Adjusting Compression by Milling Piston.. ?
 

Anybody been there done that? I have a set of JE's that measure out about 42 cc of dome volume.. I'd like that number to be more like 40cc if possible. I guess the obvious answer is: order new pistons. But maybe I can avoid that since they would either have to be custom, or I would have to buy pistons for the wrong engine and adjust my rods to suit (i.e. JE makes a piston for the 3.2 rod which has the dome volume I need, but I would have to have work done to my SC rods to use them)

One thought is, just mill it off the top, area highlighted in green. But, the JE's seem rather thin at the top and I would not want to make it any thinner. Have not measured yet, but its not that thick, maybe only a few mms. What about taking material off from the edges around the valve reliefs, circled in red.. it seems like there is a lot of material around the valve reliefs just doing nothing.. seems safer than making the dome thinner. Thoughts?


http://forums.pelicanparts.com/uploa...1393966769.jpg

I have total compression ratio paranoia now.. so I am afraid to run 10.5:1 twin plugs and I am thinking I want to be closer to 10:1 for safety margin. Perhaps that is totally unfounded. Am I a wuss?

I took the sharp edges off around the valve reliefs regardless of my compression to remove hot spots.

Hi Craig.. if I recall from countless hours poring over every last post on this forum.. you have a pretty hot 3.4 at 10.5 to 1. Is that correct? What kind of cam and induction do you run, and what octane fuel do you run? Ever had any issues w detonation w/ your setup? Thanks!

Yea its a 3.4, supercup cams stock intake, twin plugged with an andial splitter and 964 dizzy along with the SW chip. I checked the cylinder volume after assembled, or did one cylinder to get an accurate CR and it came in at about 10.4/1 running 93 octane fuel with no detonation. I think Steve leans towards the conservation side with the chip since he is going off the info I gave him and didn't have a dyno to do any tuning.

How about adding some thicker cylinder base gaskets?

Definitely considering that too.. ain't going to be cutting or machining anything until I take some trial deck height measurements... I was blown away when I realized how much a difference a 10th of a mm can make to your CR when it comes to deck height.. about a 10th of a CR point. However, the range is narrow right? 1.25mm would be very sub-optimal. I have heard people say that 1.0-1.1mm (.040-.043) is really the acceptable range. Or less if you are a Swabian Super Hero Motor Builder.

I cut my J&E's from 12.5:1 to 10:1. The crown on my 86mm bore pistons started out at about 0.600 thick. I do not recall right now how much I removed but the crown never came close to my minimum of 0.250. The compression drops quickly as you remove more material so go slow, measure often.

Just measured.. Appx 7.3 mm thick at the thinnest part of the crown, or about 0.287 in. So not as much material to play with.

And after some crude measuring and math, maybe as little as .10 or .15 mm milled off the top might get me where i need to be. I will call JE perhaps and see if that will get me perilously thin or if its ok to do that. Can I use the specific gravity of aluminum to tell me how much I have taken off, or do I have to measure their volume several times as I machine them? Hmm.

Are there not countless people running 10.5:1 on pump gas? I'm currently building a 3.0 with 10.8:1 JE FSR's, DC60 cams and a stand alone...

I would hope so..! Wouldnt the world be a better place if everyone had the good taste to rip around in hot rod porsches. I just have read posts that scare me... In your case of a DC60 I think the "dangers" are less than with a lower overlap cam such as I plan to run.. Hence my worry. However my understanding of these principles is rather poor.

10.5 on pump gas with twin plug, and long duration cams, seems to work. Did for me, both at 5,000+ feet at home, and close to sea level when I could travel.

Not clear how far you can go above that. If you had EFI and knock sensing you trusted, you have more room for experimenting, playing with ignition advance, mixing 112 leaded with pump stuff, etc.

Otherwise, how would you know you were getting knock, or detonation, short of something bad happening? Detonation posts feature how quickly rings break or other troubles occur.

I think that 1.25mm is on the edge of optimal but I would not classify it as a sub optimal rating for deck height.

Make sure you aren't confusing deck height with squish height.

You are going to get better performance with the tightest tolerance, by getting the piston as close as you can to the heads without touching the valves.

If you're going to lower compression another option is have offset wrist pin bushings installed in your rods that lower the compression by lowering the piston a little at TDC.
I think most people use those bushings the other way to lengthen the rod CTC a little and raise compression around .3 of a point but you can go either way.
Ollie's in Arizona does it for around $250 last I checked and I think others do it too.

Also round off any sharp pointed edge areas of the valve pockets so they run a little lower temp and don't cause detonation. You can do that by hand with fine wet or dry sandpaper like 400 grit and then finishing with ultra fine wet or dry sand paper around 1200 grit or finer. Weigh the pistons on a scale while doing that so weight stays equal between them if you want.

If you make the crown or top of the piston thinner then it could become too thin to transfer heat to the outer edges and rings and the top of the piston could melt and burn through.

I am going to add this here because no one told me before I blew the heads off my race motor...
The reason you can run pump gas with high compression with twin plugs is because you don't need to run as much timing as a single plug! I was running 12.5:1 2.4 liter motor, twin plug, 30 degrees total advance, using Sunoco 110 octane fuel and detonated my engine blowing 5 of the 6 cylinders. I now run 10:1 and 18 total advance, yeah I am scared.

Shane - I am confused.. I thought that the larger my deck height measurement, the poorer my squish zone around the edge of the piston? Hence the reason for keeping the deck height at a minimum without creating interference or clearance problems with the head, valves, etc..

J Fairman - Reducing the length of the rod wouldn't really change much would it? It would be similar to adding base gaskets to reduce compression in my mind. If I reduced my rod length but made my deck "correct," then I would still have the same compression. If I reduced my rod length, and left the ensuing larger deck gap there, then yes, I think I would reduce my compression.

I am going to call JE today and see what they can tell me about my idea of skimming .15mm off the top. One would hope they build a little more meat into these things than needed so that engine builders can customize to some degree. At least I hope to find that to be the case.

James,

Are you trying to get to a place more like this?
http://i900.photobucket.com/albums/a...Pistonin3D.jpg

If you have a good piston vice for your mill you could get closer. This was a piston done by CP to my spec.

Good luck.

Checking with J&E is certainly the sensible thing to do. But I, too, think there must be room there for this. It is the obvious way for the pistons to be made with the potential for a wide range of compression ratios. They build enough meat into these pistons so that the valve pockets can be made in various widths and depths.

I'm fascinated by the math one would need to calculate how much to cut to remove what volume of material. Where the cut widens at the valve pockets, that would be simple I think. But those rounded sides! Maybe computer modeling can deal with all that so you don't have to cut and try?

Of course you have to measure it, that's not tough if you can mill.

I wouldn't cut the valve pockets- you want to cut the ridge at the top, making the piston as flat as possible, to make the flame propogation easier.

If it were my engine I would stick with 10,5 to 1. When you twin plugged, did you CC the heads to take account of the increase in combustion chamber volume as result of the second plug? That will lower CR.

I did! I thought I was sooo smart! I measured them ahead of time.. 90cc, just like they should be for a stock SC head. I was really banking on picking up another cc or two.. but of course I did not realize that what cutting another plug hole giveth, flycutting taketh away. My heads were flycut I think 0.010 in.. which was enough to make the net volume of the head 89.5cc.. less than before the twin plug!

I guess I won't sweat it too much.. I think I can get 93 octane almost every place I go, just don't want to have no regrets.

You can start with a .25mm thicker base gasket to get what was taken away by the flycut. That will get the chain lengths back to stock.

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.