when to change timing belt - unknown service history
 

1997 A8Q. Bought it with 183k miles on it, fixed the trans and have put another 4k on it. No service records.

Any way to tell how old the timing belt is or what condition it's in? Should I change it now? Refrain from any more full throttle blasts till I do? What breaks them?

My 4runner 3.4 had 180K on it when I bought it. Original timing belt, original spark plugs. Very clean oil though, the PO changed the oil very regularly. Anyway the timing belt was pretty leathery but didn't really show any cracking or anything obvious. Conversely I just did the belt on one of my 968s, not a lot of miles on the belt (maybe 20K) but I had the cams out for variocam service so I put in a new belt. That old belt did not seem new or anything. If somone handed it to me I do not think I could say it had 20K or 100K on it or what. Maybe there is a way to measure the thickness of the belt (at and between teeth) and compare to new? My A8 belt has like 75K on it and my S8 belt has like 8K on it so I could compare mine.

An S8 with 180K or so on the original belt just tossed the belt recently so I would not want to run past maybe 140K on a belt, unless the car's not really worth putting time and money into.
I know on the 40v you can usually tell if the belt has been serviced, toolmarks and the like. Maybe if it looks like your belt has been done you'd figure it has at most 100K on it and is probably OK for the next 20K or so.
But if you intend to keep the car you'll probably do the belt and only do it once so why wait?

(the 4runner spark plug gaps were 2x the OEM spec, most of the electrode worn away, but damn if the thing did not run fine and ran the same after a full service, same MPG also.)

Yeah, you make a point; I do intend to keep the car. Major and costly engine damage could change my mind, so I should just prepare to do it this winter.

Modern ignition systems will fire a spark across enormous gaps; I like it!

When I bought my 01 S8 a couple years ago it had about 180k miles on it. I assumed the belt had been done once and was due again (causing the owner to sell when they got the quote from Audi service). Wrong, it had never been done and was missing multiple teeth. Eek. One of the many benefits of buying a car with an unknown service history...

Change the belt asap for piece of mind. It's about $500 in parts for the whole service and takes 1 day.

You should be able to remove the driver front timing cover with 2 hex bolts, simple. You can then inspect the belt, and I'm guessing it wasn't done. Interval is 80 or 90k so I'm figuring he didn't do it because they'd likely have told you. If it was just done, it'll look pristine. Maybe roll the motor over with a big socket on the harmonic nut and a breaker bar, and see if you can see the label on the belt? Also if the timing parts are all rusted to hek, odds are they haven't done the belt. In that case, you're running on borrowed time.