then 4th to see this for yourself).

The physics behind this is (on an inertial dyno) is that the power "wasted" accelerating rotaing parts is directly related to the mass (the rotational moment of inertia actually) and the rate of acceleration.

Short answer: the heavier the wheels you use, the lower numbers you will see on a DJ dyno, all else being equal.

[pjv911]

Yes larger heavier wheels will show less performance on a chasis dyno. I have a German made MAHA 500hp chassis dyno with lots of experience tuning cars. The difference is minimal.

Kurt Williams

[autobonrun]

How much typically is charged to dyno a car?