996Turbo Engine Build: Story+Pics

[rico]

LINKY

A very indepth and fascinating thread on the 996turbo engine. Lots of pics.

Enjoy

[Marquis_Rex]

Absolutely great!

Thanks!
You will notice that the piston ringlands are fairly generous, and there are grooves embossed/cast into them. The ring land between the first and second ring is also esepcially quite large compared to other engines of similar capacity/output. This is a trick that racing Ford Sierra Cosworths used to follow too. Well the generous ring land area is to relieve the considerable cylinder pressure. The grooves I once read about, I'll have to recap, may be there to retain oil....

[Marki]

Big to that man for taking on such a spanner fest ..... 3-4 k (i assume $) on tools

It makes me feel nervous just looking at it

[nel]

What an excellent thread!! Some peculiarities in the engine design like the dowelled intake tappets that don't rotate to spread the wear, but hey, who the am I to question porsche engine design?

[Marquis_Rex]

nel said:

What an excellent thread!! Some peculiarities in the engine design like the dowelled intake tappets that don't rotate to spread the wear, but hey, who the am I to question porsche engine design?

*tut tut* I thought you were an engineer! must be exposure to the French J/K

Ok, the TAPPETS are doweled, because this car uses Cam profile switching, the supplier for this system is INA. I've dealt with them. Bascally- the inner tappet of the inlet tappets is used for a low lift short duration profile - predominantly for low speed and perhaps part load/emissions applications. This inner tappet is no good for higher lift cams as it's width is too small to accommodate a higher speed cam. When the secondary cam is operated, a pin activates the whole tappet.
Now- the reason why the tappet is doweled to stop rotation- is because the tappet is domed, or profiled- but ONLY in a 2 D plane. The doming or counter formal contact is for tribological reasons- as you've rightly touched upon, you CAN minimise wear with a correctly domed tappet and attention to materials/ treatments. My favourite treatment is chromium nitriding and then micro polishing the tappet. A more extreme example of a 2D domed and doweled tappet is used in the new latest BMW 507 Bhp V10 M5 engine. I agree in many ways it is more ideal to have a rotating tappet- in terms of wear (Jag engines have them and even after 100,000 miles the tappet clearances rarely need doing) but I guess the onset of modern materials has allowed this move. Older rocker arm engines, (our 993s included and the old M20 and M30 BMWs) have always had a central camshaft and a curved rocker pad and usually had good wear characteristics. A curved or domed contact allows a higher valve lift for a given cam duration for a smaller tappet diameter.

In reference to the grooves of the piston ring lands- I am informed they are there primarily for sealing apparently, I'm doing some more research into this!

[Pickled Piper]

Porn for Engineers!

Actually it looks a lot like my old Beetle engine.

pp

[nel]

Marquis Rex - what about the dowelled piston ring - I've never seen that before either. It's obviously to prevent ring rotation, but is this because the cylinders are horizontal, so they never want the ring gap to be at the bottom of the bore where it would potentially let more oil through into the cylinder?

[aceparts_com]

As Bruce in 'Finding Nemo' has said; "You wouldn't want one of those to go pop!"

Looks terribly complicated!

[pesty]

Excellent. He is a brave man

I have a stupid question though. If your going to go to the trouble of stripping the engine completely why not replave the pistons and valves etc with lighter stronger versions?

Dosnt lightening the recpiricating parts make the engine rev faster more responsive?

Anyhow thanks for posting that Rico

[nel]

pesty said:

Excellent. He is a brave man I have a stupid question though. If your going to go to the trouble of stripping the engine completely why not replave the pistons and valves etc with lighter stronger versions? Dosnt lightening the recpiricating parts make the engine rev faster more responsive? Anyhow thanks for posting that Rico

It's not a stupid question - I'd agree that at a normal, high mileage obliged rebuild you'd expect to see rebore, new pistons and valves. This obviously isn't such a rebuild - it seems the only changes are the GTIIIR oil pump, stronger con-rods and head bolts.

Recall reading that the 996 con-rods are not as strong as in previous 911 engines, so in his hunt for higher power he seems to be selectively strengthening/uprating to take the extra power. The fact that he doesn't touch the valves or pistons implies that the engine hasn't done too many miles and the original components are already high enough spec for the 600 hp (at the wheels apparently) that he's getting from the lump. Car must go like a bat out of hell!

[Marquis_Rex]

nel said:

Marquis Rex - what about the dowelled piston ring - I've never seen that before either. It's obviously to prevent ring rotation, but is this because the cylinders are horizontal, so they never want the ring gap to be at the bottom of the bore where it would potentially let more oil through into the cylinder?

I've racked my brains out on this one and showed a few experts here. I'm with you on this one: The only think I can think of too, is that rather then have both rings rotate and potentially line their gaps up at the bottom when they come to rest- thus allowing wisps of blue smokes to come out on start up, this measure would prevent it.

People who buy new Porsches and Jaguars, the ones who change them every few years, can be an anal bunch. I can imagine a scenario perhaps on older air cooled motors where sometimes on standing a harmless wisp of blue smoke comes out may be once in a blue moon (I've heard this happens on 993s but never experienced it myself), and this could be the source of many customer complains potentially. It's the same fineckiness (word?) that leads to obsession with dead smooth idle control to the detriment of fuel economy and VCT response. What would have been acceptable in this market place years ago is less so now.