To add to Ryan's reply...
Hub dynos, such as the DynaPack, will be more consistent from test to test than roller dynos. With roller dynos, you can have differing air pressures in the tires, the dyno monkey can strap a car down harder one day and lighter the next, etc. Hub dynos remove at least that portion of the variability. What's left is a solid drive train that should give you results well within the statistical error range of the dyno itself (0.5-1%ish range). A roller dyno can easily give you errors in the 5% range if not set up consistently.
Whatever style you choose to take a baseline on, stay with it after modding to see what gains you achieved. If it's a roller dyno, I suggest 2-3 different days (with hopefully different guys setting it up) to average out inconsistencies in setup. Yes, this obviously gets expensive on a roller dyno, but if you want solid numbers you can either do it that way or go with a hub dyno. The same thing should be done after the mods are in place to see what was gained. If you go from one style to another, you need to take another baseline. Mustang dynos are notoriously low in numbers, but that doesn't mean they're wrong, and hub dynos are typically the highest values. You cannot compare one person's Mustang dyno to another's hub dyno run and expect to compare apples to apples.
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