Small amounts of acetone will not harm engine components however the fuel lines will swell if too strong a concentration is used. Also, 100% dry acetone is required. Acetone attracts moisture and must be kept in an air-tight and preferable amber colored container.
The good way to determine optimal concentration is on a vehicle that has the feature to show "current gas maileage" and tweaking the mix ratios. I'd also remove a fuel line periodically to check for internal problems or "softness" inside the hose.
http://www.lubedev.com/smartgas/additive.htm