CC'ing the combustion chamber

[Tbone]

I am going to CC the combustion chamber in my G head before I send it to the machine shop to get the deck milled slightly. I want to get back to the stock compression ratio (8.2:1). I did some valve de-shrouding and some edging on the sharp transitions to the squish pads. I know i took out enough material to effect the compression ratio. What I need to know is how exactly do i calculate the compression ratio. Is it stroke volume divided by combustion chamber volume?

Also, does anyone know what the stock CC's is for the piston dish and the combustion chamber for the 85 G-head? I thought i read the combustion chamber was 9cc. I know the recess is 2.2" wide x 0.190" deep. But I don't know how many cc's that would be. Anyone know??

[acannell]

9 CC??? tell me this isnt so somebody! surfacing my block and head 0.030" is equivalent to a 4cc decrease in combustion chamber , which would reduce it by almost 50%!

isnt the combustion chamber more like 55cc?

[speeduphoria]

its more like 48cc's

[Tbone]

I have read that the swirl head was 48cc. Since the G-head has more volume (I'm guessing that's were the 9 cc came from) it has to be more than 48 cc. My guess is 57 cc (48+9) but I'm not 100% sure. That still doesn't tell me what the volume is in the piston dish, though. Anyone know the answer to this?

This came from Donovan's site:

Year Head Volume (approximate) Recess Compression
2.2 1984-1985 9cc 2.2" wide x .190" deep 8.2
2.2 1986-1990 14.5cc 2.6" wide x .190" deep 8.2
2.5 1989-1992 unknown 2.8" wide x .280" deep 7.8

OR, maybe the 9cc is the piston dish volume?

[Tbone]

OK - i did some calculations... I figured that the piston dish is 11.83cc. Assuming the combustion chamber is 56.5cc (got that from an engine compression calculator that I'm using), the compression comes out to be 8.096:1. That is very close to 8.1:1. According to Donovan, the compression at stock is 8.2:1. Anyone know for sure?

Also, this is without the head gasket figured in. Is that supposed to be figured in? If I assume it is 0.040" thick, then it changes to 7.43:1. That is definately not right. If it does need to be figured in, what thickness should I use?

Thanks!

[Tbone]

One more thing I tried. Used 48 cc for combustion chamber with 11.83cc for piston dish. Then plugged in 0.050" for head gasket. I got 8.1998:1 compression ratio. That sounds right on for an 8.2:1 compression ratio. Just don't know which calc. is correct. Can someone shed some light on this for me?

[BobsBlackACR]

Seems like you are beating yourself up trying to figure things out and guess at what the cc currently is.

You said that you are planning to have the head decked anyway so why not just CC them yourself (actually measure the cc's not speculate) after the machinework to get your comp ratio 100% accurate? Or maybe see if the machine shop can do it for you?

[JamesL]

Lots of info on the net.
Calculating Compression Ratio
Engine Compression Ratio (CR) Calculator

[speeduphoria]

yeah 52cc is the max I would guess for the head CC's. Those 9cc numbers look like dish volume to me. And Yes you need to figure head gasket thickness in because it adds volume.

[puppet]

Stock gasket adds 11cc .... FWIW.

[speeduphoria]

Yep the book says .068" gasket which is ~11.0cc's. in my calculator its more like .070-.071"=11cc's

[puppet]

MP states the typical thickness @ .068"

[acannell]

i feel bad about this, but my mopar performance 4th edition book has detailed tables listing depth of dish, approximate compression ratio, compression height, deck height, gasket thickness, and head volume for pretty much every year for the 2.2, the tables are on pages 83 through 88.

i wish i had a way to scan it, i'd put it up.

if you have any specific questions about the pistons id be happy to look it up and post the answer here

[Tbone]

Thanks for the info guys. Here is my plan and why I need the head and piston dish volume separately. I removed a decent amount of material from around the valve seats. I still want a stock compression ratio, whatever that is (8.1 or 8.2). So, I want to cc the head, add that to the piston dish volume, add the gasket thickness (0.068") and see what the new compression ratio is (using a compression ratio calculator). Once I have that number, I can plug in different values for the head milling and that will bring my compression ratio back up to what it needs to be. Then I can tell the machine shop how much to mill off the head.

So, (acannell) the info I still need is:
1. Head volume for a G head
2. Piston dish volume ( I calculated 11.83 cc)
3. Stock compression ratio for an 85, 2.2L, T1 (either 8.1 or 8.2)

Also, I understand the head gasket makes a huge difference on compression ratio, but is it part of the formula to get the stock compression ratio of 8.1 (or 8.2)?
Thanks!

[speeduphoria]

The main question is Why? lower CR and more boost will make more HP on pump gas.


Anyway this is from my 3rd edition book:

milling the head .0105" is 1cc decrease
for 85T1 it says 8.10CR, 40.50mm compression height for pistons, 4.84mm dish depth which is 8cc's