Mass build cars should have about the same horse power after leaving the factory. They have the exact same drivetrain built, but there is a little room for elevation and fuel type. On the same dyno, on the same day, they will be very close.
Its the dyno that plays the larger factor. Some dyno's read higher or can be made to read higher ( because higher numbers will help advertising for the shop), and some read lower and several reasons come into play on that.
Example,
Yesterday, I was tuning a package that the owner said was previously dyno'd at 499 RWHP on a mustang dyno in January on a dyno in Virginia on a cold winter day.
So like normal with no changes made to the calibration, Tampa Tuning did the normal baseline dyno pull, and we come up with 393 RWHP and 5 degrees of knock on a 248 Dyno Jet. Normally, Id expect those numbers to be reversed on two dyno types.
After tuning the package, I build it up to 430 RWHP.
So like I do with every client after we finish up the dyno tune and the street refinement process with them driving, I always ask if they notice a differance. The smile never lies. According to the owner, the car was faster with being quicker and smoother through the RPM band now. He was able to drive with the A/C on without the car stalling, and although the numbers were differant, the car performed much better.
So, the dyno is a tool. More can go into factors on why dyno themself read differantly, compared to why the same build packages on the same make and model vehicle, have differant numbers from dyno to dyno.
The dyno only shows me a graph of numbers, My HP Tuners scanner shows me how and why im getting those numbers.
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Phil