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Originally Posted by knucklesplitter
If the effective gearing changes that are due to the tire diameter changes the horsepower that a dyno reads, then the dyno is not very good at reading horsepower. Actually dynos do not measure horsepower anyway - hp is just a calculated number based on torque and rpm. The dyno reads torque and rpm and then calculates hp from those two inputs. There should be a place to enter the tire diameter in the dyno's computer so that the difference in effective gearing does not change the hp, because changes in gearing just do not change horsepower (except in more extreme cases where the frictional losses change).
Also as far as rotating mass is concerned, if the car accelerates slow enough while it's on the dyno, the mass becomes a negligible factor. The faster the car accelerates on the dyno the more the mass will make a difference in the WHP. If a load-based dyno holds the car at a certain RPM and reads torque then the mass makes no difference.
Because of these and other factors, such as run-to-run variability in dynos and automobiles in general, it is silly to speculate about small differences in two diffent cars' HP as measured on a dyno. The dyno is a tool for tuning and should be used with extreme caution during bench racing.
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I've never seen a tuner that calculates and enters tire diameter on a dyno per run. I'm willing to bet Matt and Dean were dyno'd with the same settings.
Furthermore, dyno's are easily +/-5hp from run to run, let alone from car to car on different days. i.e. It's ridiculous for Dean to be trying to figure out why he made more hp than Matt. My post regarding the tire sizes was an attempt to demonstrate that Dean's speculating on things that probably make no repeatably observable difference, while he's ignoring things that are known to have more of an effect on numbers.
In reality, I would say anything +/-20hp on a dyno is pretty much similar enough to write off as "the same". On the track the faster car between Matt and Dean will be based on the driver, not the dyno'd hp. Trying to figure out vf39 vs. vf43 based on the data in this thread is a waste of time. According to the data and the margin of error involved, the turbos are equivalent. Same goes with all the rest of the differences between Dean and Matt's cars.