How To figure the compression ratio of your Zetec

[iminhell]

Originally Posted by illinipo

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.

[guitarseth]

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.

[cossieboy]

Guys now ım totally lost lol , so what would be my best optıons to raıse my compressıon ratıo to atleast 12 :1

[cossieboy]

On my st offcourse

[iminhell]

The first section in blue is what compression should hit by 'shaving the head', that is having a machine shop remove material from mating surface of the head to block.

The Red is the same thing but using the standard zetec head gasket.

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





So, by just using the standard zetec head gasket your compression is raised to 10.6:1.

If you want to go 11.5-12:1 you'll have to shave the head (or buy high compression pistons and connecting rods).
So you'll have to shave from 0.0800-0.1270cm (or 0.0315-0.0500in).

[sdaddy]

Is there a way to determine approximate compression ratio from a static compression reading?

[iminhell]

Dynamic you mean ...

Multiply the gauge reading by 0.0491.
That will give you an approximate dynamic compression rating.

200 - 9.820
180 - 8.838
160 - 7.856
140 - 6.874
120 - 5.892
100 - 4.910

[sdaddy]

Just had my head cc'd today and it's 46. There's a long story....

Looking at the the great info on this thread, with a stock block, pistons and Ford head gasket (.027" or 0.6858mm), if I have .020 taken off the head the compression ratio will be about 10:3:1, right?

[sdaddy]

Originally Posted by sdaddy

Just had my head cc'd today and it's 46. There's a long story....

Looking at the the great info on this thread, with a stock block, pistons and Ford head gasket (.027" or 0.6858mm), if I have .020 taken off the head the compression ratio will be about 10:3:1, right?

Well, I went with .020" off and will report here how the compression ratio ended up - as measured dynamically.

[sdaddy]

Engine is together and running like a scalded cat. Need to finish tuning the Emerald a bit for top end though. Compression gauge ordered.... will report shortly on actual figures.