Deck height measurement

[Bullet Bob]

I have measured my deck height without the copper cylinder base gasket and at TDC the piston is protruding .010" above the cylinder top. When I bolt the head on and use the solder technique outlined in Wayne's book the clearence comes in at .010". My question is, what thickness of copper spacers should I order? The pistons are JE and the target compression is 10.5:1.

I do plan to determine the volumes of both the combustion chamber and cylinder dome. Should that figure into the spacer dimension as well? What is the minimum piston/head clearance that is acceptable?

Thanks,
Bob

[MBEngineering]

Hi is it a 3.0 lt engine if so a std base gasket will do , as i have just checked the C,R on the engine i am building and it is at 10.6:1 with JE pistons.
regards mike

[Bullet Bob]

It is a 2.7 and I believe some machining has been done on the case.

Bob

[Henry Schmidt]

You need at least .040" deck height.
The more important measurement than piston to head clearance is piston to valve clearance.
That measurement should be at least .070" .

[2.7RACER]

Bob,
Looks to me like you need at least .020" and better .030" That translates to a minimum 0.50mm better 0.75mm. thick copper spacers.
From Bruce Anderson's book "Porsche 911 Performance Handbook" page 179, He recommends minimum piston to head clearance of .035" (0.90mm).
Further he recommends minimum piston to valve clearance of .060" (1.5mm).
I would expect a head volume near 65ml to 68ml.
to get 10.5:1 compression you need the following:
At 65ml head volume, the piston should be close to 24ml dome volume. 24.2ml to be exact at a deck height of 1mm.
At 68ml head volume, the piston dome needs to be 27.2ml. at 1mm. deck height.
At 10.5:1 you are in dual plug territory. Is that your plan?
Again from Bruce andersons book, to calculate compression ratio:
ADD (V1), swept volume plus (V2), deck height plus (V3), head volume minus (V4), piston dome. DIVIDE that number by V2+V3-V4. The result is your compression ratio.
Don't be surprised if the numbers don't look right. Accuracy in measuring piston dome as well as head volume and deck height is critical.
I use a spread sheet to plug in all the numbers. Makes it a lot easier to calculate and understand.
Get a copy of Bruce Anderson's book as well as Wayne's book. Each of these are filled with Golden info.

[Jim Williams]

I have seen lot of postings in various threads on compression ratio measurements. In most of them, a lot of emphasis has been placed on measuring deck height, and using the volume determined with this number to include into the compression ratio calculation.

I have also seen a lot of variation in determining the deck height, seemingly based on the different methods used, solder, etc.

Different knowledgeable engine builders seem to have different recommendations on minimum deck heights and piston to valve clearances, so a lot of importance is given to this number or that number for deck heights. While a proper deck height is important in a rebuild, the deck height volume does not have be treated on it's own in determination of the compression ratio. IMHO, doing so only adds to the complexity of the formula.

Also, I have looked at the piston dome volume in a little different light. Starting with the method in the BA handbook, I found that I understood the whole compression ratio determination a little better when I looked at the various volumes in a different way. I came up with a different formula. I eliminated the deck height volume as a separate step, and included it as a part of another volume. Also, I use a different term, which I call the Volume Around the Piston Dome. After all, it is really the volume *around the dome* that is a part of the overall volume in the cylinder. Treating it in this way eliminates the need to determine the volume of the piston dome. After all, when you determine it, with what I'll call the long formula, you only have to subract it out.

I posted a link to the formula that I use on a couple of other Pelican threads several weeks ago, and either no one bothered to look at it, or they couldn't understand how it worked. I take that back, I did get one comment that it looked like a good approach.

Also, people seem to resist changing from a set way of approaching things, even when it simplifies them. When I mentioned my formula to BA at one of his engine schools, and offered to explain it and get his opinion, he said he had been using the method described in his handbook for years, and didn't see any reason to change.

So if anyone is interested, I offer the link again, and encourage anyone who is interested to look at it. As a matter of fact, since some people seem to like to find reasons why something won't work, to those folks I would offer a challange to find a mistake in it (friendly challenge, of course). And if you find a problem, of course, post it.

[2.7RACER]

Jim,
Yes your method works and is perhaps a easier way to determine the compression ratio of a cylinder and piston combo.
Bruce Anderson's formula incorporates the deck height since the awareness of that dimension is important in determining the thickness of the copper spacer/gasket at the base of the cylinder.
The measurement of the deck height with the solder process provides the information necessary to properly size the base copper spacer/gasket.
Once this dimension has been determined and the proper base copper spacer gaskets are installed your process does indeed then give a simple method to determine the actual compression ratio.
However, we as engine builders need to be aware of the deck height and the solder method is a simple way to measure it.
I have no argument with your method or formula. Thanks for your work and explanation. It is another way of looking at things and perhaps removes some of the mystery to the mathematically challenged.
Ultimately, I think most errors are found with measuring head volume and piston dome volume inaccurately. This of course leads to wrong results or at least poor accuracy.
With a Porsche engine where the builder needs to understand the implications of the copper base spacer/gasket as opposed to say a V-8 where the cylinder relationship to the block is fixed some understanding of deck height becomes more important.
Thanks again for your work and explanation.

[Jim Williams]

Doug,

No argument with what you state. Everyone who builds one of these engines should be aware of the deck height issue, more especially if they are making any engine mods at all, rather than just doing a stock rebuild. Including the deck height measurement in the compression ratio formula should make those that might not otherwise condsider it, realize it's importance, and the impact of the base gasket size.

Of course the deck height is not the only thing sort of lying under the surface that can affect the compression ratio. The actual center-to-center rod length would be another. Admittedly, this also affects the deck height.

My intent with the formula initially was only to help me understand that there are really only two volumes involved: one with the piston at bottom dead center, the other with the piston at top dead center. Once one understands this, it makes the whole thing a little less confusing (or it did for me). But my point remains, while the deck height is an issue not to be overlooked during a rebuild, it isn't necessary to determine the compression ratio.

Thanks for taking the time to look at the formula, and for your comments. The more we discuss this stuff, the better we all appreciate and understand it.

[davidppp]

Hello Bullet Bob.

I'm confused..

You say the piton sticks up above the cylinder, but you have 010 clearance when the head is fitted?

How ?

Where is your solder placed to get this number?

I have made fine engines with less deck height than most will recommend, but never with negative deck height!

Kind regards
David

[Bullet Bob]

Well, my heads were off to the shop being ported and I wanted to go ahead and get the spacers on order so I grabbed a spare head and gave it a shot. Apparently the spare head was for a bigger displacement cylinder so the piston would in fact fit up inside. I now have my heads back and they clearly will not allow the piston to clear. I did not order the spacers based on the original measurement, though I will go ahead and order 1.5mm worth of spacers.

Thanks,
Bob

[304065]

Originally Posted by Jim Williams

Treating it in this way eliminates the need to determine the volume of the piston dome. After all, when you determine it, with what I'll call the long formula, you only have to subract it out.

link
* * * And if you find a problem, of course, post it.

You still have to determine the volume of the piston dome by measuring with a burette. Sure, you don't have to do any math to independently calculate the dome volume so you can use it in the equation, but the equation is the easy part, we all have spreadsheets to do the math anyway.

Another good thing about independently determining dome volume to a specific value is that it allows you to check that your pistons were manufactured correctly. Not so much an issue with factory Mahles, although there is the occasional confusion about which bump is which, but in my own case, having just had some custom JEs made, the volume must be measured. Not for lack of confidence in the vendor or JE, but because EVERYTHING gets measured.

[Jim Williams]

John,

I hope I didn't come across as trying to convince anyone that everyone would want to drop the "long" formula and use mine instead. Just to be accurate, if one measures the volume *around* the piston dome using method I posted the link to, then subtracts that number from a calculated number, one gets the dome volume. Using a method like this is the only way I'd feel comfortable with the result. I'm not aware of any way do it differently, unless it would be using solid geometry to calculate the volume under the dome of a (mostly) irregular shape. And I suppose one could arrive at the compression ratio by carefully measuring every aspect of the interior of the combustion chamber and swept volume, without ever cc'ing anything.

I really didn't come up with the formula to avoid doing math, but just to better understand the compression ratio concept. For someone who wanted to understand it better (me), or even to just simply measure it (me again), it has served the purpose well.

Anyway, to each his own methods. Both probably have their place. The author of a well-known performance hand book said at his engine school that he would probably stick to the method published in his book (after I explained mine), because he had been doing it like that for years. Others certainly may do the same.

[Grady Clay]

HeHeHe,

Good discussion guys. The saying “There is more than one way to skin a cat” applies here.

Probably the most important thing said above is when starting from just assembling stock Factory parts in original configuration to building anything else is a great leap.

This discussion may be about CR but there is one extremely overriding principal that can’t be emphasized enough:
The piston can never touch the head under any circumstances of dynamic temperature, RPM, load or any other condition – PERIOD. How close you go is up to you. Closer is generally better performance until the piston touches the head (even the slightest contact under any unique condition) and disaster strikes. How close can you go?

911 engines are very expensive if you trash one. They are wonderful race engines and similarly on the street if you build them right. How do you play it ‘safe’ and still get the performance these engines are capable of? You measure everything. Skilled builders know what to do (well usually). First-timers need to apply every caution.

Searching Pelican on terms; compression ratio, deck clearance, squish, variations and more will yield a wealth of useful information about 911 engines. Yes, it takes time and organization to realize this. It is worth the effort.

Best,
Grady

[efhughes3]

A related question. Closer is better when it comes to head clearance, yet a bit risky. Is 1.65mm way tooooo much? It certainly isn't risky...