Two Rotax 185 engines with the "stacked" bi-props

Pilots flying Lazairs from high altitude airports can get a bit more thrust to compensate for the reduced air density by drilling four 3/8″ diameter holes in the top of each muffler. This will increase the maximum propeller speed by approximately 150 RPM, but since It also produces a corresponding increase in noise, it is not recommended unless you’re sure you need the extra thrust.