Compression Ratio Measurment on 2.7 RS copy
 

I am finally getting time to assemble my 2.7 RS replica motor. It is based on a 73E 2.4 7R case and I'm using Mahle RS 90mm pistons and cylinders. As youare all aware these are advertised at 8.5:1 compression ratio. I felt it might not be unreasonable to expect to bump these to 9:1 when the head mating surface was machined .010" while being bored to fit the larger cylinders. I have set the deck height to .040" (1mm).

Unfortunately, when I measure the comression ratio I come out with much lower numbers even than the 8.5:1 advertised numbers. Various posts I have read here over the years indicated that Mahle might exagerate their compression ratio numbers a bit which seems odd but I can accept that might be the case. However I'd like to hear from anyone who has seen similar results on a similar motor as I am now contemplating options such as selling the RS pistons and buying JEs.

Compression ratio was measured in two ways:

method #1:

With engine on stand rotate so that spark plug hole is vertical and high. At TDC fill combustion chamber (to bottom of spark plug threads) burp to remove last air. Piston sealed with grease. Remove ATF and head, clean. Turn to BDC re-grease install head. Refill with ATF from burette, burp and measure total volume.

Combustion chamber (at TDC) volume = 63.5ml
Total volume (at BDC) = 518ml
compression ratio = 8.15:1

method #2:

As per Bruce Anderson's book

V1=swept volume= 2687/6=447.8ml
V2=deck height volume= .7854x.1cmx9.cmx9cm= 6.36ml
V3=combustion chamber vol = 68ml
V4=piston dome volume = 10.6ml

compression ratio= (V1+V2+V3-V4)/(V2+V3-V4)

=(447.8+6.36+68-10.6)/(6.36+68-10.6)= 8.02:1


Allowing for some rounding errors and errors in measurements these two results are close enough (2%) to offer some corroberation.
Can anyone see something I am overlooking? Theses results are dishearteningly low and I am looking for opinions on a diresction to take.

Thanks for taking the time to read this.

Scott Johnston

Scott, I ran the numbers like you and agree with what you have. It looks like the only way to get 8.5:1 compression with what you have is to run a .014" deck height.
I would be tempted to talk to someone representing Mahle to understand how they come up with 8.5:1 with a very marginal deck height.
The only way the Mahle piston runs at 8.5:1 is with a dome volume near 14.6ml, with a 1mm deckheight.
The numbers don't lie. So whats the deal?

In your deck height calc:
Isn't cly volume Pi X *R* squared X H?

JP he's not using pi, if you use pi R squared h you still get 6.36ml. Not sure what the deal is with .7854 but the 6.36 number is right.

Guys;

The (Pi)xRxRxH formula that we all learned in school does indeed give the same result that one the Bruce Anderson gives in his book which uses .7854. Got me as to why?

2.7Racer;
My dome volume is no more then 10ml by direct measure. I assume that I do have RS pistons as I bought them from EBS, they are 90mm and do have a slight dome with valve cutouts.

Thanks for your input.

Scott

Scott,
I would contact EBS and discuss your problem with them. I've done a little business with them and I think everyone on this board would agree they are the "good guys".
Ultimately, I expect for you to have true 8.5:1 pistons working in a 68ml head you will need a piston with a dome height closer to the 14.6ml as calculated.
Anything less than 14.6ml dome would require you to run less than 1mm (.039") deck height.

Scott,
I believe the recommended max. cut from the cyl. head sealing surface is 0.040". Yours was cut "only" 0.010". If so, calculate the compression ratio increase. Don't forget to remove a commensurate amount from each chain box mounting surface.

Sherwood

Today I made another measurement of the various volumes using the Bruce Anderson method. I have nearly 100% confidence in the accuracy of this test and it yielded a compression ratio of 8.26:1.

2.7racer:
I will talk to EBS about this. I too have bought a lot of stuff from them over the past 10 years and always found them to give great service. My piston and cylinder box has the correct part number so I am sure they are the correct part too.

My heads probably started out with 68ml combustion chambers but I didn't measure them before having machined this time and this engine had been apart once (at least) before I tore it down so the heads may have been machined in the past.

My combustion chambers are 66ml now and with the 10.6ml dome I can see no way to achieve the desired compression ratio of around 9-9.25:1. As you note it will take a substantial increase on dome volume to get to the advertised compression ratio with a 68ml combustion chamber. I calculate that it would require about a 13ml dome to get me to 8.5:1 with my 66ml chamber.

Sherwood:
My head may have been cut in a previous rebuild but it sure looks like taking another .030" of the surface would bring me really close to the valves.

By my measurements and calculations each .010" cut on the heads will only reduce my combustion chamber volume by .36ml. Taking the full max cut of another .030" would only get me a reduction in chamber volume of 1.1ml. No where the 3ml I need to even get to the advertised 8.5:1.

I sure do appreciate the input you guys are giving. It is causing me to think more about this and spend more time on my measurements and calculations.
Thanks a lot,
Scott

You pretty much have to measure the cylinder and the head separately - doing so together may not yield the proper result. See some pictures here for details:

http://www.pelicanparts.com/swapmeet_pics/Bruce-Jerry/Page2.htm

-Wayne

Wayne;

I did use exactly the method described and used by Bruce Anderson (and shown in the photos you linked to), with the plexiglass plate covering the combustion chamber as well as the cylinder during their successive fillings with ATF.

I also used the method of filling the assembled cylinder and head more to make a comparison. This yielded a result within 2% of the BA method.

Thanks,
Scott

These are the numbers for my RS clone

V1= 447.8ml
V2= 4.14ml
V3= 66 ml
V4= 13.8 ml
Compression Ratio: 1:8.95

I am using standard Mahle P/C, heads were machined, engine case was machined and then align bored

I am very pleased with the result

When comparing Scott's numbers with Peter's I can see why peter is happy. First Peters deck height is less by just over 2ml (4.14 vs 6.36). Second by machining his heads Peter has reduced the head volume by 2ml (66 vs 68). Third Peters unmachined pistons have more than 3ml dome volume (13.8 vs 10.6).
All of these numbers work toward increasing Peter's compression ratio as compared to Scott's.
I still see a Piston dome volume difference between pistons.
As I see it Scott did NOT get 8.5:1 pistons.
Someone owes Scott an explaination.

Preterfrans:
After rechecking my measurements I found that I too have 66ml combustion chambers (V3). Of course my swept volume is the same as yours at 447.8 (V1). What was the deck height you shimed your cylinders too? Mine was 1mm which according to my calculation of;
pi(4.5cm)(4.5cm)(.1cm)=6.36cc (V2)
The biggest difference I see is that my piston dome volume measures 10.6cc (V4) as opposed to the 13.8 that your does.

Doug:
Good observations you made. Jon at EBS has gaciously offered to check the volumes in question in order to confirm my numbers. I'm sending him a piston, cylinder and head in the morning.

Will let you know what we find.
Thanks, Scott

Something does not seem right here, but I don't think Mahle made any other lower compression RS 2.7 pistons.

-Wayne

Scott,

My deckheight is 0.65mm (including the 0.25mm shim). This gave me a piston to head clearance of 1.05mm and a piston to valve clearance of 1.76mm (both measured with the solder method).

Odd that the dome volume on mine is higher than on yours. I attached a pic of the piston (its the left one).http://forums.pelicanparts.com/uploa...1127142304.jpg

Re: Compression Ratio Measurment on 2.7 RS copy
 

FYI, there is no such thing as a 73E 2.4 7R case. All the 7R cases from 1973 were from the 1973 1/2 911T CIS engines. The 911E engines used 4R and 5R case halves. In general, these are pretty valuable if they are uncut (not bored out to 2.7L). I recommend most people look for a 1974 case to build a 2.7 RS-spec engine out of...

-Wayne

Higher dome height *should* give higher compression, but you must make sure that the distance from the center of the rod to the top of the flat of the piston is the same. This distance is different for many pistons, and is the reason why installing the 2.2S pistons in a 2.4 motor increases the compression.

-Wayne

Hello Wayne.

Thats very interesting information about the 7R 2.4 cases.

Because I have a 7R case 2.4E...

Now what I'm wondering is was this a factory rebuilt motor on the later case?

Kind regards
David

Wayne;

Perhaps you are correct about there not being any E 7R cases, but mine sure seems to be.
My case has a 7R part number. It also has the 911/52 type number stamped into it. My enginew number is 6230902. As you no doubt know the type 52 is the E designation. In the engine number x2xxxxx indicates an E engine and the xx3xxxx indicates 1973 year of manufacture. All of these numbers appear original and not to have been altered in anyway.
I find it curious that my 73S has a 5R case and it has an engine number a few hundred numbers earlier. I have always assumed that Porsche changed to the 7R some time in mid 73.
Regards,
Scott

It was my understanding that the 7R cases started showing up on production cars as early as '72 (granted, very few), and from then on there was no real rhyme or reason to when they used 4R/5R or 7R cases. Thus you can have a '73 with either, though it seems all the '73.5 T's had 7R cases.

This is at least what I have gathered from reading other threads on this subject.

Brooke

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.