mark kibort

Here is the best compression similar ive seen

remember, use inches vs the mm we all use. 97mm = 3.818" 100mm = 3.937" 79.8mm = 3.10"
1mm =.039"

http://kb-silvolite.com/calc.php?action=comp

mark kibort

using the calculator
based on :
52cc head US
48cc head euro
.039 gasket thickness
97mm bore (3.81") or (3.74" for a 4.5 liter 95mm bore)
78.9 mm stoke (3.10")

CR for all the 928 engines in question are:
4.7 US 9.5:1
4.7 euro 10:1
4.7 euro '84 11:1
4.5 US 9.3:1

the variables are the deck clearances and piston volume. i do know that the euro 84 was near 2cc total, and its flat as a pancake! euro 83 was near 8cc and US was near this value as well, maybe a little more, but not more than 10cc. the 83 pistons have very deep valve pockets. thus, the 8cc volume.

for reference, i used the 84 pocket depth for the 5 liter conversion using the 85 4 valve pistons. the valley was already 6cc and adding the new cuts for the 2 valve 84 relief pockets, the total was 8cc for the 5 liter 85 pistons to work with a 2 valve euro head .

can you find any errors in my calculations or the calculator itself?

Mk

Jim bailey - 928 International

The Euro S heads are slightly different with the LH style having steel insets in the exhaust ports plus as I recall a small notch near the spark plug in the combustion chamber. I do not have any here to look at right now.

mark kibort

thats interesting. my part euro 5 liter used heads theoretically from a 82 euro and didnt have those steel sleeves. (cc'ed at 48cc). scots 82 euro M11 heads did have the sleeves, but we will never know if those heads really came on that original 4.7 euro M11 block. (as the heads were already fitted with the B1 cams). anyway, they CC'ed at 48ccs too?

Mk

John Veninger

My 80 EuroS has steel sleeves in the exhaust ports. Was a PITA to blend them

hacker-pschorr

Care to explain a bit further on this process? My friendly machinist thinks I'm nuts for even suggesting the idea.

918-S

My heads have those also!

mark kibort

I used the blue tape used in interior painting and plastic wrap, to protect all openings around the cylinder. of course, mine was much easier having it out of the car, but i was not about to disassemble the engine for a 1mm of grinding. there is nothing crazy about it. sure, it doesnt look like a skilled machine shop did the mod when you are done, but who cares, as long as you take 1mm off the valve relief half circle, whats the issue??

an air grinder makes quick work of the job, and you have to be careful to not take too much material, so a steady hand is good.just make sure you take off 1mm and do it all the way to the bottom of the valve relief. you can take an old 45mm and 40mm valve set and make sure you took off enough. put the heads on , and push the valves down .5". if they touch, you got a problem. if they dont, mission accomplished.

Mk

drnick

there were two different euro s engines, the S and S2. both 4.7 litre but the injection system, cams and compression ratios are different. the S2 has 310 bhp vs 300 for the S and higher compression also, it uses LH injection rather than k jet.

FlyingDog

I doubt any of those are correct. Every other measurement I've seen shows that actual compression is lower than Porsche spec. All of your measurements above are above Porsche spec except the 4.7 Euro S (80-83) which is the same as spec.

John Veninger

Herr-Kuhn

I measured an 81 at 8.1:1. The Euro 4.7 merasured out to 9.75:1...it will be 8.1:1 when I am done with it.

mark kibort

How is it possible for the numbers to not be correct? the calculator is not wrong, as it takes into account more factors as well.

there are not too many variables here. 48cc vs 52cc for the heads (us vs euro)
pistons 2cc euro 85, 8cc euro 83, and near 10cc US 4.7/4.5

what else is there???? where could there be differences and what have you measured? Ive a few heads and lots of pistons now, and dont see how the numbers could be off.

let me know if i have made any errors!

Just checked them again, and went out to many decimals.

looks like all the cars are pretty close to spec except euro 85

4.7. US 9.3 * based on 10cc piston, 52-3cc head
4.7 euro 82 10.1 * based on 6-8cc piston 48cc head
4.7 euro 85 11.1 * based on 2cc piston 48cc head
5.0 euro heads 9.9:1 * based on 8-9cc piston 48cc head

how is it possible for the euro 85 not to be 11:1 vs 10.1. the only thing that changes is piston valve reliefs. 2cc vs 8cc!! i I have both of these pistons, and you certainly cant be off by very much. its 2ccs, 3 doesnt fit!! euro is more like 6, but even if you go as high as 8, you still end up with 10.1!!
all of the deckhights are similar too. scots was a little above stock when we shaved the block slightly. that only raised the CR by about .2-.3. stock is pretty darn flush with the deck!


MK

mark kibort

Im still very curious, how the euro 82 and euro 84 can be only .5 CR ratios apart (ie 10 vs 10.4) with the only change of the pistons!! 2cc vs 6-8cc.
mk

slate blue

Hi Mark there is differences in the head too. The little notch in the combustion chamber for one isn't there on the Euro S2 heads. Unforntunately I don't have a pic of my later heads handy.