Could you see any harm in including this if it does not impact the measurement on certain engines?

So, getting back to my question. I can setup the deck height to find my desired compression so long as I maintain 1.0mm piston/head clearance?

n.b. I am using John Cramer's & Bruce Anderson's definition of deck height being the piston edge to cylinder top at TDC.

andy

Walt is also wrong. He's a great machinist with a great reputation, but he is absolutely wrong when he uses the term "deck height" to describe, in great detail, the measurement of piston-to-cylinder clearance.

Suppose you and he are correct. Suppose you remove a cylinder head and use an angle grinder to remove material from the top of the piston. You have increased piston-to-cylinder head clearance, right? But you haven't altered the deck height in the slightest.

Take a look at the formula for calculating compression ratio. You can find it in Bruce Anderson's book, but it's a very common formula. Look at the deck height parameter and note how the overall compression ratio varies with deck height, which is a function of stroke, spigot height, cylinder height, rod length and compression height. Piston to cylinder head clearance has nothing to do with that calculation and is not used in calculating compression ratio.

Look at the definition of compression distance, this is defined as the distance between the centerline of the pin boss and the deck of the piston. It is not a function of dome height, or piston to cylinder clearance.

Hope that clears it up.

I don't understand the question.

Andy,

Let's go back to your original question before this thread took a semantic turn.

Your 2,0 S pistons can be thought of in two pieces. A perfect cylinder, for the bottom piece, and a dome, on the top. If you were to take a bandsaw and cut the dome off, so that there was exactly 34mm of distance between the center of the pin bore and your cut, you would end up with a flat-top piston and then a dome. If you measure the volume of the dome, you can use that measurement in computing compression ratio.

Now, in practice this is not what we do. Some pistons, JE being a good example, actually have a flat ring around the outside of the piston top, to make it easier to measure deck height. You can actually set up a dial indicator on the edge of the cylinder, zero it out, then move the indicator over to this ring and measure deck directly. On a typical 911 engine, the deck is "positive" which is a bit misleading-- positive deck height is when the piston's theoretical deck is below the top of the cylinder.

This is the measurement that Steve is referring to when he says you want 1,0mm deck. This 1,0 mm deck is a good place to start, you don't want the piston sticking too far out of the hole, as you will end up with very high compression indeed AND will begin to impinge on a healthy piston-to-cylinder head clearance.

Interestingly, but not relevant, is the fact that 964 pistons actually have negative deck, they slide up into the head.

All right, how do you measure deck when you have a 2,0 Mahle piston? Look at the edge of the piston where the cylindrical wall stops and the dome begins-- it's just a smooth radius, there's no ledge to measure from! If you look in Bruce's book you will see a photo he took where he uses a height gauge and a test indicator to measure the distance between the height of the dome and a spot on the piston wall that is 34mm up from the center of the pin bore. (He probably subtracts half the diameter of the pin from 34mm and measures from the top edge of the pin bore.) Not perfect but it works.

A couple other points-- the 3,2 motor used a 32,8mm compression height- I guess they moved the pin up when they lengthened the stroke. Not a factor for you.

Also-- we think that Mahle used a ZERO deck to compute the stated compression ratios. This would make sense because deck height varies, so if you zero it out it gives you a reference for everyone's engine, and the actual static compression works out lower in practice. I credit Kenik with discovering this.

SO- to answer your original question- the factory achieved 1,0mm deck with normal crank, rods, spigots, spec cylinder height and 0,25mm base gaskets. S pistons have something like a 45cc dome, this is how they got to 9,8 to 1.

Hope this helps! Email me if you need the calcs.

Re: Deck Height yet again
 

Let me make it simple: it couldn't hurt anyone to measure with it in there but it could hurt some if it wasn't.

Thanks for the detailed response John, I cc'd my piston but only got 37.4cc for the volume of the dome. I did this by pressing a straight edge down on a piston until the very top of the crown was level with the cylinder walls.

Then I used the glass plate and burette to fill the void with 43.5cc of liquid. I must re-measure the dome height but my initial measurement of 16.1mm gave me a cylinder volume of 80.9cc.

Subtracting one from the other and I was left with a dome volume of 37.4cc.

I'll come back once I have double checked the dome height, perhaps an error here is why I am currently looking at <1mm deck height to get 9.8:1 compression.

andy

Re: Deck Height yet again
 

Uh, wow. Piston design and chamber design specifically in CC volume certainly do impact compression significantly. Quench effect and ignition be dismissed, removing material from a piston and leaving other variables unchanged will decrease compression. 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.

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

Andy,

80mm bore
66mm stroke
72cc combustion chamber (this was my '66, what is yours?)
With a 37.4cc dome volume, you end up with 9,4 to 1 with a 1,0mm deck height

To get to 9,8 to 1 compression you need a deck of 0.616mm

For reference, my JE slugs were 37cc dome for 9,5 to 1, but that was with a 1mm overbore to 81.

Here is my thread (from SIX years ago! :)) showing my pistons, see how the tops are flat? That is 37cc dome. Do yours look more like a 906, is there a sharp ridge on the top? That is a higher dome than mine. Look at the last photo in the thread-- those are 906 pistons I think, although some say that the 69S pistons were the same.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/368320-jugs-slugs-confessions-je-piston-owner.html

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

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

Agreed.

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):

http://www.precisionmatters.biz/imag..._installed.jpg

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.

Re: Deck Height yet
 

You're doing a great job explaining to yourself so don't sell yourself short.

So if all things are equal and the primary concern is piston to head clearance...which is only changed by adjusting deck height... then it makes sense to use the only non-static metric which is the deck height. It's not hard to understand.

To add my BS here.

To me "deck height" is the difference between the plane of the piston's horizon line (the point of reflection from the piston's side to crown) and the top plane of the cylinder. This is a nice number to have for calculation of deck volume for CR number. I like Steve W's method for measurement of this however one must consider base gasket, if any, crush.

This may NOT be the point of closest interference between piston and head however, for example the piston crown may be close to the combustion chamber above the plane of deck height.

The important thing is that no point of potential interference between the piston and head should be less than .040" = 1 mm is what people say and seems time tested.

One has to look for these interference points when putting odd combinations of head and piston together, particularly with bore increases. For these points I use solder and a head assembly.

Having said that, I have "gotten away" with .035 (Carillo + Mahle forged) at a few points at the combustion chamber edge, but didn't like it.

It turns out that using dome height 15.9mm, which I accurately measured using a genuine made in USSR height gauge.

My deck height without a base gasket installed is right about 0.018" or 0.45mm. Add the base gasket and I have 0.7mm deck height.

Based on this my compression is about 9.4:1, if I were able to offset bush the rods 0.5mm I'd get pretty close to bang on 10:1. Opinions?

This would make my deck height only 0.2mm but there's plenty of piston to head clearance.

andy

I cut a perspex disc on the lathe to make a more accurate head volume measurement and got 71.2cc which brings my CR up to 9.7:1 without changing the deck height. I'm happy with that CR.

andy