A non eddy current chassis dyno cannot load the rollers to simulate the amount of resistance that the car will see of the street. This causes the engine to react much differently that it will do as compared to a street tune.
Our Mustang Chassis Dyno uses eddy current, you plug the type of chassis you are going to dyno into the dyno computer, and it knows the basic weight of that chassis and the drag coefficient. Our dyno also has a weather station and it takes this info into consideration as well. We can also run a car through the 1/4 mile on our dyno, and surprisingly most cars are within several hundreds of a second of what they run at the track. Not tenths, but actually hundreds of a second. Our '95 Formula is so consistent at 11.31 to 11.32 at the track, that we use it to verify our dyno.
Where this leads to is you can accurately tune a forced induction equipped car on a chassis dyno that uses eddy current. Since it places the correct load to the rollers, the timing will come in just like it does on the road, and we can tune without having to worry about the typical road hazards. We tune a lot of 900 to 1000 whp cars, and street tuning can be downright scary.
As long as you monitor inlet air temps and keep them consistent with what the car sees on the road, everything will be extremely accurate. You have to keep in mind that inlet air temps on the street are going to vary depending on weather conditions, so a dyno tune with a little higher inlet air temps is not a bad thing, as you never know if someone is going to be in extremely hot conditions.
We don't dyno tune Ecotec engines, so I'm not on here trying to drum up business. Bob

 

you're failing to see airflow differences.

also, while mustangs are good to use for getting everything dialed in. not everyone has a mustang dyno. dynojets are cheaper and easier to find.

 

Check out my second paragraph, the key is in keeping an eye on the inlet air temps. Bob