Rev, you are certainly correct that it is
possible, but it is not usually the case. Using the EPA's published load tables for most cars has resulted in loading conditions that have been just about identical to actual road testing in our experience. Our own testing has shown that loaded a/f readings on our dyno are typically within 0.1 ratio of actual readings on the road under the same load where a dynojet would show anywhere from 0.5 to 1.0 a/f ratio difference (on the rich side on the dynojet). This is the reason that the EPA and companies like Roush use load bearing dynamometers to do research and engine development work. Again, this is where a tuner's experience comes into play. I have no doubt that an experienced tuner can recognize this quirk when tuning on an inertia only dyno, and can leave the appropriate amount of "safety factor" in the tune to keep it alive on the street. Knowing exactly how much safety factor room one has in such a tune is another story though.
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Greg Banish
President,
Detroit Speedworks, Inc.
'93 LX Turbocharged Road Racer
'00 Bad-Ass F150