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Originally Posted by Lapkritis

Uh, wow. Piston design and chamber design specifically in CC volume certainly do impact compression significantly.

Yes, you are right. And nobody here, including me, has said anything to the contrary.

Quench effect and ignition be dismissed, removing material from a piston and leaving other variables unchanged will decrease compression.

Also correct and has nothing to do with the discussion of deck height.

This is common knowledge for anyone who works on an engine with carbon build up in the combustion chamber and knows about the pinging/detonation that results.

Agreed.

Piston to head and relation to deck height... who would seriously take a grinder to a piston?

I don't know anyone that would. The point of the example is to illustrate to you that you can change piston-to-cylinder clearance all you want, and you still haven't altered the relationship between the top of the cylinder wall and the position of the piston.

Look, the reason it's called "deck height" is that the term originated as the measurement of the distance between the centerline of the crankshaft and the "deck" or top of the cylinder block on a watercooled engine. If you look up the specifications for, for example, a watercooled V-8 Ford Windsor, you will find "deck height" of, for example, 9.480". Of course this is not the piston-to-head clearance.

In the Porsche world, we don't bother expressing it that way, but it's the same concept-- distance between the piston height excluding the dome and the top edge of the cylinder. This is a shorthand way of expressing it that gets the engine builder to the same place, rather than having to say "198mm deck height." Practically only machine shops will have a mandrel that goes in the main bearings to truly measure spigot height. Here is what that looks like (on a 356 motor):



If you search here you will also find many other references as to why this is so. Take a 2,0 liter engine like Andy's-- 80x66 with a 130mm Rod and a 34mm compression height on the piston.

Half the stroke is 33mm
Rod length is 130mm
Compression height is 34mm (same for all 2,0-3,0 pistons)
This all adds up to 197mm, that is the height of the reciprocating mass without the piston dome. This is the "magic number."

For the cylinder side--

Spigot height 115.55
Base gasket height 0.25mm
Cylinder height 82.2mm

All adds up to 198mm. Subtract the height of the reciprocating mass from the height of the cylinder and you get 1.0mm deck height.

Another reason why we use this terminology is that it's pretty constant across years. If you look at the 2,4 motor, the stroke went to 70.4mm, the rod length dropped to 127.8, and the cylinder height was the same.

70.4 /2 = 35.2 plus 127.8 rod +34 compression distance = 197mm, the magic number.

Try it with a 3,2 motor-- 74.4mm stroke plus 127.0 rod length plus 32.8mm compression distance = 197mm. The magic number again.

Hope this helps, I can't explain it any more clearly.