I8UR4RD

iTrader: (16)

Is more needed to get the true answer?


This is what I have so far, and its the only one that has not given me a crazy answer like 7:5:1







I'd like to know the formula for finding the real answer rather than plugging it in, or make my own calculator. I have looked at the other threads, and did not come up with much other than dead links, and programs requiring excel which I do not have.

soundengineer

iTrader: (16)

I use a calculator built into some engine software....
assuming head gasket of 4.030 as well...
and assuming the #'s in the left column are correct for your setup

I get 9.59:1

soundengineer

iTrader: (16)

also, 3 of the ones I just randomly chose using google, also returned answers in the 9.53~9.59 range

wildcamaro

iTrader: (15)

^^^i got the same thing when I calculated it by hand...

ckpitt55

iTrader: (4)

This calculator is making the assumption that at TDC, the piston is flush with the deck surface of the block. This may or may not be true for the particular setup you're running - you would also need rod length, head gasket bore, and piston compression height to get an accurate number.

static compression ratio = (cylinder volume at BDC) / (cylinder volume at TDC)

BDC volume = (swept volume)+(gasket volume)+(combustion chamber volume)+(piston dish)
TDC volume = (combustion chamber volume)+(gasket volume)+(piston dish)+(piston-to-deck volume)

bww3588

iTrader: (49)

Most online calculators are going to be just rough estimates. It can get really fuzzy when you factor in ring land volume and actual gasket bore and thickness. The formula above is the jist of it for static. Dynamic is another animal.

Sales2@Texas-speed

iTrader: (5)

The calculator he used asks you to add or subtract the piston being in or out of the hole in the same line as gasket thickness. If the piston is out .005", you take the .005" from the .051" thick gasket, if it's in the hole, you add it to the thickness of the gasket. They just saved one step/calculation.

Sales2@Texas-speed

iTrader: (5)

Exactly, they are just tools to get a good estimate of compression. 9.52 vs 9.59:1....who cares?

ckpitt55

iTrader: (4)

ah gotcha. missed that, sorry.

Sales2@Texas-speed

iTrader: (5)

No big deal! I was looking for it as well, and didn't find it until I slowed down and read all the descriptions.

wildcamaro

iTrader: (15)

Yea usually when I calculate compression I assume a couple different scenarios as far as piston being out of the hole or in the hole and usually it is a negligible effect, like other guy said 9.52 or 9.59 same difference..

bww3588

iTrader: (49)

Of all the variables to be assumed, you pick the most critical...yea, it won't change compression by much, but there is no reason at all that you should have to guess on that. You should know deck clearance exactly.

I8UR4RD

iTrader: (16)

thanks for the replies. yeah I kept being told my compression is 9:3:1 (and im betting it is)

rods are 6.125
i have ls9 gaskets
lq9 block with (ive been told is a 9.225 deck)
heads are 3/4 deck
any more info i dont really have other than cam profile. 232/240 .595 /608 115

for some reason I thought all of that would come together to get the true result......but I am really more interested in the math behind it. Wanted to get the formula for the real answer.


Example

RL+DH/GT= Compression

bww3588

iTrader: (49)

Using that equation, you have a 183.52:1 compression ratio...

I8UR4RD

iTrader: (16)

yeah but thats not the equation thats just an example. with 183:1 CR I may as well have a bomb.

bww3588

iTrader: (49)

The actual calculations are in post 5. A few minutes with a calculator and you should be able to ballpark it pretty good.