help? deck height, cylinder volume, compression and a book misprint?

[911ducktail]

Hi guys,

hoping someone can shine some light into a few questions I have regarding deck height, compression ratio, cylinder volume and the book

1983 3.0 w/ JE "10.5:1" pistons (PN 274049) stock cylinders. heads have been drilled for twin plug.

in the book on page 147 it talks about the crush-solder method for measuring 'deck height'

I follow those instructions, took the acid out of a 3mm piece of solder, laid it out on the pistons, and crushed it. I got 2.22mm




obviously thats out of the 1.2-1.5mm 'spec' that should really be 1.0mm on a hot engine, right?

but isnt that measurement actually measuring the clearance between the top of the piston dome and the cylinder head? isnt 'deck height' the measurement between the top of the piston (ignoring any dome) and the lip of the cylinder?

I started searching here and found this post which seems to reaffirm my belief

Originally Posted by Eagledriver

The solder method tells you the clearance between the head and the piston at TDC. It was mistakenly called "deck height" in Waynes book. The deck height is the height of the edge of the piston below the top of the cylinder. It is easiest to measure with a vernier caliper. Measure it at the 3 and 9 positon since the piston will rock in the 6 to 12 axis and affect the measurement.

The deck height should be between 1 and 1.5mm. The way you measured will indicate a greater height because the head has a chamfer around the edge. The solder method will only tell you if you have a safe distance between the top of the piston and the cylinder head, it will not tell you the deck height. You need the deck height for your compression calculations.

-Andy

so what I did is I took dial indicator and installed it on the block so I could measure TDC - then I could effectively measure the 'actual' deck height by dropping the piston down and using a level to get a value that I then subtracted the stated height of the JE piston from their spec sheet

this gave me a deck height measurement of 1.648mm, which included 0.33mm for the actual measurement of a mic'd Victor Reinz gasket that is included in the rebuild kit (I am assuming this is supposed to be the .25mm gasket and it has some kind of coating on it that makes up the difference)

so again, this number is out of spec for both 'standard' and high CR engines

from there I went to CC my heads. somewhere along its life my engine picked up a bank of heads from a 1982 engine to go with its original bank of 1983 heads :lol:

I cut a circular piece of plexiglass, drilled a pour and breather hole, grabbed a 100ml syringe and green alocohol, and took an average of 10 measurements over each of the different cylinder heads

my averages for one bank was 93mm and the other bank was averaging 94ml. from other threads here I have read most 'stock' SC heads are CC'ing around 90ml - I doubt that the void for the 2nd plug takes up ~3ml?



yes I know there are bubbles here - this is to show my process, I did this 10 times and made sure the bubbles were gone when I actually measured the syringe


and the JE piston spec sheet




so, what am I doing wrong with these measurements? I dont see how only changing out pistons should result in that big of a different? If im measuring everything correctly how do I go about getting the deck height measurement closer to 1.0mm?

[boosted79]

Set piston at TDC, use DI to measure distance between top of jug and lip on piston in line with the pin on both sides . make sure cylinder base gasket is in place and cylinders are snugged down. That is the deck height,.distance between outside edge of piston and the top of the cylinder

edit - actually a caliper, not a dial indicator

[911ducktail]

thank you for replying boosted - Ive seen you comment a lot in these type of threads

if I understand you correctly that is how I came up with 1.648mm figure





I also did some calculations if I wanted to get back to 10.5:1 or at least 10:1 - assuming I swapped out the 0.33mm VictorReinz gasket for an actual 0.25mm OEM gasket. that at least gets me down to a deck height of 1.568mm and then I'd have to get ~1mm cut off both heads - is that too much?

is it possible the pin bushing in the rods are too far 'off center' changing this dimension?

[Trackrash]

Do you still have the old pistons? Did you check the deck height on the old pistons?

Did you actually measure the dome volume on the pistons?

I left out the base gaskets on my current motor to get my deck height right.

Porsche allows for 1mm to be removed from the heads. Have they been cut before?

Edit: this is what I was told many years ago. However, in the 84-89 Manual, Porsche says .25mm max can be removed.

If you do both those things you will have to adjust your chain box to cam alignment and your chains may have more slack. Depending on your cam there might also be valve to piston issues.

[911ducktail]

checking the old gaskets and the deck height on old pistons is the next step!

I was futzing with measuring tools while sending some parts, including an old piston, through the ultrasonic today....

edit:

I dont believe either set of heads has been cut. Thats excellent to know about the 1mm figure, thank you. Dumb followup- whats involved with adjusting the chain boxes - are we talking milling work being needed?

the cam is a 3.2 reground to 964 spec by dougherty, I went mild with the plan to go EFI, although I still kinda fret over not getting the GT2-102s

How did you seal you cylinders without the base gaskets? if I just pulled them but left the heads alone I end up with 1.318mm deck height - and 9.5/9.6 compression - better but not the 10-10.5 I was going for...

[boyt911sc]

Originally Posted by 911ducktail

thank you for replying boosted - Ive seen you comment a lot in these type of threads

if I understand you correctly that is how I came up with 1.648mm figure





I also did some calculations if I wanted to get back to 10.5:1 or at least 10:1 - assuming I swapped out the 0.33mm VictorReinz gasket for an actual 0.25mm OEM gasket. that at least gets me down to a deck height of 1.568mm and then I'd have to get ~1mm cut off both heads - is that too much?

is it possible the pin bushing in the rods are too far 'off center' changing this dimension?

Ducktail,

As 'boosted' explained how the deck height is determined, your calculation is flawed. The piston dome height has nothing to do to get deck height value. The distance between the cylinder edge to the flat edge of the piston @ TDC determines the deck height. You could measure it directly with a depth gauge. You are making a simple measurement too complicated. Forget the dome height value.

Tony

[911ducktail]

Thank you Tony, I'm still confused then...

The dome height is necessary since I'm unable to directly measure the deck height, as you suggested, with a depth gauge because the piston isn't flat - this is in the book right?

Since I know the stated dome height from JE (figure a in my red pen calculation) and I know how high 'above' the top of the cylinder the dome goes from using a Dial indicator (figure b in the drawing) I'm able to calculate the deck height as a-b

If I'm having the same difficulty measuring the deck height with the depth part of a caliper (I get 1.05 to 1.07 and I know it's not sitting flush) a depth gauge won't help?

[Trackrash]

Originally Posted by 911ducktail

checking the old gaskets and the deck height on old pistons is the next step!

I was futzing with measuring tools while sending some parts, including an old piston, through the ultrasonic today....

edit:

I dont believe either set of heads has been cut. Thats excellent to know about the 1mm figure, thank you. Dumb followup- whats involved with adjusting the chain boxes - are we talking milling work being needed?

the cam is a 3.2 reground to 964 spec by dougherty, I went mild with the plan to go EFI, although I still kinda fret over not getting the GT2-102s

How did you seal you cylinders without the base gaskets? if I just pulled them but left the heads alone I end up with 1.318mm deck height - and 9.5/9.6 compression - better but not the 10-10.5 I was going for...

On my motor I elongated the three holes in the cam end plate. (not sure if that is the correct name) That allowed the cam housing to move toward the case compared to the chain box. I have heard of the chain boxes being milled.

I used curl-t on the bottom of the cylinders. It was a long time ago. I would suggest Yamabond. Others may disagree. After many years mine aren't leaking (much).

[Trackrash]

From Bruce Anderson's book.

[JJ 911SC]

LMFT4U:

[VFR750]

It is very likely the twin plug modification increased the head volume 3cc. 3cc is very small.

[boosted79]

Yes, take some modeling clay and plug the 2nd plug hole and then cc the head to see what you get. You are confusing deck height with dome to head clearance. Deck height is easy on those pistons because they have a nice lip around the circumference. Just take the end of the caliper and rest it squarely on the cyl. and measure down to the piston lip. Deck height gives you squish to create turbulence in the chamber to get a better mixing and a better burn. Too much deck height and you don't have enough squish and can lead to detonation pockets at the periphery. The solder just tells you if there is enough clearance between the dome and head.

[boosted79]

Cometic can make you some custom copper base gaskets if necessary. probably not cheap but cheap and Porsche never go together.

[911ducktail]

Originally Posted by Trackrash

On my motor I elongated the three holes in the cam end plate. (not sure if that is the correct name) That allowed the cam housing to move toward the case compared to the chain box. I have heard of the chain boxes being milled.

I used curl-t on the bottom of the cylinders. It was a long time ago. I would suggest Yamabond. Others may disagree. After many years mine aren't leaking (much).

this is good to know that someone has done this - when you say 'doesnt leak bad' are we talking 'leaking' as it happens on a 911 engine or in general

I wonder how thin of a gasket CE can slice - if we're talking Kramer feeding Elaines' neighbors' cat thin or something else

Originally Posted by Trackrash

From Bruce Anderson's book.http://forums.pelicanparts.com/uploads25/deck_height_0021463370783.jpg[/img]

thank you for this - I didnt realize those calcs were in that book. I have been living out of Waynes. I will have to check this out.

Originally Posted by VFR750

It is very likely the twin plug modification increased the head volume 3cc. 3cc is very small.

Fair enough, I guess I was looking around at other threads and people were measuring the 2nd plug port around 0.5cc-1.0cc I am going to try some putty tonight/tomorrow

Originally Posted by boosted79

Deck height is easy on those pistons because they have a nice lip around the circumference. Just take the end of the caliper and rest it squarely on the cyl. and measure down to the piston lip.

this is my ultimate issue I guess - using a caliper's depth gauge on that chamfered lip isnt a repeatable measurement as far as Im concerned.

the best way I came up with making this a repeatable measurement was sticking the DI onto a 3" thick piece of steel on a flat surface of the block and then point the tip at the flat top of the piston dome

again, pic for illustration purposes - I know the cylinder needs to be bolted down to the case to take this measurement


1. Zero out the DI at TDC for piston 1
2. As the piston drops down from turning the crank I took a machinists straight-edge across the opening of the cylinder.
3. As the piston is brought back up it touches the machinists straight edge - that measurement was .480" (12.192mm)
4. JE has stated the piston dome height is .525" (13.84mm)

5. now that I know how high the piston dome is, and I know how far the piston travels from TDC to 'flush' it is simple subtraction to get me the actual deck height

13.84mm - 12.192mm = 1.648mm deck height (with the as-actually-measured 0.33mm "0.25mm" base gasket)

this is the only measurement that I feel comfortable with because it seems to be the only measurement I can make repeatable - with the 'solder as deck height' measurement it moves from 2.18mm-2.30mm and with the 'caliper depth finder' method it varies from 1.02mm-1.07mm

I really dont think Im confusing 'deck height' with 'dome piston interference' its just that Ive been given the piston dome height and I can use it to back into deck height using the process I outlined above.

---

Sticking JE pistons into an otherwise stock 3.0 engine cant be some crazy new build- there has to be other people that have done the same thing and yet I cant really find other people here that have made threads regarding this

thank you again to everyone, if Im being hard-headed I apologize. I just feel like maybe Im not explaining something clear enough

[Trackrash]

As far as my cylinder bases seeping? I can post a picture, but I built my motor in 1985, so yea, there is some seapage. No drips or runs. Keep in mind that I have a 3R case bored for 90mm P&Cs so you should be better off. I was told the base gaskets don't do a heck of a lot, but others may disagree.

I am about to order J&Es for the SC motor I am currently building. However I'm planning on using 9.5s. I am hoping to get to about 9.8, so I'm following this closely.

I would be interested to find out why your pistons are "shorter" than spec. I don't think you are measuring your deck height correctly with the solder. There is a small flat lip at the edge of the piston. That is where the deck height should be measured?

I question J&Es spec for the dome volume. That is what I think you should confirm.

I wonder if others have had this issue.

[boosted79]

I think you are worrying too much about the deck height. Because of the piston/wall clearance you may get some variation using the depth gauge technique because the piston has some rock to it. If you are getting 1.02-1.07 that is a variability of only .05mm out of a target of say 1.3 mm or about 4%. being off by .05 will make no difference. Get as much of the caliper end resting on the cyl. lip as possible, turned 90 deg from what it shows in Wayne's book on pg146.
One comment on the cc'ing. A burette is more accurate than a syringe because it is much longer with a much smaller diameter, that allows you to read to the bottom of the meniscus. Get a 100 ml so you can do one head with one fill.

[boosted79]

here's an easy way to set your mind to rest on the deck height.Make a tip for the DI that will rest on top of the piston lip, probably get a screw that threads into the end and then grind the end down so it will rest on the piston lip with piston at TDC. Zero the DI on the cyl top then indicate down to the lip, that will double check your caliper readings.

[911ducktail]

im concerned about the deck height because im 10% over the top end of spec for a standard, non high compression rebuild. Everything Ive found here on the forums says preDet is a seriously issue if my numbers are right

so, Ive ordered a burette and stand, I've ordered some small tips for my DI, and my BIL is stopping by tomorrow with *real* measuring gear to see what we can come up with

in the meantime, I mic'd my old copper base gasket - 0.27mm and its not flat so, im pretty certain it originally was 0.25mm

I also tried using a caliper - I set the piston to tdc and took 3 measurements at 12,3,6,9 across piston1

Google Photo helpfully made a gif of some pics I took ha!




again, the problem is lack of repeatability.

12: 1.14, 1.20, 1.27 = 1.20avg
6: 1.30, 1.34, 1.32 = 1.32avg
3: 1.27, 1.23, 1.30 = 1.27avg
9: 1.17, 1.20, 1.22 = 1.20avg

that all averages out to 1.25mm which is 25% off the number i got using known variables (obviously JE can be wrong, Im hoping to have the equipment to really measure the dome height when the BIL stops by)

the lowest number in that dataset results in 9.8:1
the highest number in that dataset results in 9.6:1

so Im still almost a full point below the JE spec - so I definitely need to have higher resolution when measuring the cylinder head volume.

Also, again, thank you to everyone who has been commenting, its really appreciated!

[mskala]

Your pic showing the method of determining deck height is the same method
I used. JE gives their height number and you just subtract it out. My 86mm
pistons looked like yours, there is less than 1mm of surface around the outside
so it would take a tiny, flimsy gauge to get in there, and it won't be as good as
your way.

[snbush67]

The problems are to much deck height and to much head volume to get the compression you want.

Have you considered not using the base gasket? It is really just a spacer, it just takes up space it doesn't seal anything.

Then you get within 1mm for deck height, and you decrease your squish area. Redo the calculations and you may come out where you need to be.

The JE pistons are shelf items designed to provide an "advertised" CR with stock 90cc heads. Since you have 93cc heads you are getting lower CR.