AEM Dyno-shaft permanent inline driveshaft dynamometer

[nrubenstein]

you can do that without a $1200 tool if you can datalog already. all you need is an IAT measurement. :)

With all due respect, IAT isn't the only factor, and may not even be the biggest one on a car in constant motion. IAT doesn't measure flow or turbulence, or whether or not the car is actually making more power. It just measures temperature, which only really matter when you're driving slowly unless your airflow is unusually bad.

[nando]

I've actually done it, you can measure a significant difference even at 25mph. You can see it from secondary factors to, for example, the car will go leaner because there is more air. you don't need to know the exact flow rate if you know what to look for. :)

if you only changed the intake, and needed to add fuel to reach the same AFR (because of cooler temps), or can see that MAP increased significantly (less restriction), then you can know you're making more power. Don't need a dyno for that, you just can't quantify the exact difference without verification (I've done that too). But even then, you can get pretty close with good assumptions and math (10% more MAP, about 10% more air& fuel, about 10% more power, minus efficiency losses).