Deck Vs. Squish...
 

Because the 911 pistons are shaped hemispherical and protrude into the combustion chamber the edge or shoulder of the 911 piston is used to determine deck height.
They way that I understand it is that deck height is the difference between the shoulder of the piston when it is at TDC, and the top of the cylinder. The shoulder of the piston is the "deck". Whereas if you had a normal flat piston you would use the top as the "deck". To determine deck height is to determine the distance between the deck and the top of the cylinder. I may be stating this wrong as some people refer to the top of the cylinder as the deck, either way deck height is the difference between the two.

The squish area may be different depending on the shape of the head and the piston. For example the shape of the head where it meets the cylinder is initially the same shape as the cylinder wall so it is a continuance of the cylinder, and in some heads the pistons deck can continue past the seam into the head.

I hope you can see the difference, the thing is I do not think this is the way it is explained in Wayne's book. And after saying all of this I really don't think deck height by itself matters, only the squish actually matters.

That is trading deck height for dome volume to lower compression. Too much deck height and you get detonation. If the compression is really too high it makes sense to flatten the pistons out.

Wayne's book, as great as it is, does not do a great job of explaining this.

Your explanation is correct: deck height is the distance between the "deck" of the cylinder (the term originated long ago-- think about the cylinder head mating surface of a Model T iron block to visualize "deck") and the theoretical top of the piston WITHOUT THE DOME. Some JE pistons have a flat land machined into the edge of the piston to make it convenient to measure. Others, you have to measure then subtract the dome height. But it takes you to the same place.

Many confuse head-to-piston clearance with deck height which is only an accurate use of the term with a flat top piston. Think back to the Model T example. Or 930 turbo pistons. . . :)

Reading Wayne's Engine building book on Page 147 states (the solder method) "This method technically measures not the deck height, but the height to the top of the combustion chamber."

However he goes on in later paragraphs to refer to that measurement as deck height.

So, I think you can disregard everything I have written and just consider squish and deck as the same.

Just for the record; real deck height, and the squish height will most likely not be the same.

Thanks for the explanation !

Would you also agree that actual deck height does not matter? And that the squish height is the only measurement that means anything?

And also can we agree that for now on deck height when referenced in the air cooled forums means the same thing as squish height? SmileWavy

I found that Bruce Anderson's book, as well as looking at other engine rebuilding texts for V-8's gave me a better understanding of the concept of deck height. Technically the deck height is the distance from the center of the crank to the top of the cylinder.. and that 1mm deck height number Porsche engine builders shoot for is of course the difference between the deck height and the distance from the center of the crank to the "top" of the piston.. not really the top, but the start of the dome, as 304065 (John?) explained.. which can be thought of as half the stroke, plus the rod, plus the piston's compression height. So maybe the correct thing is to not call that target "1mm" figure "deck height" per se, but the amount of ideal squish around the edge of the piston / cylinder, to reduce propensity for detonation but to also afford enough room to avoid piston to head contact,, at least at the edges..

Anyhoodle, called JE this afternoon.. I think I talked to someone named Dan. He told me kind of what I was expecting to hear.. "people modify pistons all the time, but I don't recommend it." Damn. I was hoping to hear my plan to flycut the pistons was a great idea. He said they are a "hollow dome" forging.. which I guess means they are real thin at the top because there is no meat in the dome, maybe someone can explain better what that term means, we had a poor connection and I wasn't doing a great job of understanding exactly what he meant. Then again, getting custom pistons made wouldn't be a huge undertaking according to him. I shudder at the thought of building a high CR engine and then building up some carbon over the years and having something that detonates like crazy with the hot weather we get here etc.. Maybe I will consider some custom slugs. Any of you racers want to buy an unused set of 10.5 JE's?? :)

IF your going to re order and you havent gone FSR then now is a great time to do it. Im getting my 10.5-1 FSR JEs in the mail tmrw. Im down in MD so I was concerned about the 10.5 but after talking to a few top builders they seemed to say that the 10.5-1 wasnt a big deal. I almost dropped to 10.3 or 10 but I was told it was almost unmeasurable. Any thing over 10.5-1 is treading into dangerous area. Im twin plugged with the 3.6 and going to be running carbs with a cam so it shouldnt be a biggie.

With your measurements, 10.3:1 CR is what I come up with if you go with 1.25mm squish.

1mm puts you at 10.6:1.

If the dome is 42.5 then your up to 10.7:1 CR at 1mm.

I think you will end up great with 1.25mm squish.

I also think EBS has a stock option for you, as the 9.8 CR pistons will end up getting you at 10.1:1 with your 89.5 head volume and a tighter squish.

I have been running 10.5 CR in my 3.0 RSR spec motor with 46 mm PMO's, DC60 cams and Bosch twin plug for about about 8 years with no detonation on 93 octane pump gas.

I think you will be fine at 10.5 with twin plug.

Tom,

Do you recall the deck height? AKA squish, AKA piston to head clearance for your engine?

Shane

The problem with 10.5 on street gas is that street gas is inconsistent and will change in the future.
As soon as the "green terrorists" figure out that the best way to get old cars off the road is to reduce octane, you can bet they'll do it.

By reducing your compression, you increase detonation resistance and loose very little performance if any.

Henry, thanks for your advice.. I think I am going to do what I can do get things into the 10.0 to 10.2 territory. Do you think that is reasonable?

And regarding the future of old cars.. what are we to think of E85? Boon or bane? High octane on one hand, complete political and environmental boondoggle on the other! If anybody wants to get our old cars off the road, its the car companies.... We luddites are bad for sales it seems. :)

10.0- 10.2 Reasonable? sure
We are going even further. We are building twin plug street engines in the 9.5 to 9.8 range and getting the snap we're looking for with cam choices and ignition timing. This allow us to custom fit timing to fuel quality. Are we leaving some performance on the table? sure, but unless we're building a 10/10 race engine, we're always sacrificing some performance for drive ability.

I know less than nothing about the pro /cons of E85, sorry.

As for car companies worrying about our old Porsches, not so much.
They are worried about car retention for those who buy a new car every three years not a third or four old Porsche that lives in the garage.

Perhaps the most important thing to consider when selecting a compression ratio is that the piston is only part of the equation. Dynamic compression not static or measured compression controls performance. Cam selection effects compression as well as dome volume.

I'm curious, is this a new approach within the last 6mos for you?

As for E85... very unfriendly to nearly the entire fuel system on the old cars and not just 911's. It's corrosive in practical use and carries less energy potential than gasoline. You have to run a richer stoichometric ratio than gasoline for the same power output. It does have a higher octane rating than regular pump gasoline and tends to keep things cooler due to the higher fluid density in the combustion chamber for the proper stoichometric. This makes it popular with the racing crowd... the subsidies make it popular with the alt fuel crowd. Could you retrofit a compatible system to an early car and desire for long term performance as a street car? Sure, anything is possible if you're willing to pay the right price. Some folks just tune for it and fill the tank up figuring when the fuel system begins to fail it'll be far enough down the road to be out of their hair. Beware of those outfits if you don't want a mess on your hands in the future especially with these older cars that have softer metals and pre-E85 compatible rubber in the fuel system.

Yeah, E85 is a can of worms that I have no interest in opening. Maybe one day when I get turbo fever I will perhaps look at it in a different way.

Yes, dynamic compression ratio is an interesting concept and learning about its existence is what prompted me to start thinking about my engine in a different way. A static ratio I guess does not take into account what effect the rest of the engine geometry, including the cams, has on the actual compression and pressure.. I learned that low overlap cams make more dynamic ratio than higher overlap ones. I still have a lot to learn, and I don't know if you can really trust that figure to make comparisons between engines, but here are some numbers that I have been looking at..

A 964 in stock trim has a DCR of about 8.1:1 to 8.2:1. Now of course these engines have twin plug and knock control. And this assumes a "real" static CR of about 10.7:1

My lowly SC in original form had a calculated DCR of about 7.38:1, single plug, stock.

The engine I am building now, including a custom cam which I plan to get from camgrinder, would have a DCR of 7.4:1 at a static compression of 10:1, or a DCR of about 7.6:1 at a static compression of 10.5:1.

I have read on the internet some guidelines for DCR, but I think they really applied to V-8's and not our oddball hemi head detonation prone engines, so I am hesitant to apply those rules of thumb. My hunch, based on comparing the SC and the 964 above, would be that a DCR between 7.4 - 8.0 would be "safe" but that's just because its the range between two known combinations that work.

No Andrew, coming from a racing background, I've always understood the concept of compromise (risk vs reward). We have always adjusted / compromised build specifications to suit our needs and the needs / desires of our customers.

What we don't do is to compromise the quality of our builds by using substandard parts in these endeavors once the faulty quality of the parts has been established.

Pardon the hijack, I can't miss this opportunity to hear from detonation experts!

I just finished replacing the pins on my ignition's distributer's centrifical advance camshaft on my 1969 911s. The pins had wear grooves preventing full advance. Luckily my '69 had brazed pins not induction welded. I tapped the holes and installed shouldered bolts that screwed down and hit home. I used blue Locktight.
If it's still stock it has 9.9-1. It has Webbers and I'm running .65 idle jets an increase of .05 (just today) on advice to stop sniffing and flat spot coming off idle circuit with E10 here in New Jersey.

Setting up the timing today I was glad to see my repairs were holding total advance until just over 5000RPM.

Question-

Would I be able to hear preignition? Would it sound like a chain rattling in an iron pipe like other engines?

My spark plugs are white and clean.

Am I safe to run the suggested 30 degrees total advance?
Initial advance is above 5 BTC stock setting-

Is it true to only tune to total advance.

Car runs like scalded-dog (but not scary faster than before).

Faster than a surprised biker today!

Thanks

Don't confuse pre-ignition with detonation.
Pre-ignition happens when something in the combustion chamber causes the air fuel mixture to ignite before the spark. This is more damaging than detonation.
I believe the rattle you are talking about is detonation. This is when the fuel air mixture combusts too fast. This can be from to much advance, too low an octane, among other things.
Detonation and Pre-Ignition

Thanks Ed.

I don't hear any rattling noise in my engine. I'm concerned about a danger that can not be heard.