Hey Frank:
What engine, what run, and how were you checking for sameness?
If you're not trying for a stunt run it probably doesn't matter.
If you're running an engine tuned for a 2-4 break, I'm very surprised that you didn't notice a difference. I would expect that without a uniflow tank, possibly running on atmospheric pressure, that you'd start with a good 2-4 break and end up in a solid 2.
If you're running an engine that needs a wet 2 run (i.e., an LA 46 the way it's typically run), then unless the difference is extreme the only way to tell is by timing laps. My LA 46 generally speeds up from about 5.4 second laps to about 5.2 second laps over the course of a flight, with a non-uniflow clunk tank on muffler pressure. I have a Tower 40 on a similar tank that speeds up about twice as much. I would like the runs to be dead consistent.