Sorry I got in on the thread a little late. To shed some light on the question of the .7854 number, it comes from the factor of pi and the 2 going from radius to diameter. In the formulas, when the 2 is squared yielding 4 and divided in to pi, you get the .7854. This is a constant that exists in all the volume formulas.

If you want a little more straight forward method of calculating the compression ratio than in Bruce's handbook, try my formula described here:

http://members.rennlist.com/jimwms/Tech/comp_ratio.html

It's also in the OCT '97 Panorama.

Regarding the 4/5/7R cases. First of all, nothing is ever set in stone. Porsche is and was a small car company, and if they had extra cases lying around, no doubt they would use them. It certainly is possible that your E came from the factory with a 7R case.

However, the 7R case has specific bosses cast into it that allow it to work with the CIS injection system - the 4R and 5R cases do not. This has to do with the accelerator linkage setup on the CIS injection, versus the MFI setup. That is why, if you decide to use an early 2.0 aluminum case with a CIS engine, you will need to have the bosses added / welded onto the case (not a huge deal actually).

I do know that replacement cases from the factory were 7R cases - after a while, that's all they had. So, if you ordered a replacement case for your 2.4 motor, you got a 7R case. It would be my best guess that this engine case that you have is not an original case, but is a replacement case.

Ehh, it doesn't really matter...

-Wayne

Wayne et al

Firstly, my 73E has a 7R case (ex factory) mind you if was a UK RHD car (This may make a difference). Also re deck height, This I found very difficult to measure (solder method is best) but with these shallow dome pistons in a deep hemi head, my piston looked almost flush with the cylinder but when you put the head on you have 0.9-1mm, go figure, I think its because the dia of head seating edge is slightly larger than the bore and the piston is slightly smaller this combined with the 'initial' angle of the head and the slope of the piston give you clearance, this 'deck height' is totally different to a flat top piston and head with a large squish area, but my result was the same as yours with my E pistons and machined heads I hoped to increase the comp ration but barely scraped up to 8.0

HTH
Neven

Comp Ratio
 

Scott

Further to my last post, I played with your numbers and concur on your calc/measurements. I think this is excessive 'deck cleariance'. But remember that 1mm equates to 6.36 cc at this bore, So If you machine another .75 mm from your heads you drop the head vol 4.77cc, These make big differences to the comp ratio, and you can see that machining/setup can alter the comp ratio so the Porsche Mahle figures are probably right but we don't know what their target deck height was, The conventional thought is 1mm which is fine but you can only measure this with the head on by crushing solder. Another option (and i'm sure I'll get flamed on this) is to 'zing' 0.5 mm at the edge of the piston (at the head angle) which increases the dome vol by 3cc if you can set the piston higher. This always depends on the valve to piston clearance which is critical. The basic problem is that unless you stretch the rods (by offeset boring the wrist pin bushing) you are limited to 1mm in machining of heads. But if you could increase the rod length by 2mm and machine the heads by 1mm and zing the pistons (and possibly deepen the valve pockets) you could get up to 9.5

As an aside my method of checking the valve to piston clearance was to assemble a head with 1 spring in it, put a dial gauge on the rocker and as you rotate the motor press the valve down with a screwdriver till it hit the piston measuring the clearance every 10 degrees or so, a lot faster as you don't have to back up and reset all the time

HTH
Neven

NevenH,
The question remains the same. How does Mahle rate this piston set at 8.5:1?
Even with Scott reducing his head volume from 68ml to 66ml, he would need to run a deck height of .68mm(0.0267") to get as advertised 8.5:1.
So the question remains unanswered at this time.
I agree with your numbers, however I would want to understand what my valve clearances were before I performed any further machining of heads.
"Zinging" the pistons to gain 3ml of dome height is interesting. Rather than lengthening the rods I would look into fly cutting the spigots. I don't know how much can be gained by cutting the spigots so it might be required to cut something from the cylinders as well.
I'm not familiar with the cost of lengthening rods. Perhaps you have some experience with this.
Ultimately, I want to understand this discrepancy with the rating of the Mahle pistons.
With a proper piston that actually delivers 8.5:1 at normal deck heights and stock heads, Scott should be able to accomplish his original goal of 9.0:1 without excessive machine work and the cost that goes with it.
I still want to hear Mahle's side of this story.

Doug

If you calculate backwards from Scotts figures a comp ratio of 8.5 would require a 'deck height' of 0.35mm, This is the theoretical height diff between the top outside of the piston the the cylinder top, as I have found assembled with these shallow pistons this would probably give you enough clearance between the piston and head. I would not be surprised if porsche ran these this tight.

As for flycutting the spigots this unfortunately has the effect of moving the camshaft toward the centre and therefore suffers the same restriction as cutting the heads, cutting the heads is more productive (tho on a mag case you are advised to cut the spigots as you find the cylinders have 'hammered' a ridge in them)

HTH

Neven

NevenH,
I ran the deck height figures in my spread sheet. The .68mm number I quoted is for his 66ml head.
The .35mm number you quote is for a 68ml head.

Thanks for all the input guys:

As regards Waynes comment that mine might be a replacement case: It could be, as I am not the original owner. However it seems somewhat unlikely that someone would have gone to the trouble of restamping it with exactly the same style stamps as the factory used and with both the correct engine and type numbers. Maybe...but there seem to be several other posters here who also have 7R cases in their Es. But as Wayne pointed out...It doesn't really matter.

I just heard back from EBS with the results of their measurments on my head, piston and cylinder volume. They measured my head volume at 65ml vs the 66ml that I found but the biggest difference was that they measured 13.1ml dome volume which is very close to the number that Peter got (13.8ml) and pretty far from the 10.6ml that I found. So I will remeasure the dome volume.

Using the EBS numbers I get a compression ratio of 8.68:1.

Neven:
You mention that your results were the same as mine. Can you be more specific? Was your measured dome volume also in the range of 10ml? I measured mine twice and got the same dome volume so I'm not sure where I might have lost the 3ml that EBS found. I too though of "zinging" (I like your term) the top of the piston just outside of the dome volume in order to raise the dome volume. I figure it could loose .25-.5mm without much harm.

I figure the whole discrepency is due to this dome volume difference. I'll be back with more info soon.

Thanks again for everyones input on this.

Scott

Scott,
I've had fun with this and learned a few things too. I made a spread sheet using Bruce Andersons formula to compute compression ratio. It has been very informative.
NevenH,
Very good input. I spent a month in Christchurch several years ago on a job. I have a great deal of respect for all Kiwi's and the resourcefulness required to live on your beautiful islands so far from everyone.
I breathed the freshest air ever. Just a spectacular country.