I think a larger engine is more likely to show improvement with Auto-RX that can be measured on a dyno than a smaller one. A larger engine has more cylinders to have gummed-up ring packs, and more horsepower to loose due to the effects of the dirt which Auto-RX removes.
If, hypothetically speaking, each cylinder with a gummed-up ring pack loses 1% of the horsepower...that means my 225HP 5.0 has lost 8% of it's factory-rated horsepower, or 18HP!
That sort of difference *SHOULD* show up on a dyno. On the other hand, an 88HP 4-cylinder that lost 4% of it's horsepower would only have lost 3.52HP, and that wouldn't likely show up on a dyno due to the margin of error.
If we're talking about smaller engines, the effects of sludging/gummed up ring packs/etc. that Auto-RX cleans up are likely to be less, and therefore, less likely to be outside of the margin of error of a dyno.
When you have a test instrument that has a margin of error, you scale up what you are testing so that it is out of that margin of error. That is why I think a larger engine (with a LOT of miles on dino oil) is ideal for a dyno test of Auto-RX effects.
