mark kibort

I know, brake hp, so it could have been a brake dyno!

I agree with you though, there have been some 85s at 280 at the wheels and thats 330hp flywheel. do we have a dyno run for an LHjet 84-85 euro 4.7???

If jps was the older CIS, it would be interesting to see the rear wheel dyno of the 11:1 or so CR later S model. I also would be willing to be all euro S 4.7s would run a little faster than any US 85s, certainly, the US 85s will run strong, but those euros are rippers!!!!! im sure they have at least the rated 10 to 20more hp, but are a little lighter as well and have the better gears for how most folks run our cars (see, i said it Heinrich ole pal!!)

in building up scots engine, im more impressed with the 928 engineering than ever. just that bottom end alone is a work of art!


MK

heinrich

Mark ..... dynojet is all I use.

FlyingDog

Did you actually take any measurements? It has been established that S4s are actually 9.3:1 (same as 85-86 Euro catalyzed cars) instead of the published 10.1:1. I wonder if LH Euro S engines are really 10.4:1 or are actually less.

John, forget lowering the CR, just manage the boost better. Look at SAAB, Audi and the new Mercedes... Big CR + BIG BOOST

mark kibort

anyway you slice it, the euro comes out to close to 11:1.

ive measured the combustion chamber of the euro many times. folks its 48ccs
take that with 1mm (.040)gasket (thats 7.6cc) and the piston cut outs are 2-3cc for the euro 85 and 8cc for the euro 82ish.

plug in the numbers and the euro 82 is 10:1 and euro 85 is close to 10.8.

have a hard time beliveing that the S4 is 9.3:1. i think Marc T. posted that the combustion chamber is 38cc, but the pistons have that big dish, i think its like 15cc.

the only other engine that is different than stock CR specs is the US 84. have a hard time believing the 9.3 :1. pistons are dished with at least 8ccs, the combustion chamber is 54ccs and the rest is the same. works out to about 8.7:1.

plug in the numbers and see what you get!

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

it will ask you for inch versions
48cc
8cc
.39" gasket
3.83 bore (ie 4.7 liter 97mm)
3.106 stroke (ie 4.7 liter 78.9mm)

this gives 10.3:1 compression as the spec for porsche shows.
BUT, when you change ONE thing like the piston cuts, as i have a euro piston, used for my 5 liter valve cut outs, as 82 euro were too deep for the 85US piston, and its 2 cc for the cuts. even if you call it 3ccs, its still 11:1 COMPRESSION!!!!!!

I know two things for sure, 48cc for euro head combustion, and 54cc for US heads for 4.7.

be interesting to see if we plug in some known values for the S4, what we come out to!

MK

mark kibort

for an S4, i plugged in:
38cc combuston chamber
.039 gasket
20ccs for piston with dish and cut outs
then the normal 3.937 for bore and 3.106 for stroke ( 100mm x 78.9mm)

comes out to 10.4:1. since an 85 piston is only 6-8cc with its smaller dish, i cant imagine the S4 piston being more than 20ccs. anyway, if it is, the CR can be lower, but to get to 9.3:1, i have to be WAY off! am I?

heck with the 85 US piston use in my part eur5 liter, the piston had 6cc for the dish and i added the 2ccs with the 2 valve cuts for the euro 2valve heads for a total of 8ccs. turned out to be the same as the euro 80-83 piston volume of 8cc.

MK

FlyingDog

How much do the pistons sit above or below the deck?

mark kibort

stock, i believe it looked right at the deck level. that does make a pretty big diff, as every .005 is like thats coming off the gasket and worth about .3points on the CR. so, if the S4 is truely 9.7:1, the piston will have to sit about .01 below the deck, and i dont think it does. anyone? anyone?

MK

FlyingDog

https://rennlist.com/forums/928-forum/107282-piston-mods-and-machining.html
CC seems to be 42 and pistons 24.
I didn't know what to put in for the gasket bore, so I made it the same as the cylinder bore and came up with 9.310 using 42, 24, .040 gasket, .003 piston to deck, and the rest of your numbers for an S4.

Using your Euro chamber and piston numbers, I got 10.147.

mark kibort

41.6 is what i saw on that list post. (good stuff!!) and 24 sounds about right a little high. like to test that out myself some day (not soon!!!!!)

so, where did you get the .003 below deck on the pistons? if so, thats worth about .2 CR.

on my euro numbers, yes, 10.1:1. but for a euro 85, with the 2cc cuts, its more like 11:1. did you confirm that?

MK

mark kibort

i used water with my cc test and used a thin sheet of plastic. if these numbers are a little high, which they could be, say 40cc instead of 42ccs and 22cc instead of 24cc, take the gasket and allow for .005 compression and suddenly you get to 10:1.

However, (just read more of that other thread) TONY, who did the cannola oil test!! ha ha , found the combustion chamber to be 40cc. there is part of it, and we are up to 9.75:1 now.

in doing my tests of the 82, 85 euro pistons and US vs euro heads, there definitely is a range of accuracy depending on the fluid injecting method!

i would have hard time believing that porsche could be that far off with their specs.

but you never know , or maybe we do!!

MK

FlyingDog

Tony posted a picture on the second page of that thread with a .003" feeler guage on the piston, below a straight edge. Water is a bad choice for these types of measurements because of the high surface tension.

FlyingDog

Yes, I get 11.091:1 with 2cc, but I doubt the piston cut outs are that small on even a Euro.

mark kibort

actully works well, if you use a sealed top. (which i did). surface tension is only an issue if you don have it contained. ill try the cooking oil on some of the euro pistons tonight and see if there is a diff. also on the US 4.7 heads.

MK

mark kibort

Have you seen the piston cuts of a euro 85 piston? ill do a picture with some canola oil for you!! (like tony did) even if they are 3ccs, which they are not, its in the 11:1 range. by the way, my 85 pistons had a 6cc valley and the cuts were an additional 2cc, but they cut into the existing valley!!

Just checked my notes, 2.2cc for the euro 85 piston cuts on a flat top piston.

Mk

heinrich

OK Boyz ... when I unplug the white-green wires (going to the same 2-prong plug at the top) she dies..... so, where is the greenie?