Re: Stock horsepower variance???
Understand what you are asking in your statement. My opinion of your problem is to use the same dyno, before and after mods for the best differential results. If it was me, I would dyno on the same day as close as possible to remove the most variables. Stock, then mod intake. This yields the best "real" differential measurement. You really don't care what is before or after, you care the most about the difference or the amount gained
Are the HP's identical?
At the factory there is supreme process control regarding the engine operation, dyno torque measurements, calibrations, etc...
When you leave the factory, you introduce so many variables, your percentage of accuracy goes through the roof. Meaning, my WK2 dyno'd on 100 dyno's would make 100 different readings due to different measurement devices and multiple calibration methods, etc...
In my opinion, I believe the factory could test +/- 1% of rated torque (hp calculated from TQ). I doubt any dyno operator (dude with a dyno) actually calibrates his measurement instruments, keeps certificates of calibration let alone properly maintains it.
I can test a 40,000 HP +/- .75% torque measurement on a dyno using scientific instruments, and guarantee it to ASME standards. Our equipment requires 3 hours of maintenance for every hour of use.
2012 WK2 4x4 Limited 5.7L - Brilliant Black - Waiting for programmer + stereo
2000 Dodge Dakota Sport 4.7L - Flame Red - Lowered - 4x10" Kicker L7 Solobarics - 1250W Kicker 1250.1SX - 2x6.5" Infinity Kappa Perfect Components - 900W Kicker 900.4SX