deck height, dome volume and compression ratio
 

I have a few related questions, all related to deck height and compression ratio.

I am rebuilding my '85 911 3.2 to a 3.4L. Walt Watson at Competition Engineering did all of the machine shop work on the heads, crank, rods, etc. (no work on the case). he also balanced all the components.

I have measured deck height using the solder method and have gotten consistent averages, right around 0.6mm, net of any cylinder base shim. My concern here is the difference from side to side. A typical measurement with a 0.25mm gasket in place was 1.02mm on the side closest to the flywheel and 0.66mm on the fan side, a difference of 0.36mm. others were slightly less. Is that normal? Should I be concerned?

The much bigger question i have is about my compression ratio. I'm using Mahle P&Cs, 98mm bore, "Max Moritz"style pistons. This setup is supposed to yield a 9.8:1 CR. Trouble is, some of the published numbers don't seem to add up.

One view: Mahle says the dome volume is 28.4ml. With 98mm bore, 74.4mm stroke, 1.25mm deck height, 90cc head the CR is 8.89:1 - Way Off.

Alternate: Mahle says the dome height is 9.3mm, which is consistent with my measurement. I "cc'd" the dome volume by positioning the piston in a cylinder so the top is even with the top of the cylinder, used a burette with windex to measure the remaining volume to net out the volume of the cylinder dome. I get 33.1ml for dome - very different dome volume, but the CR is looking better. That dome combined with the same assumptions as above for other dimensions yields a CR of 9.47:1. Better but not 9.8. If I used a 1mm deck height, i get 9.7:1. I know my volume measurements aren't perfect, but it seems more consistent with 9.8:1.


My questions for the crowd are:
- Anyone have experience with the Max Moritz/98mm setup to confirm the CR that you got? Does my measurement of head volume seem right?

- How aggressive can I be on deck height? I've seen suggestions that 1.25mm is a minimum for stock or street rebuilds. I have seen other suggestions that 1.0mm is ok. Is that just a difference of opinion or is it situational?

Any advice is welcome. Thanks

Distinguish "deck height" which is the distance between the theoretical top of the cylinder (without the dome volume, like a flat-top turbo piston) and the edge of the cylinder, from "piston-to-cylinder head clearance" which is what you are measuring with the solder method.

Fortunately you have the published dome height- you can set up some gage blocks on the cylinder edge and measure to the top of the dome, then subtract 9.3mm and that will give you the deck. Try it first with the standard 0.25mm gaskets in place.

This is not to suggest that your piston clearance measurements aren't helpful, they are, but for a different (very good) reason-- not hitting the head!

Ok, so you put the piston in the cylinder with the rings greased, set the piston top even with the cylinder top, so that gives you a volume like this:

98mm diameter, so r= 49mm
height = 9.3mm (from top of dome to theoretical deck)
vol = pi r^2 h = 70.15cc volume of the measurement cylinder

Of course you can check this by filling it all the way to the top, then recording what the burette says, it should be close. Subtract what goes in from the burette for the volume, it should be very close to what Mahle publishes. There may be some distance between the top ring land and the "deck" but the grease ought to prevent much from getting down there and screwing up the reading.

On the "how aggressive" you can use 1,0 and be comfortable, and Grady tells us that when he was doing race motors, he could sneak the deck down to less than that by practiced assembly techniques, tight tolerance control and frequent teardowns. For a race motor, this must occur. For a street motor, as long as you keep the detonation margins adequate by a deck around 1,0, I would call it cool.

Bill, how are you making out on this? I am in the nearly the same situation.. I am rebuilding an 82 SC into a 3.2 using Mahle Moritz wedge pistons. If the piston dome vol is well and truly 28.4 cc, then I believe if I have done the math right for my motor, I will have a whopping 8.68 static CR to look forward to assuming a 1 mm deck. With zero deck height my math gives me 9.62 CR, but of course running zero deck height is not a possibility. Where do they get this 9.8 number from that they spec them out as? I am hoping that when I measure the actual volume of the dome I get something more like 37 cc, but that seems very unlikely............ have I done my math right? I assumed the head volume was 90cc, I suppose there is a chance that could be smaller than that figure once I finally measure it. Kind of disillusioned with my project now that I have realized this.. are all the Moritz kits running around out there with sub 9.0 CR?

Any New information?
 

I was wondering if there was any new information as I am traveling down this exact path. To add to the discussion, I emailed Mahle and was told there were two different piston made from the 98P11 forging. Here is the reply I received.

"There were two similar pistons with the wedge style dome made from the 98P11 forging. The Max Moritz piston dome is 10mm tall, with 35.4cc net volume. There is a shorter one, 9.3mm tall, with 28.3cc net volume."

I'm a little confused by all the calculations, but wouldn't this be a sure fire way to figure just what you have. Calculate the volume of the swept area of 1 cylinder (pi x (1/2 bore)squared x stroke). Assemble 1 cylinder and a head with the piston at TDC. Measure the volume of the combustion chamber. Add that volume to the volume of the cylinder and divide by the volume of the combustion chamber (as calculated above with piston at TDC). That should give you your static compression ratio.
Bob B

Yes that will do the job. The problem is how to do the measuring. You'd have to seal the rings with grease and rotate the engine to make sure the spark plug hole is straight up and fill with liquid. hoping you don't have any bubbles. In general it's easier to measure the individual components on the bench. The math is not that hard.

-Andy

I know this is an old thread, but I wanted to reply as the info in this thread threw me astray with my max moritz 3.4l build. To confirm, your numbers look good. Mahle motorsports just confirmed to me that the volume of the 3.4l piston top (9.8:1) is 35.4cc.

I did the same calculations you did, and with a deck height of 1.05, and valve reseating, i got a compression ratio of 9.8-9.9:1.

The 28.4cc number is the dome volume of their 3.0 to 3.2 max moritz pistons.

Can you clear this up for me. Both of these pistons you reference are 98mm, correct? You are saying the 28.3cc domes are for a 3.0-3.2 and the 35.4 for a 3.2 -3.4?

The 35.4 would be 3.2 going to 3.4 This yields 9.78/1 with 1.25mm deck and a 90cc combustion chamber

Edit; If for a 3.0 going to 3.2SS, 35.4 would yield 9.3/1 and would use a 22mm wristpin

The 28.4 doesn't make sense as it yields pretty low compression unless you ran a 3.6 crank and 1mm deck...(nets 9.3/1)

BTW, this should be for a 23mm wrist pin.

That's what has a few of us confused.
 

Ha ha hence the reason for my drinking.......

My set measures a 9.3mm dome height vs the 10mm. My compression math works out to be LOW and my wrist pin is 23 mm.

For me, this all started by trying to avoid this and sourcing numbers from the people who made them.

So the last point to prove is the dome volume by actual measurement.

What are you building?

'89 3.2 to 3.4 with set of Mahle MM 98mm P&C.

I haven't seen any confirmation regarding the 3.0 to 3.2 domes being 28.3cc. This is speculation until it is actually measured. The OP of this thread measured a 98mm MM piston with a 9.3mm dome height and got 33.1cc.

"Mahle says the dome height is 9.3mm, which is consistent with my measurement. I "cc'd" the dome volume by positioning the piston in a cylinder so the top is even with the top of the cylinder, used a burette with windex to measure the remaining volume to net out the volume of the cylinder dome. I get 33.1ml for dome"

Within a margin of error of .5cc, this puts the 3.0 to 3.2 98mm MM piston with 9.3mm dome height and 1mm deck clearance at 9.3:1 compression, which seems consistent with what I have read. Eventually, I'll get around to doing this measurement myself to confirm.

Thanks for the reply. This is essentially where I am at. I just need to make sure I measure it correctly either that way or the Bruce Anderson way.

The other point I am not sure about is "declaring" the 9.3 domes are for a 3.0 to 3.2 conversion. I have this set. The domes measure exactly at 9.3mm(consistent with what Mahle told me) and they have a 23mm wrist pin which is consistent with a 3.2L. You said yours had a 22mm pin which is what you would expect for a 3.0, correct?

3.0 to 3.2 conversion piston set should indeed have 22mm pins.

This is what Mahle told me.
"Called Motorsports for you this morning & confirmed part # & specifications for you. Compression height is 32.8mm. & volume is 35.4cc & this is based on the 3.2 to 3.4 application. The actual part # is PP98-013. The 17635-01 is the print-drawing #. Motorsports Tech. Line # is 888-255-1942. Hopefully this will answer your questions concerning this Mahle performance piston. Thank you. "

Below is from another thread. Take it all with a grain of salt. I deduced that the 3.0 to 3.2 is the 28.3cc volume based on the email reply to Tie Rod from mahle below. Still, seems to be lots of conflicting info, but Mahle did confirm my 3.4L piston had the 35.4cc piston top volume, so I assumed the rest. If we can't trust Mahle to give good info... then cross your fingers for my engine. Mind you, see my warning on bilstein.
______________________________________

Here is where I got some of my info that raised my question of the true dome volume for these pistons. Sadly, EBS, who supplied my pistons, had no more in stock to give me measurements, and his Mahle rep was gone for the weekend. They wil be getting back to me early next week. I also emailed Mahle, to see what they say.

Looking at his measured dome height volumes, it would appear that the larger of the two dome volumes (35.4cc) correlates to the 3.2 to 3.4L MM pistons. However his measurements also point to the 9.3mm dome height, which Mahle says should correspond to a 28.3cc dome volume (see email from Mahle below).

Here is a supposed word for word response from an email inquiry to Mahle. Mind you, I have posed questions to Bilstein about shocks and got a rep who gave me bad info. After calling them on it an re-inquiring, I got the correct info. Numbers are only as good as the data given.

NOTE: the 3.4L max moritz pistons are PP98-013 so there may be x2 different 3.0 to 3.2 max moritz pistons.

I interpret the above response as Mahle referring to both of their MAx Moritz piston styles... the 3.0 to 3.2L option, and the 3.2L to 3.4L option.

Interestingly, my forging number seems a bit different. Presumably just a different production run.
http://i1168.photobucket.com/albums/...pshrojvjqi.jpg

http://i1168.photobucket.com/albums/...pssgcsref7.jpg

Considering the fact Mahle has been making versions of this piston for about 35 years with different dome heights, compression heights, pin diameters and for different customers (Max Moritz, Ruf, Porsche Werk 1) anything is possible and the only true way to know is to measure.

Here is a 97mm version that was developed by Werk 1 as a power kit for SC Euro customers that weren't to thrilled by the drop of 20 hp from the Carrera 3.0. It used 97mm turbo cylinders and displaced 3.1L. It made a factory claimed 210 hp with no other changes. Everything I've read regarding this rare engine is that it had a claimed 9.5:1 CR. with a 97mm bore and a 70.4mm stroke, it would need roughly 36cc dome volume to achieve that. That piston doesn't look any different than mine. Read more about it here: Point One | Issue 180 | Excellence

http://i906.photobucket.com/albums/a...psyxtezwkh.jpg

Josh, is that a busted stud on the lower right?

I hope when measuring you guys are using a plastic disk to lay on top of combustion chamber (head). Or on the top of cylinder.... 2 holes in plastic disk to get rid of bubbles.... Otherwise you are getting a higher reading due to capillary action.....

Don't forget to install the spark plug you intend to run as well,,,,,,,,,,,,,,,,