I contacted Spence about the motor mounts and his initial response was to go with the polyurethane. - But we discussed the possible benefits & shortcomings of each design. We assume there must be an improvement in vibration dampening if you went to the rubber but are wondering if there would be a noticeable difference in engine movement. With a blower we'll allow extra clearance in the hood opening for motor movement but will there be a need for more clearance if we use the rubber mounts? Did you notice a change in how much your engine rocked between the two? We've come to the conclusion that we'll go with the rubber ones if the difference isn't perceptible. - Otherwise we'd want the polyurethane so we can keep the hood opening as small as possible.