The kind of cross-country highway cruising you guys did is not the kind of driving that causes timing belt failures. On the NSX, we're really talking about slipping a few teeth on the belt rather than it snapping completely. I know of no NSX that has actually snapped a timing belt- even those running on their original, floppy 30-year old belts that look like a loose rubber band when you remove the timing covers.
What does you in is running at high RPM and/or rapid acceleration and deceleration into the high-rev range. This puts a fast dynamic load on the belt and, if the belt teeth are sufficiently worn or the tension is too loose, it will skip a tooth (or a few)- typically on the rear exhaust cam gear. Because the C30A is an interference type design, the pistons will meet the valves and result in major engine damage.
I like to think about the service intervals from two perspectives: time and mileage. The time limits are intended to address the material degradation of the rubber on the belt. This can affect the positive engagement between the rubber belt teeth and the metal teeth on the cam gears and drive pulley. Things that affect the rate of degradation are temperature, moisture, contamination (oil, grease, debris) and usage conditions. A heavily tracked car in Phoenix that has none of its timing cover gaskets is going to degrade much faster than one of Kaz's meticulously serviced cars that just cruises up and down the M1 at 65 mph. Honda tried to set the time limits (6 or 7 years) based on what it thought most U.S.-based cars can get away with in terms of degradation. History tells us there is a lot of safety margin here. I think for most cars, a 10-year time interval is ok.
The mileage interval is mostly intended to address belt tension. The NSX does not have a self-adjusting tensioner. Once you set it, you have to live with that tension for the entire service interval. Over time, just from running, the belt will stretch and lose tension. We know that the mileage limits also have a safety margin- there are hundreds of NSXs out there running on their original belts and well over 100,000 miles. But, I think the Honda mileage limits are a good metric to use. Practically speaking though, who will actually drive their NSX 90,000 miles in the next 7 years?
Thus, we're really talking about the time limits. I think the 10-year number is a reasonable one. It also lines up nicely with the Stage 1 engine refresh (heads stay on) and crank pulley replacement.
