997 cylinder liners

[hartech]

Nelly, they seem to start around 20K to 50K but there are not so many that have exceeded this anyway so time may well alter that picture. We also don't get to see any lower mileage ones that failed within the manufacturers warranty/guarantee perod - so there may well be even lower mileage examples that have similarly failed although those owners had a new engine and may not have been too concerned about the accuracy of the description of the cause of the failure.

The 3.6 996 or 997 is basically very similar to a 3.4 996 except that the 3.6 seems to have the least proportion of coolant diverted to the cylinders with the 3.4 having the most and the 3.8 being in between - this relationship roughly following the incidence or number of failures.

The other difference compared to a standard 996 3.4 is that the torque at low revs is higher in the 3.4 Cayman S, the 3.6 and the 3.8 and whereas the basic cylinder casting is the same for all those (in terms of shape and coolant passages) except for the 3.8 which has a larger bore and a larger outer cylinder sleeve fitted into the same basic block - so reducing the volume of coolant around the cylinders.

Baz

[Ian_UK1]

hartech said:

FOR THE BENEFIT OF OTHERS RESPONDING - I believe Gen 2 engines do not have Lokasil bores.

I just wanted to pick up on the (perfectly correct) statement above that Baz made earlier in this thread. Porsche has made some very substantial changes to the new DFI block/crankcase, both in terms of design and materials. Here are some salient quotes from the Technical Service Manual for the Gen-2:

The new 911 models feature a two-part, vertically split crankcase with an integrated
crankshaft thrust block. The advantage of this design is that smaller components
can be used, while the separate bearing saddle with cast-in cast iron elements is no
longer needed, thereby reducing the overall weight of the engine.

The actual crankcase on the new 911 engines is made completely of an aluminiumsilicon
alloy (ALUSIL).
This procedure offers the following advantages:
• With ALUSIL, the crankcase can be made from one cast, without cylinder
sleeves and without having to coat the cylinder bores afterwards.
• ALUSIL is an excellent heat conductor and thus allows high specific engine
output values.
• ALUSIL has excellent friction properties. Since the pistons and piston rings
slide on the exposed silicon crystals, they have a low tendency to seize.
• ALUSIL does not present any recycling problems because the crankcase
does not include any foreign materials, e.g. cast-in cast iron cylinder sleeves.
The listed advantages of the alloy are certainly important arguments in its favour.
Indeed, the low-pressure chill-casting procedure, which has since proved to be the
best solution by far for casting ALUSIL, is an important prerequisite for reliable,
mass-produced crankcase cast parts.

Cylinders are now connected differently in the cylinder-head cover area. The individual
cylinders, which originally stood freely in the water jacket (open deck design),
are now connected by a closed cylinder deck (closed deck design). The advantage
of this design is high cylinder stability, particularly the cylinder shape (roundness
and low cylinder deformation) over a wide load and temperature range. This has the
added advantage of reducing friction and thus reducing fuel consumption. Even
piston and piston-ring sealing has been improved as a result of the higher retention
of roundness of the cylinders. The entry of oil from the crankcase into the combustion
chambers and the entry of the fuel-air mixture from the combustion chambers into
the crankcase is reduced. This both improves consumption and reduces performance-
inhibiting overpressure caused by blow-by in the crankcase.

These changes seem to be exactly what was required to address the main issues being experienced by owners of the M96/7 engines (with the possible exception of cooling of the cylinder walls - there's no specific information in this regard, other than an oblique reference to Alusil's heat-conducting properties).

This (to me as a layman, at least) would suggest Porsche was more than fully aware of the design defects in the M96/7 engines and has gone all-out to eliminate them in the new motors. I also think this view is further reinforced by Porsche's use of a clean-sheet apporach for the Gen-2 motor. If Porsche believed the M96/7 design was sound, wouldn't it have been more cost-effective to adapt it for DFI rather than start again from scratch?

I'd be interested in (more knowledgeable) people's opinions on this.

Ian

Edited by Ian_UK1 on Monday 18th October 09:51

[hartech]

Yes Ian - all my comments relate to the older engine design and clearly the main issues have been addressed in the new engine.

I believe there are some issues connected to the pistons cocking up but I expect the botton end to be greatly improved.

Alusil is good for making a stable crankscase and good for cylinder lubrication and long life.

Baz