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Old 02-20-2006, 01:16 PM   #71
patr
Scooby Specialist
 
Member#: 97
Join Date: Jul 1999
Location: Vancouver, BC, CANADA
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what changes is based more on the rotational inertia and mass. i.e. wheels are never the same weight/mass so that has a bigger effect than anything else.

so lets just assume for the sake of argument that you somehow were able to find 2 wheel and tire sets that magically had the exact same weight and rotational inertia (not possible, but lets just assume it was), and they were of different diameter.

the engine output remains the same, if you used wheel speed to determine RPM the back to back dyno plots will be "shifted" slightly to the right or left, but the shape will remain the same. At least with our dyno. The number difference torque-wise should be negligible, however, since power is torque x RPM, the "peak" power may shift but its actually misleading since the torque has not changed. BUT since we use a true RPM pickup, not wheel speed, you get minimal if "negligible" changes. In other words, it doesn't change much. Not much AT ALL compared to the changes you will see from using heavier/lighter wheels.

This i further backed up by the fact that you get the exact same plots if you do pulls in 3rd, 4th, 5th (assuming temps are all inline for each pull, and you dont get out of control on the road speed since the frictional losses are a bit higher) - in other words, lets say up to 180 km/h, doesn't matter what gear you are in, you get the same plot - but reality is that temps are hard to control so and the frictional losses go up at very high speeds.

BUT, one thing to understand is that a LOT of dyno operators use wheel speed to get and RPM number, which means that the dynos plots will move A LOT if you change wheels. We use a direct RPM probe. You can tell if someone is using wheel speed if the HP and torque axes do not cross exactly at 5252 RPM.

But in reality, wheels are not the same weight. You can tell a difference from 15": to 18" ! But its not because of size, its because of the weight.And with the increase indiameter, you get slightly higher frictional losses because the wheel speed are up, but if its a small change, you won't see it much. If you are testing, for example, a small change, like one pipe and another pipe with a 1/4" difference, I would stick with the same wheels, but otherwise, I wouldn't worry about it.
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Last edited by patr; 02-20-2006 at 01:41 PM.
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