Stock Mahle 9.8:1 Piston Dome Volume

[mikedsilva]

Has anyone determined the exact dome volume of the stock Mahle 9.8:1 piston?

I've tried searching but don't seem to be able to find it...

Thanks in advance
Mike


Pic for reference...

[faapgar]

Well after the 1st valve job if the seats were not cut too deep and new valves were used CR would be 9.6-9.7.If the valves were resurfaced about 9.2-9.4 CR.The Club Sport intake valves used to be flat on top with no dish.Every little bit helps.Best way I found is to CC the volume then look at all the parameters when you take it apart.Sometimes if there have been 2 valve jobs 9.8 can be 8.2.Ciao Fred

[mikedsilva]

Originally Posted by faapgar

Well after the 1st valve job if the seats were not cut too deep and new valves were used CR would be 9.6-9.7.If the valves were resurfaced about 9.2-9.4 CR.The Club Sport intake valves used to be flat on top with no dish.Every little bit helps.Best way I found is to CC the volume then look at all the parameters when you take it apart.Sometimes if there have been 2 valve jobs 9.8 can be 8.2.Ciao Fred

Hi Fred
head volume was cc'd at 89.. but I did not measure total volumes with piston in the cylinder.. I naively assumed there would be a factory stated number for the piston dome volume for the given compression ratios

[Peter M]

Mike:
http://forums.pelicanparts.com/911-engine-rebuilding-forum/602795-82-euro-sc-rebuild-2.html

Read it right through as he makes a mistake measuring it the first time

[Walt Fricke]

Mike - if the engine is on the stand, why not CC with the piston at TDC, both valves closed, through the spark plug hole? Bust out the center of a plug, glue in a clear plastic tube, put a mark on the tube and measure how much fluid it takes to fill the tube alone, then screw it in and fill the chamber. That covers head and dome volumes in one fell swoop. You can do it as a bench test on a short block easily, or even on an assembled engine (on a stand) though you have to work harder to remove the fluid.

[mikedsilva]

Originally Posted by Walt Fricke

Mike - if the engine is on the stand, why not CC with the piston at TDC, both valves closed, through the spark plug hole? Bust out the center of a plug, glue in a clear plastic tube, put a mark on the tube and measure how much fluid it takes to fill the tube alone, then screw it in and fill the chamber. That covers head and dome volumes in one fell swoop. You can do it as a bench test on a short block easily, or even on an assembled engine (on a stand) though you have to work harder to remove the fluid.

Good idea Walt.. but it's already in the car 'bustin bugs'!
https://youtu.be/dooDP_tCgRU

sounds tappety in the vid.. it isn't actually.. just a poor phone recording and the exhaust note...

[Walt Fricke]

Mike - it is possible to measure CR on an engine in the car, but it takes a special tool called a Whistler.

With a cold engine, pull a spark plug, screw the Whistler tube into the plug hole, rotate the engine to firing TDC (so valves are closed), and the machine sends compressed air into and back out of the combustion chamber. It uses acoustic principles to determine the pitch. You set the CC (actually, the cubic inches) of the cylinder on the machine, which tells it what the pitch means. Pretty darn accurate, and can be used to find TDC or confirm the pulley/case marks are spot on - move the crank a bit one way and the other to find the highest reading, and that is true TDC.

30 or so year old technology, but works.

You aren't in the Outback, but maybe no shop handy has one of these? They are expensive in terms of DIY use. NASCAR uses them, and so does PCA Club Racing (which is how I know about them).

[mikedsilva]

Originally Posted by Walt Fricke

Mike - it is possible to measure CR on an engine in the car, but it takes a special tool called a Whistler.

With a cold engine, pull a spark plug, screw the Whistler tube into the plug hole, rotate the engine to firing TDC (so valves are closed), and the machine sends compressed air into and back out of the combustion chamber. It uses acoustic principles to determine the pitch. You set the CC (actually, the cubic inches) of the cylinder on the machine, which tells it what the pitch means. Pretty darn accurate, and can be used to find TDC or confirm the pulley/case marks are spot on - move the crank a bit one way and the other to find the highest reading, and that is true TDC.

30 or so year old technology, but works.

You aren't in the Outback, but maybe no shop handy has one of these? They are expensive in terms of DIY use. NASCAR uses them, and so does PCA Club Racing (which is how I know about them).

Wow, never heard of something like that before.
Using the numbers posted by the fellow in the thread linked to by Peter M, I established my compression ratio was likely close to a true 9.2:1 using my deck height and measured head volume. Very safe. You are right though, I should just do it on the stand. I just figured that since these are factory pistons from Porsche, there might have been some documented numbers from the factory.

[mikedsilva]

I'm working on another motor which has been built at least twice before....

Heads had to be machined so this time I thought it would be best to work out the dome of the 9.8 pistons so it would be easy to use online compression ratio calculators.

I installed the rings on the piston and pushed it to the top of the cylinder so the top of the dome was touching my plexiglass. I made sure the deck of the piston was even on each side. I then brushed some grease in between the cylinder wall and the piston to make it leak proof. Wiping away any excess that would affect final results.

Using my burette and isopropyl alcohol, I was able to measure 88.2cc filling the cylinder perfectly around the piston dome and under the plexiglass with no bubbles.

Using online volume calc showed that volume of the cylinder from the deck of the piston (without the dome) would have been 126.06cc - 88.2c = 37.86 cc dome volume.

I've only done it on 1 cylinder but I think this is all I need.

[Rosco_NZ]

Mike the cc number sounds highish .. JE quote for a 90mm 9.5:1 piston, around 23 and 27 for 10.5:1.

[targa72e]

I think the difference is he is working with a SC which is 95mm bore. JE piston for 95mm bore on 3.0L with 9.8 compression is listed as 38.5.

Thanks,

john

[mikedsilva]

Originally Posted by targa72e

I think the difference is he is working with a SC which is 95mm bore. JE piston for 95mm bore on 3.0L with 9.8 compression is listed as 38.5.

Thanks,

john

Good to know, I guess my measurements are close then?

Interestingly, many people who have had experience with the Mahle 9.8 pistons, do not believe that they are truly 9.8 but are in fact much lower. I am not quite sure what forms that basis.

[mikedsilva]

Originally Posted by mikedsilva

Good to know, I guess my measurements are close then?

Interestingly, many people who have had experience with the Mahle 9.8 pistons, do not believe that they are truly 9.8 but are in fact much lower. I am not quite sure what forms that basis.

OK
i did the measurement again, not being happy with how my first one went..
it took me 3 goes to do it without spilling too much alcohol.

This time I calculate a piston dome volume of 36.62cc.

Using this info and the assumption that deck height on a stock motor is 1.5mm and that cylinder head volume is 90cc, then true compression would only be 8.76:1 (using a 9.8 piston)

In my particular case, my heads measure 84.6cc average (they have been machined a couple times) so I have used some thicker base gaskets to get me to 1.55 deck height.. I needed this higher deck height to make sure i have sufficient valve/piston clearance and I do have sufficient clearance using a 964 came.

This gets me to 9.48:1 compression which I think I am safely happy with.