compression ratio for euro heads on 4.7 or 5 liter bottom end. questions!
 

calulating compression ratio of the euro heads with US or euro pistons (4.7L ) and euro heads on a US 85 bottom end.

from my last project notes:

48cc euro combustion chamber
7cc gasket
2cc valve cuts *euro 85 style *
6cc dish of 85 piston (US 4 valve converted)
total 63cc

Total displacement of 5 liter cylinder is 619cc. compression ratio equals 619/63 or 619+63/63??? (either 9.8:1 or 10.8:1 )

(total volume of 4.7 liter cylinder is 583cc)

i have a hard time believing the later 10.8:1 ratio, as the euro '82 piston cuts are 8cc total for its deep valve cuts and that works out to be the same volume of the '85 US piston with the shallow new 2 and existing 4 valve cuts and its valley. ( ie 2cc + 6cc = 8cc)
That euro '82 engine with 8cc cuts is rated at 10:1 but 4.7liter , which does work out to 10.25 :1 using stan's and JV's math. hmmmm. If its a euro '85 2 valve cut , this calculates to 11.23 :1 so that doesnt work. using my math, the euro 85 4.7 becomes 10.23:1 which is close to the 10.5:1 factory rating.
(i.e. 583cc /57cc)



so, ???

how do you calculate compression ? stroke volume plus combustion chamber divided by combustion chamber? or stroke volume divided by combustion chamber volume?

the interesting thing here, is with a US 4.7 piston, the compression should be somewhere between the euro 85 and the euro 82. (ie between 10.5 and 10.1) up a full point from the stock US 4.7 9.3:1 numbers. (due to 54cc combustion chamber) Just found some more notes on US 4.7 pistons. actually 11cc as is.
(3cc more than euro '82 plus the 6cc more in the combustion chamber for a total of 9cc more or 72cc !! vs '82 euro of 63cc)

just taking the combustion chamber differences of a US '84 at 72cc vs the euro smaller combustion chamber of 57 or 63 ( '85 vs '82 euro) you get by just dividing into the volume of 583cc you get:

10.23 at 57cc for the euro 85 (583/57cc)
9.23 at 63cc for the euro 82 (583/63cc)
8.1 at 72cc for the US 84 (583/72)

so,

watcha think?

mk

http://kb-silvolite.com/index2.php works pretty well, and gives some extra info, like effective dynamic compression, etc...

Very nice program! thanks for the link

anyway, that proves that the compression ratios from the factory are pretty optomistic.
4.7 us is 8.6:1
4.7 euro 82 is 9.6:1
4.7 euro 84 is 10.5:1
5 liter euro 2 valve head is 10.1:1]
and finally, the US 4.7 bottom end with the euro heads is 9.4:1

its strange how the ratings for the US is way off, but right on for the euro 84. (ie 8.6:1 vs 10.5:1) the good news is for scot is that we know from the comparitive components,that he should have about 1 point better compression vs the stock 4.7US and equivilant compression to the euro 82 engine we are fixing up. (ie 9.4:1 vs 9.6:1 calculated)

based on:
48cc euro head
54cc for the US head,
2 cc for euro 85 piston cuts
8cc for euro 82 pistons or 5 liter piston
11cc for US pistons.
along with a 1.5mm gasket thickness or .06" around 6ccs

mk

PS, so it looks like the euro 5 liters are around the 10:1 compression range. anyone care to dispute this? stan, JV, DL, no need for 100 octane anymore

This made me curious so I grabbed a scrap '80 Euro piston an checked the relief volume. Got about 1.75tsp or 8.6cc into the cuts.

Looking at the inputs, where'd you get 1.5mm for the gasket thickness?

I certainly wouldn't mind saving on the race gas, but I won't be hasty.

One issue to consider is that we run 30+ degrees advanced timing, and my understanding was this also creates the need for higher octane.

I will ask David how he came up with his numbers.

use some packaging tape to close off the piston cuts and then add the water or oil. you will not get that surface tension bulge and it will be closer to 8cc. anyway, thats the CC of the 5 liter euro piston with the 2cc 2 valve cuts like the 85 euro pistons.

either way, you can see that its right around 10:1 compression with our (my old) 5 liter euro . 619 / 63 is pretty close to 10:1. any other number has to be wrong, as its not that hard to measure the CCs. in fact, you measuring the 8ccs of the '80 pistons, shows that it could even be lower!!

i usually use 2mm for the gasket, but im sure its crushed to 1.5mm otherwise all the porsche specs of compression would be even FURTHER off.

right now, the only one that meets the porsche spec is the euro 84 4.7. (ie 10.5:1) the rest are pretty far off and lower than spec.

SO, save your money on gas, run 92. i ran it and got 293hp at the wheels in my 5 liter part euro with the AFM to boot!! plus, i also verified that going farther than 42 degrees full advance lost 3hp but gained 3 useless ftlbs of peak torque. basically, ran the stock 20degrees at 3krpm with vacuum disconnnected plus 3 degrees.

so, why does the 84US cc out to 72ccs and give a compression ratio of 8.6:1!!!

how can it possibly be 9.3:1??? maybe with no head gasket!! ha ha

MK

I was poking around the web an found that just one reference to a 928 gasket thickness. That's at http://www.cometic.com/ . It shows 0.045" which yields a 10.065 CR on the calculator. A slight change in gasket thickness changes the CR a lot. 0.06" is on the thick end of specs looking at head gaskets, from what I found.

On the relief volumes, I thought "8cc? it can't be that much" and proceeded to pour in a bit more. The relief was full with no meniscus. Not real accurate, but certainly gave a number near that ballpark.

yes, so its more like 10.5:1 compression with the thinner value head gasket. (i was estimating the 1.5 to 2mms)
since its more like .045 or near 1mm, i plugged in the numbers and got 10.5:1 for the euro heads and US 85 5 liter pistons with shallow 2 valve cuts
48cc head
8cc piston
.045" think gasket close to 5cc
619 ccs for bore and stroke volume on a 5 liter ( 3.11 stroke, 3.94" )

for the euro 4.7 ' 82 year, it becomes 10.1 :1 imagine that!
only changes from above are the ccs or 3.11 stroke and 3.82 bore

so, the revised numbers using the new gasket size thickness. (as it does change the numbers.

4.7 US is 9:1
4.7 euro 82 is 10:1 (scot's was 97.5mm pistons, stock 97mm is 10.0:1)
4.7 euro 84 is 10.8:1
5 liter euro 2 valve head is 10.5:1
and finally, the US 4.7 bottom end with the euro heads is 9.6:1

Now we are not that far of the porsche spec.

Scot is only loosing .4 of compression by using the US 4.7 pistons and the euro heads!


Mark

IIRC, 85US AT OEM gaskets are either 1mm (.039") or 1.5mm... the 1.5mm was available for resurfaced heads and block decking to keep from raising compression after a rebuild (I think)... that was the crushed size... not sure if they used other sizes for diff. MY's... could explain compression variences...

I was told to pay more attention to the dynamic/effective compression as cam timing can dramaticly affect ultimate compression... could also explain published diff's...

:cheers:

Hi Mark,
Just so I get this straight... for my '83 US... I can slap on a set of Euro heads, have my existing pistons cut with deeper reliefs, and I'll be at about 9.6:1? If I ever have to pull the engine, this sounds like a good deal!

By the way, is this with the Euro '82 or Euro '84 heads, and what's the difference?

Thanks!
Rick

Hi Rick,

yes, thats what we are seeing. the euro 82 through 84/85 heads are the same. only difference on these models were the piston cuts. (84 euro had the shallow 2cc cuts vs the 8cc cuts on the 80-83 euro)

you could even do what we are doing with the US short block. we are going to just follow the valve reliefs and use a air grinder to take off 1mm at th circumference. the valve pockes are 46mm wide on the euro and 44mm on the US pistons. (intake valves are 45mm and 43mm respectively)
this may not work if we didnt take some material off. BENT VALVES!
however, it still may be non interference since the depth of the valve pocket is close to the euro 82 on the US 4.7 pistons.

Mk

correction on the formula. Looks like the website with the automated calculator takes total swept displacement, adds combustion chamber, and then divides by combustion chamber.

we are seeing more differences with the change in expected head gasket thickness. Looks like from my old notes, i used 7.2cc , based on 1mm gasket.

all that said,

the compression ratios are a little different still, as the gasket makes the largest difference.

FINAL values from the compression computer:

4.7 US is 9:1
4.7 euro 82 is 10.3:1 (scot's was 97.5mm pistons, stock 97mm is 10.2:1)
4.7 euro 84 is 11.2:1
5 liter euro 2 valve head is 10.66:1
and finally, the US 4.7 bottom end with the euro heads is 9.8:1

again based on:
48cc euro head
8cc piston (euro 82 or 5 liter with 2 valve flycuts)
7.25cc gasket (1mm thickness or .039")

(US head 54cc, US piston 11cc, euro 85 piston cuts 2cc)

all looks believable, except the stock euro 84 with the shallow cuts. this one works out to be 11.2:1 compression vs spec of 10.5. all the others are close to the same range.

key thing to take away with is the relative differnces with the different components especially with the euro heads and the 5 liter pistons with flycuts.

Hi Mark,
Great info! In that case, the whole project could likely be done without removing the engine. Very interesting potential there that opens up a lot of options re cams, intake and/or SC. An SC feeding through the Euro heads, for example, but at less-than-Euro compression might be pretty wild.
Thanks!!
Rick