Here's specifics....and exactly what we do. (Factory or "standard" Gates belt.

Adjust new belt at 1,500-2,000 miles.
Re-adjust belt every 15,000 miles.

The "Internet" has made the "entire belt problem" on a 928 much more of an issue than it really is. In the entire time I've been working on 928's, having done literally hundreds and hundreds of timing belt replacements, I've never had a failure of a belt that I've replaced, using factory parts.

Never.

I've had one Continental belt, on a race engine, that allowed the engine to skip cam timing, when the engine backfired.

Done correctly, with the proper parts, this system is virtually bullet proof.

......That being said, I've redone hundreds of very "fresh" timing belt jobs, from other shops or individuals. And I do not "proactively" do these jobs. They get towed to me, with timing belt "issues". If an aftermarket belt or tensioner device is installed on an engine and functional, I leave it alone, knowing that I will see it back, in the future. Yes, I used to "argue" about aftermarket belt tensioners, aftermarket belt brands, and other belt "ideas". No longer. This alienated people and was completely counter productive to my business interests. I now LOVE these pieces, yet don't use them.

You can figure that out....

I only use the factory belt tensioning tool. I own several, in case we don't trust the calibration tool or if we have a reason to compare one tool to another. New belts get adjusted to 5.0 "belt units". They will be noisy and we "back them off" if they get set to 5.2. (Which is how we learned to adjust to 5.0.)

The "belt warning light" is amazingly simple, yet amazingly reliable, repeatable, and predictable. The light will come on between 2.8 to 3.0 "belt units". "How" the warning is activated is an important detail. If the light comes on, on a long deacceleration event (like getting off the freeway), can be reset by shutting off the key and restarting the engine, and does not come back on with normal driving, this generally means that the belt needs to be adjusted. Any other "warning" needs immediate attention.

A new belt will generally be at 3.2 "belt units" at 2,000 miles. It is very rare for the light to light before 2,000 miles, but occasionally will occur, if the new belts was installed on worn gears.

Thanks Greg, that pretty much matches my experiences. I've got about 240k miles on the 944S since factory new and the only belt failure I had was with a first generation -00 belt from Porsche. But I followed the 2500/15/15/15/Replace schedule pretty religiously. Then of course we found the chain tensioner pads go at 160k miles. Oops.

From what I have been told the 928 has the updated belt formulation and since the engine essentially loafs (At 60mph the 928S is running 2k rpm in high gear, the 944S runs at 3k in high gear) wear is significantly reduced overall. So there should be a pretty good safety margin in there (even the water pump is running at lower RPMs per mile of travel)

Interesting stuff.

About 15 years ago I had an intermittent warning light problem with my GT. It turned out to be a fault in the wiring on the sensor. Someone had (not so obviously) pulled on the wire instead of the connector and broken the wire inside of the insulation, or it was a factory defect that simply broke over time. None-the-less it was tricky to diagnose since the tensioner reading was perfect and it rarely flashed on.

It did result in my purchasing the factory tool and learning how to check and set tension, so all was not lost...