Help: The Protrude of 951 Pistons at TDC
 

Help needed.....

I read somewhere that at TDC the 951 pistons sticks out of the deck (block)? Is it true? Can someone please verify this? If it is true, is it the same case for 968?

Trying to figure out my "boost leak into water system" problem.

Boost leak in over flow tank seems to indicate head gasket.

About the pistons i can't say for sure one way or the other?

Assuming, it is true that the 951 pistons protrudes a little bit out. Why did Porsche design it that way? And if this is true, then the 8V combustion chamber must have been designed to accomodate the extra piston sticking out ...

OK, here is a 951 scenario.... where the plug fires at TDC and exhuast valve opens shortly after (not shown in picture)... Notice that pressure is not acting directly at the mating surface between head and block?

Now here is a scenario, where the plug does not fire at TDC. Firing either before or after TDC.... combustion = expansion and pressure could've escaped thru the headgasket?

Yes i see you're point, but when piston is pushed down after combustion pressure is still going to side of gasket.

Plus i don't think the piston pertureds that far in the head if it does.

Top dead center means flush to top of block, unless engine is 350cin chev with 12 to1 compression

I have a 3L motor built using a stock 968 bottom end (pauter rods stock length) with a modified 8V 951 head.

If the the 968 (NA) pistons are not design to protrude out of the 3L deck, like what the 951 (turbo) piston do in a 2.5 block........ this could well explain why the water system is always pressurized under boost.

IIRC a couple of other 951s on the board have had similar problems... and no one seem able to find the cause... perhaps boost leak is simply a result of "wrong timing"?

What type of head gasket are you using WF or Metal with the metal gasket quite a few people were having problems put the gasket in and for what ever reason there is a problem.

pk951, WFHG is used. I have already replaced 3 good gaskets in the past month, had the head resurfaced, block checked, retorqued countless times...

More inputs please... anyone?

P.S> I am really really not drunk. :roflmao:

The 951 pistons doesn't protrude at TDC. If they do, it has to be minmal, like a millimetre or two.
I don't have a technical data to back this up, but I have turned the engine many times with the head off and at TDC the piston seems to be flat with the cylinder top.

Duke, isn't a millimetre or two considered a lot? The HG is less than a mm thick.. IIRC.

By the way, 8V head seems chamfered at the sides to accomodate extra piston movement, what about the 16V head?

Also, the 951 head combustion chamber does not have room for a protruding piston.
But the question is if the diameter of the combustion chamber has been modified to mate the 104 mm cylinders?
Would be great to get a comparison pic of the combustion chambers from both the 951 and 944 NA 2.7 head, since the latter head is the most common one used for 8v 3.0 liter turbo apps.

You're right, a mm is quite lot :rolleyes:
But I really don't think it protrudes at all, if it did I think it would hit the head.

In a stock application the piston doesn't protrude beyond the deck at TDC.

The pistons are designed to be flush with the head – however a milled block is not uncommon. If your pistons are over .005” out you are asking for contact with the head. A possible solution is to go with a custom head gasket from Cometic – you can specify the head gasket thickness.
The general rule is a .035” clearance between piston and head is a minimum. This allows for rod stretch and piston rocking.

Chris White www.944enhancement.com

What do you mean by boost leak into water system? What problem do you have?

At any rate, there is no protrusion of piston walls past the top of the cylinders in any car that I'm aware of. In some high compression engines the piston dome does protrude into the head, but turbos are all low compression and use flat or dished pistons.

If you are losing coolant after shutoff, that's a good indication of a bad head gasket.