Why doesnt this add up? - Comp Ratio...

So the engine i bought has JE pistons with a C/R of 9:1 with stock 58cc heads. I do not have a part # on the pistons but in looking all of the ones they could be the largest dish is 20cc. So lets assume it is that. Here is my problem from the forumula and #s that my head porter used he gets a C/R of 9.4:1 with larger 60.2cc heads http://web.camaross.com/bb/confused.gif the C/R should be under 9:1 if the chambers are larger. Here is the data he gave me that he used:

piston dish = 20cc
deck hieght = 0.20 in the hole = 4.18cc
head gasket (Felpro 1074s) = .39 = 8.15cc
combustion chamber = 60.2cc
cylinder = 783.52cc

So he added the first 4 numbers together-
20
4.18
8.15
60.2
------
=92.53

Then add that with the cyl area-
92.53
783.52
------
=876.05

Then devide-
876.05/92.53=9.46 compression ratio

So with a bigger combustion chamber than stock i am getting a much larger compression ratio than the advertised 9:1. Please tell me that the formula is wrong or i have one of the numbers wrong! http://web.camaross.com/bb/smile.gif

Thanks!

Whenever you calculated your first compression with the old chambers, you did it wrong..

you are right, your compressionw ill be 9.4:1... with a ton of quench http://web.camaross.com/bb/wink.gif

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-94 Black T-Topped Z28 M6, 10s, and daily driven!
-96 CBR F3, modded of course
-92 Talon Tsi AWD, also too many mods..
-99 Tony Kart :)
http://www.fastblackcar.com

Ahh so the answer to my problem was that he thought the piston was actually a difrent size dish. It is a -31cc dish. And the heads ended up having 62cc chambers in the end so:

888.85/105.33=8.43

Perfect. http://web.camaross.com/bb/smile.gif

[This message has been edited by kmook (edited September 10, 2002).]

ok i'm confused.

You said that your piston is a -31 dish and with a stock 58cc head that would give you 9:1 well if you plug the numbers into your formula you gave i dont get 9:1

piston dish = 31cc
deck hieght = 0.20 in the hole = 4.18cc
head gasket (Felpro 1074s) = .39 = 8.15cc
combustion chamber = 58cc
cylinder = 783.52cc

31
4.18
8.15
58
------
=101.33
+783.52
--------
884.85/101.33=8.73

Now 8.7 is close to 9:1 Maybe they are just rounding up? I don't claim to be very good at this i am just trying to learn something!!!

Thanks scott

Got me http://web.camaross.com/bb/confused.gif

Thanks to Mr. Mudge http://web.camaross.com/bb/wink.gif for the calc.

It looks like JE is figuring a 9:1 c/r out of the -31cc pistons via a 383 with 0 deck hieght, stock 58cc heads, and stock .049 head gasket. Comes out to 8.9:1

Looks like i'll be depending on what my actual deck hieght is... lets do extremes- 0" 8.75:1, .025" 8.37:1

Glad that worked out http://web.camaross.com/bb/wink.gif

How do you convert deck height to cc's?

thanks

Here's a link to a nice CR calculator:

http://www.camaroz28.com/misc/index.shtml

Mike

I kinda thought that it might be something like that.

thanks

<font face="Verdana, Arial" size="2">Originally posted by Mikey 97Z M6:
Here's a link to a nice CR calculator:

http://www.camaroz28.com/misc/index.shtml

Mike</font>

according to that my Cr is 12.45 right on


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1994 Z28 A4
383 scat 9000 series crank, eagle rods, wiseco pistons,12.5:1cr ,TCI Streetfighter,HOOKER LT's,Meziere H20 pump,LT1 Edit,95+ opti
1/4:11.89
11.64 corrected
MPH:115.23
1/8th:7.4x
60':1.77

[This message has been edited by AHARTZ28 (edited September 12, 2002).]

<font face="Verdana, Arial" size="2">Originally posted by AHARTZ28:
according to that my Cr is 12.45 right on

</font>

What data did you enter?