Any ideas on how much your numbers will vary from one dyno to another?

 

There does seem to be a difference between the dynomax vs Mustang dynos.
I did some research a few months ago and the Mustang tends to be about 10% less than the dynomax. Otherwise, it would depend on how well each owner of the dyno maintains them.
:chevy

 

if they're all the same model of dynojet, they're terribly consistent. i've done same day, different dyno comparisons here in houston.

you're basically at the mercy of the weather station. some of the older dynos rely on the user to input weather data. if they only bother to do this the first time they use the dyno (for the day), then forget about comparing corrected numbers. also, unscrupulous shops can plug in incorrect weather numbers in order to make their before/after numbers look better.

the newer dynojets have an automatic weather station that updates in real time. numbers can still be fudged, but it's harder to do so. you'd have to put a heater and humidifier blowing over the weather station, or figure out how to tell the software to ignore the weather station.

some of the oldest dynojets did have a different inertial weight. you might see a few hp difference on those, relative to the newer ones. (changes the rate of acceleration, therefore changing actual power that goes to the wheels relative to power absorbed in the engine's/drivetrain's internal inertia change.)

also, a well-used one might have slightly freer bearings, but i'd call that a 1-2hp difference at most.

spoken from someone who's logged close to 1000 dyno pulls...