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Old 05-19-2009, 02:32 AM   #1
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How To figure the compression ratio of your Zetec

I always have trouble trying to find this info and I'm not the best with wanting to do the math. I found a good write up on a different site that I'm working off to give us good info. Here goes ...



The advertised static compression ratio for the US spec Zetec is 9.6:1, note the US spec and that we are not talking about the SVT version of the engine.

Typically to find the compression ratio you'd use this formula: but we don't know all the required information to make that formula work so we'll have to work it backwards. (we don't know the combustion chamber volume)
b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket)

We know the total engine volume is 1998 Cubic Centimeters but we only need the volume of one cylinder to complete the formula. So we divide 1998 by 4, 1998 / 4 = 499.5 Cubic Centimeters.

Now to find the Combustion Chamber volume simply divide the total chamber volume by the compression ratio, 499.5 / 9.6 = 52.03125 Cubic Centimeters.

Now remember this is a total chamber volume. That is with any relief in the piston and including the head gasket. We can remove the volume of the head gasket because we know what it is.
The standard US spec head gasket thickness for the Zetec is 0.6858 mm thick with a bore of (about) 84.8 mm. That means using the formula for the volume of a cylinder we can figure the volume and eliminate it.

Pi (3.14159 for all practicality) times the radius (half the diameter of 84.8 or 42.4 mm) squared times the height (which is our 0.6858 mm).
3.14159 X 4.24^2 X 0.06858 = 3.873281546 cubic centimeters

Now we can subtract our head gasket volume from our total volume and are left with the cylinder head volume (roughly), 52.03125 - 3.873281546 = 48.157968454 cubic centimeters.




Now lets run through the SVT version for kicks.



The SVT has a static compression ratio of 10.2:1 and uses a head gasket that is 1.0414 millimeters thick,

1998 / 4 = 499.5
499.5 / 10.2 = 48.970588235
3.14159 X 4.24^2 X 0.10414 = 5.8262884378
48.970588235 - 5.8262884378 = 43.144299798


So now we've found out where the extra compression for the SVT comes from. Now it's time to play with some more numbers and make a chart for gasket swapping and milling the head, but remember these are merely approximations and may not be actual, use caution.


Standard Zetec Raise Compression:

shaving the head (in cm),

standard head gasket (.027" or 0.6858mm)


0.0254 --- 50.596702491 --- 9.872184854
0.0508 --- 49.162154982 --- 10.16025437
0.0762 --- 47.727607473 --- 10.465640883
0.1016 --- 46.293059964 --- 10.789954269
0.1270 --- 44.858512455 --- 11.135010339




SVT Head Raise Compression:

shaving the head (in cm),

standard SVT head gasket (.041" or 1.0414mm),


0.0254 --- 47.536040727 --- 10.507816645
0.0508 --- 46.101493218 --- 10.834790050
0.0762 --- 44.666945709 --- 11.182765960
0.1016 --- 43.232398200 --- 11.553835105
0.1270 --- 41.797850691 --- 11.950375240




with just using the .027" (standard Zetec) head gasket

0.06858 --- 47.017581344 --- 10.62368556


with Zetec head gasket (.027" or 0.6858mm)


0.0254 --- 45.583033835 --- 10.958024466
0.0508 --- 44.148486326 --- 11.314091186
0.0762 --- 42.713938817 --- 11.694074905
0.1016 --- 41.279391308 --- 12.100469124
0.1270 --- 39.844843799 --- 12.536126444








*Please be advised these are only approximations*




I figured the compression ratios by taking the displacement divided by 4 divided by the chamber size. Not sure if that is correct but I'd guess it will get you close.
Also being no one has ever CC'd the head while shaving it everything is just a guess.

Open to criticism and/ or a better chart/method.


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Last edited by iminhell; 05-22-2009 at 09:20 PM.
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Old 05-19-2009, 09:07 AM   #2
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Even though its a approximation, I need some kind of ideal of my compression ratio in my Zetec built short block w/ 9:1 pistons, SVT head. Nice work.
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Old 05-19-2009, 02:05 PM   #3
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great info!!!, but i'm not sure its totally right, only reason is i was told my compression was 10.7:1 on a head shaved .060". Not saying ur wrong by any means, more than likely, my tuner is wrong, but it seems logical!
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Old 05-19-2009, 10:14 PM   #4
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Old 05-19-2009, 11:10 PM   #5
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The biggest factor here is the size of the combustion chamber... the SVT has a smaller CC and thats why it has a higher CR.
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Old 05-20-2009, 07:30 AM   #6
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Im curious if that math would work the same once pistons , deck height , valve job is done etc , I would think you would need to know CC`s of head , CC`s of deck height , CC`s of pistons

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Old 05-20-2009, 03:19 PM   #7
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^^You would. From the way these numbers are figured all we know is the total combustion chamber volume, that is head, head gasket and piston dish volume combined. If you'd CC the head you could eliminate it and would be left with the volume of the piston alone. Then and only then you could figure in actual compression from a piston change.
BTW, that Performance Hookup vendor has CC specs for all the pistons listed on their site.
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Old 05-20-2009, 05:51 PM   #8
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looks good, but it looks like some of your numbers in mm are actually in cm. .040" is about 1.016mm. not 0.1016mm.
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Old 05-22-2009, 09:24 PM   #9
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Quote:
Originally Posted by illinipo View Post
looks good, but it looks like some of your numbers in mm are actually in cm. .040" is about 1.016mm. not 0.1016mm.


Heading to work after I posded it dawned on me what you meant and where I went wrong. I fixed it, I hope .



I'll move it to the Zetec specific stickie at the top eventually. I was just waiting for someone to verify my work. Once we agree that it is close enough to say it's correct I'll move it.
I'll also label the columns as to what they mean, just don't have time right this minnute.
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Old 05-23-2009, 08:13 AM   #10
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I might have to pick up a junk head and see how much my machine shop would charge to cut 0.010" slices and cc the head after each slice. Take it out to 0.060"s, so we could have some idea. Its only ~$20 to get it milled, but I'm sure that would be per cut and ccing would be extra. I had him cut 0.020" on the head I'm having done.

Your calculations assume that 0.010" takes out a completely cylindrical section of combustion chamber, which may not necessarily be true (once you get past the first couple thousandths), but its definitely a good approximation. I ran through the math for my engine and will end up with around 11.3:1. That's with 10.6:1 pistons and 0.020" milled off the head. That's actually pretty close to what I was expecting it to be. Like Tom (and you) said, you really need to know the cc of the head.

Also as an additional note, its not necessarily clear in your post that to calculate the final compression ratio, you don't want to subtract the head gasket volume from the calculated combustion chamber volume. I kept subtracting head gasket volume and was at like 13.4:1. I knew that wasn't right.
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