Deck Height & Head Gasket
 

I'm resealing my 3.3l turbo engine. The cylinders are C2 Turbo units with 930 factory heads. Right now the heads were mounted dry with no head gasket. Question: Shouldn't the C2 Head Gasket thickness be calculated into the deck height equation?
Also, using the C2 Turbo head gaskets shouldn't pose a problem with the factory 930 heads, correct?

Thanks in advance,

Head gaskets do not enter into deck height measurements; they are unrelated.

Base gaskets DO affect deck heights so that should hopefully be obvious for anyone performing these procedures. :) :) :)

Measure deck height and adjust as necessary using the various thicknesses of base gaskets. Make sure you get as close to .040" as possible to minimize the chance of detonation.

Hi Steve,
Thanks for the reply. Ok, now I'm confused. :)
Many times when the conversation of deck height comes up, many talk about achieving the ideal squish as the gods@Porsche designed it. If deck height helps achieve the designed squish, why won't the head gasket be a factor as well?
Are the C2 Turbo heads different than the 930 heads in their combustion chamber design?

Deck ht and "squish" (compression) are unrelated. Deck ht is the distance from the cylinder lip to the piston edge at TDC. The piston should be lower than the cylinder lip. Too much deck ht and you have a region around the perimeter of the cylinder that is prone to detonation.

Head gasket thickness adds to the combustion chamber volume and will reduce the "squish" and compression.

Thanks Tom and Steve. Got it. I guess the heads are the next thing to address on the list of mods....

Tom said it great!

I think you meant to ask, shouldn't the head gaskets be included in the measurements in determining volume and Compression Ratio?

That answer is yes. But it is increasing head volume not deck height.

...the head gasket will not be a factor because the head gasket is above the edge of the cylinder, so therefore will not increase deck height, which is the distance measured from the edge of the cylinder to the shoulder of the piston at TDC.

Squish is the distance measured from the top of the piston at TDC to the head.

It has become confusing because many references refer to measuring squish as measuring deck, using different meathods and terminology etc.

Well, the first 911s had an actual head gasket - something compressed between the top of the cylinder and the head, and (if I have this part right) preventing the cylinder and the head from touching each other. Like the way water cooled cars do it, though without water passages.

Then Porsche came up with a crushable ring (CE ring), which fit in a slot in the top of the cylinder. When the head was torqued, the head and cylinder surfaces touched each other and were tightly clamped together. The ring crush did not affect any calculations of the volume of anything. Everything was determined by piston configuration, the thickness of the cylinder base gasket, maybe rod length, head dome volume, and whether the head's sealing surface had been flycut or not. The 3.2s dispensed with the CE ring altogether. Then something like it made a comeback until the advent of water cooling.

Some turbos have used a more robust ring system, and slots in the head as well as the cylinder (?), but doesn't the torque on the head studs compress it so that the head and cylinder surfaces are compressed into each other? So it is still irrelevant for setting mechanical clearances, squish zones, and CR?

Here's the head gasket in question. It appears from the invoice from the previous owner that the heads were cut, but it didn't say how much.
The head gasket is .5mm thick. This gasket might have come about through Porsche's efforts to address the head-to-cylinder seepage that appeared on a number of C2 Turbo engines.

http://forums.pelicanparts.com/uploa...1422318910.jpg

Sounds like you'll have to set the proper deck height and then use the solder method to check the squish clearance with the gaskets in place then cut your heads if the squish is too large.

The solder method does not measure compression. It can used to determine piston to valve clearance if the solder is laid down across the valve pockets.

You will have to use the volume measurement method to determine actual compression ratio with the gasket in place.

I'm totally aware it doesn't measure compression, it can be used to measure squish clearance. The purpose of setting the deck height is to get the desired squish and valve clearance assuming no head gasket is used. If no head gasket is used then deck height = squish. If he sets the deck height to 40 measuring to the top of the jug and then he uses the head gasket the squish will be 40 + gasket thickness assuming no compression of the gasket when the head is torqued. ccing the head will not tell you squish. Take four short pieces of solder laid on the squish area of the piston, with radial orientation and 90* spacing. Torque the head with the gasket in place and turn the engine over . Remove the head and measure the solder, that's the squish clearance. The head gasket can be viewed as an extension of the cylinder assuming it is non-compressible. The squish can be decreased by using a thinner base gasket if he uses a head gasket.

I had never heard the term "squish clearance" used in relation to the 911 engine. I thought squish and compression were the same. Thanks for the explanation.

The squish (or quench, take your pick) is what squeezes the mix from the perimeter of the piston in towards the center of the CC and plug and the high velocity of the gas moving out of the squish causes turbulence in the chamber to get a better mix and burn. If the squish clearance is too large then the mix is more prone to detonation because the poor turbulence results in pockets of the mix with different A/F density. If the squish is too small then you risk piston to head contact at operating temps. Water cooled can run a tighter squish than aircooleds. So in summary, a head gasket affects the squish clearance and it can be adjusted with base gaskets, head gaskets, flycutting the head or a combo of all three. :)

Dave, just get it together and turn up the boost

I'm going to get a graduated burrette and measure the combustion chamber volume. Anyone participating in this thread tell me the volumes for;

Std 930 Head in cc's, .25mm mill in cc's and .5mm mill in cc's?

The heads are twin plugged so keep that in mind.

Stock single plug 930 head is 90cc. In theory, a .25mm cut on a 97 mm bore would reduce the chamber volume by about 1.85 cc and a .5mm cut is double that. In actuality the cc reduction will be slightly less than this because the chamber surface curves in so the cut doesn't remove a perfect cylindrical volume from the chamber.

boosted79,
Thanks.

Looking forward to your cc'ing results as I am going to twin plug also. What CR are you shooting for?

boosted,
Not shooting for a CR. I'm trying to determine how much the heads were shaved by the previous owner. The repair sheet mentions shaving the heads, but didn't say how much. So, I'm working backwards, cc'ing the heads then determining from that how much was removed.

Got ya. I'm not sure how much the second plug adds to the chamber volume, probably very little. You could see how much it adds by using the burrette to fill the volume above the plug to the chamber surface.