In 1994 Porsche introduced the type 993 engine. This was an update to the 964 engine and the bulk of the parts were carryovers with some notable exceptions, most of which are improvements:
- Hydraulic Lifters with pressure fed shafts
- Lighter more durable crank and rods, these were used unaltered in the 996 twin turbo
- Carryover plastic intake from late 964
- Hot film anemometer – i.e. Mass Flow Meter. Better reliability, flow, resolution no moving parts.
- Revised cam towers with plastic composite valve covers
- Electronics: Revised ECU with onboard dual coil channels, better wiring harness, coils mounted to engine, etc.
- 49/42.5mm valves with 8mm stems. All other 911s since 1964 used 9mm stems. Gudas were soft phosphorus-bronze material to minimize valve stem wear
With the varioram, valve diameters were increased to 50/43.5, and on RS models the intake valve was increased to 51.5mm and intake ports and the intake manifold diameter was increased several millimeters.
The problem is that the 8mm valve stem, which shold allow more flow and lighter weight, also had 12% less contact surface with the guide. The guide not only keps the valve in place so it’s centered in the seat, but is responsible for transferring all the heat from the valve to the head on the exhaust side.
That, combined with a softer guide, dramatically reduced the service life, especially for the exhaust valves, which run much hotter. Ignoring exhaust guide wear will cause the valve to overheat and ,ay also cause it to land off center or stick open, causing bent valves and almost immediately complete failurer.
The general consensus is that exhaust guides should be replaced at 100,000 miles while performing a top end rebuild. After the rebuld yiou are good for another 100,000 miles or so..maybe more, since the replacement guides are not as soft as the factory guides.
This is why we do not sell 993 engines with more than 80,000 miles. At 75,000 you still have 25,000 trouble-0free miles to go.