After reading Boomins post about the 50k rebuild claim I thought it was worth clearing up a few issues.
I can give first hand knowledge of engine life; I worked as a Mazda technician from 1991-1998, where I fitted many engines under warranty and rebuilt several for customers on UK fd\'s long before people started importing Rx7\'s and years before any Fd\'s were seen outside of the Mazda dealership network, from 1998 to 2001 worked part time at RX Motors where I have examined hundreds of blown FC and FD engines and from 2001 to date have been rebuilding engines for my own customers. During this time I do not believe that I have seen more 13b turbo cars than I can count on one hand that have allegedly made the 100k miles mark, and some of those I think were rebuilt/replaced prior to that point.
In general most FD\'s are doing well if the engine is untouched at 60k miles, and does indicate more motorway mileage.
Hopefully this will help answer the expensive \'what happened to my engine\' question
In the early days the need for rebuilds was mainly due to seal failure on circa 60k miles, however as we started reaching the 8-10 year old phase of most of the fd\'s out there, it is more common to be due to either water seal failure or water channels cracking, there are several reasons as I see it for different cars obtaining different mileages and reasons for failures:
1/ Tip failure due to wear, this can differ from car to car depending on the type of driving, long motorway journeys vs. short stop starts (multiple heat cycles from hot to cold)
2/ Tip failure due to detonation, more common on modified cars but not unheard of on poorly maintained stock cars (plug/lead failure, fuel system problems etc etc)
3/ Water seal failure, as Carl says in boomins post, degradation can be hugely advanced due to leaving a flooded engine full of fuel, which attacks the seals. There is also definitely a connection to corrosive antifreeze attacking the metal surrounding the seals, and general degradation of ten year old seals, again effected by multiple heat cycles and aging cooling systems (poor water flow through rad, water pump cavitation, lack of ducting etc etc)
4/ Water channel failure (cracked plate around the seal channel) this seems to be due to difference of thickness of material around the outer edge of the seal channel, the thickness varies considerably, this is compounded when corrosion sets in and attacks the already thin metal, causing a fracture point. (This is again not helped by poor water flow etc)
5/ Warped plate/plates and shrunk housings, this is primarily due to severe overheating due to prolonged driving once overheating has occurred (due to lack of water, as in a leak, poor water circulation or a fault as listed in 3 and 4)
Sorry it is a long post but hopefully clears up a few issues