A dyno is like any other tool, it can be used for it's intended purpose or mis-used.......
When I dynoed my 331 race motor, the dyno operator (also an engine builder,with a lot of expirence in small block Fords)told me it would probably do the best at 30 to 32 degrees total timing,we started at 30.....he also told me my jetting should be real close....after the break-in and cool down, we began making pulls, jetting was perfect, we adjusted the timing from 26 degrees to 34 degrees making pulls, it made the most HP and torque at 30 degrees total timing....from 30 to 34, there was a 22 hp and similar loss of torque, same when we went down to 26 degrees....there's no way to "feel" that or know it without a dyno....I tried different carb spacers and even air filters till we found what worked best on my engine......
An engine builder and dirt late model racer (passed away a few years ago) had a dyno in his shop, almost within earshot of my house. He'd call me from time to time on a satruday and I'd give him a hand with his race engines....we spent 2 long days on the dyno with one of his race engines, changed headers,carb spacers and used them with a combo of the 2 race carbs we had...after 2 long days, we found a total of 76 hp and about 65 foot pounds of torque over the baseline run, just by trying different carb spacers with different carbs and headers........
They are a good tool if used correctly........

David

I've gone from dyno to dyno with the same engine numerous times doing the Engine Masters deal. They are both always DTS units, and have never read the same - whether going from Detroit to New York or to Ohio.

Simply too many variables involved. Some of the variables are obviously in the engine, but others involve the dyno room setup (intake, exhaust, air handling) and the calibration of the software (there are several rates of acceleration, temperature and air compensation standards to choose from).

The dyno is a directional tool as opposed to an absolute - unless you hold all the variables under control - same cell, same software settings. Very similar to a flowbench, where I can get heads here that flow a certain number and improve upon them - while still being lower but outpowering some other heads that flowed more on the internet somewhere. Some head characteristics do not show up on a flow bench but make a pretty big difference in the engine - such as combustion chamber design.

You will learn more in a day on the dyno than you can learn in a half dozen track days.