As I've said...these are tools. There are standards and if (a huge IF ) would all use the same SAE standards, and control for environment we would have all the disparity between dyno's and flow benches.
Same with micrometers. Good mic's come with a standard that is certified. Good machinists verify their mic's on a regular basis. There is absolutely three different machinists, using different mic's all set to a certified standard should have any more than a .0002 discrepancy at the most.
One big issued I've found over the years in the difference between crank and wheel dyno's is that most engine dyno's are in a controlled space so they don't use ambient air. Where a chassis dyno almost always uses ambient air.
Also, the engine dyno is often done with dyno headers that are much better than what goes in the car. The chassis dyno will use the headers that fit the car. Lots of guys have a dyno carb, a known working piece to establish a base line. That's all good. But you need to test with the actual carb to be used in the car, like a chassis dyno would.
Never overlook ignition systems. Engine dyno shops usually have an ignition system already on the dyno. It's usually a tricked up piece, or at least it's been well tested. On the chassis dyno you are dealing with the ignition system in the car. Who knows what the grounds are like, or the wiring in general.
As I've said before, testing, testing accurately is very expensive. A local dyno to me (way back in the day) were stumped when I brought in a brand new ignition to run with my engine on the dyno. We sorted everything out on his dyno then wired up my stuff, including the coil. The operator assured me that my ignition would be a power loser because his box was warmed up by a guru.
Big disappointment when my off the shelf ignition was better across all the RPM range.
You have to test like you intend to race of drive. Whoever coined the phrase paradise makes perfect was an idiot. If you continually practice, but practice incorrectly you will only get perfect in doing it wrong.
Perfect practice makes perfect. Doesn't matter if it's bowling, golf or building engines. Duplicate real world as close as you can and your results will correlate between the flowbench, dyno and track.
