Ok to make a long story short I am trying to figure out the compression ratio on the engine I just took apart. I went to a few different web sites and used the calculators and even did it the hard way by using the formulas for volumes (ie..head,pistons,gasket and such). The problem I am running in to is that If I figure the compression by hand the long way I don't get anywhere near what the calculators say. Could someone explain what I am doing wrong.
Heads 119 cc oval ports
pistons KB203 2.17cc dome/.133"
head gasket .039" compressed volume 7.17cc
piston to deck clearance .009
4.31 bore
4.0 stroke
Please help, this is driving me crazy.

Dustynn

Dustynn, I get 8.59 to 1. (cyl vol+combust chamber vol+gasket volume+deck height vol-piston dome volume)/(combust chamber vol+gasket volume+deck height vol-piston dome volume)

Hope this helps.

Tom

I get 8.58:1 by hand and by using a compression ratio calculator.

Chamber volume = head volume (119cc) + head gasket volume (7.17cc) = 126.17cc

cylinder area = pi (aprox 3.14) * diameter (4.31) = 14.59

cylinder volume @ TDC = cylinder area * deck height (.009) - dome volume (2.17) = -.02cc

cylinder volume @ BDC = cylinder area * deck height + stroke (4.009) - dome volume (2.17) = 956.32cc

total volume at TDC = -.02cc (cylinder) + 126.17 (chamber) = 126.15cc

total volume at BDC = 956.32cc (cylinder) + 126.17cc (chamber) = 1082.49cc

CR = 1082.49cc / 126.15cc = 8.58:1

Casey, You beat me before I could correct my post. I too did it by hand and got 8.59 to 1.

Tom

Ok, so I am getting the same comp as you are but the catalog advertises the comp ratio for kb203's as 9.1 with a standard bore. With the increased bor the compression should also increase correct?
If so then where is the missing link? I am having to rebuild my 468 because a lifter lost a retainer and trashed the crank and bearings. After I got the engine down I got an interest into building and blueprinting the next engine so I used the old numbers as a test. I thought my compression ratio was around 9.5 to one with this setup but the calculator is telling me different.
049 oval port heads 119cc
kb203 pistons 2.17cc dome
.039 head gasket
.009 piston to deck height
I get around 8.58to1 compression but that catalog says this should give me 9.1.
This is my problem...If I order a set of pistons that advertise 10to1 compression what am I going to get????

Originally posted by TX-Gearhead:
Ok to make a long story short I am trying to figure out the compression ratio on the engine I just took apart. I went to a few different web sites and used the calculators and even did it the hard way by using the formulas for volumes (ie..head,pistons,gasket and such). The problem I am running in to is that If I figure the compression by hand the long way I don't get anywhere near what the calculators say. Could someone explain what I am doing wrong.
Heads 119 cc oval ports
pistons KB203 2.17cc dome/.133"
head gasket .039" compressed volume 7.17cc
piston to deck clearance .009
4.31 bore
4.0 stroke
Please help, this is driving me crazy.

Dustynn -
First, a 4.310 x .039 gasket will be 9.33cc's
As I recall that .133 dome should be 12cc's & if so your compression comes out to 9.04
If it's 2.17 it's 8.45
The compression ratio program I use takes into account piston top diameter & also how far down the rings are so it will be a bit different than some of the online stuff.

The number I get for converting to cc's is 16.387
If you multiply .133x16.87 you get 2.17. Now is this number already cubed or do you have to cube the number. I thought the .133 was cubic inches which would mean it is a straight conversion. I thought of this and 2.17 cubed is 10.21 which gets me back to the advertised compression ratio but on the kb site there is a compression calculator that shows 2.17cc=.133". I feel like I have opened pandora's box on myself here :confused:

.133" is cubic inches and the 2.17cc is cubic centimeters. To convert a cubic measurement (volume) into a square measurement (square) or vice versa you need a 3rd element (usually height).

Dustynn,
You cannot use that formula for a dome. All they are giving you is the height of the dome & you need to know how many cc's that is either by measuring or from KB & assume they are right.

I think Mike is right and it's just a coincidence that .133 * 16.387 = approx 2.17.

Ok so maybe I am crazy but I don't understand why you can't convert directly from cubic inches to cubic centimeters. Disregarding and engine and pistons it seems they would carry the same volume regardless of shape, or am I trying to simplify it a little to much. I went to a website that converts from ci to cc and i got the same .133ci=2.17cc.

Dustynn

ok,,,so i was wrong...I got to looking in a Summit magazine and they give cc's for the speedpro pistons. Although I run the kb's there was enough info to determine that the kb203's have alot more cc's than 2.17, I figure it would be closer to 15.......

KB uses zero deck clearance in their calculations. You have .009. That is why they show a higher compression ratio than what you are coming up with.

KB uses a .040" quench for their CR estimates.

Try it again, but this time use .022" for a set of cheap steel shim headgaskets!!

Should hopefully pick up say 3/10th's a point???

pdq67

Originally posted by TX-Gearhead:
Ok so maybe I am crazy but I don't understand why you can't convert directly from cubic inches to cubic centimeters. Disregarding and engine and pistons it seems they would carry the same volume regardless of shape, or am I trying to simplify it a little to much. I went to a website that converts from ci to cc and i got the same .133ci=2.17cc.

Dustynn Dustynn,
You are right, you can convert directly from ci to cc but what the .133" number they are giving you is, is just the height of the dome not the number of cubic inches ;)

If it were cubic inches your calculation would be correct.