Zetec Compression Ratio Calculations

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Davespages.co.uk will not accept liability for any loss or damage caused by deviation from the manufacturers configuration. The content below is gathered from my own investigations/calculations and must be used as a GUIDE ONLY I cannot guarantee its accuracy.


Firstly you need to be able to distinguish between STATIC and DYNAMIC compression.

Static compression

This is your stated COMPRESSION RATIO. This involves one of two calculations, the more complicated using the bore, stroke, head gasket volume, total cylinder volume, pi and the combustion chamber volume to give the compression ratio, the other ignoring the stroke IF you know the compression ratio for a standard version of your engine.
Most cars run between 9-10:1 ratio... this means the air/fuel mixture is squashed to one ninth or one tenth its original volume.


Dynamic Compression

This is the readout you get when you do a "compression test" by screwing a pressure tester into the spark plug's hole in the head.

This reading is effected by many things,
- STATIC compression, the higher the static compression, the larger the effect it has on your dynamic compression.
- Engine condition... are the valves seating (sealing correctly) are the compression rings around the pistons sealing properly, is the head gasket still in tact, is the head free from hairline cracks? All these effect how well the air/fuel mixture when compressed is retained in the combustion chamber ready for the ignition stroke (spark) If the valves do not seat/seal correctly, this means the mixture can escape... same with the compression rings around the pistons.. or excessive wear in the cylinder causing the rings not to seal.. air/fuel mixture can be lost past the rings into the crank case.
- Cam duration. It sounds silly but really it isn't, the duration of the camshaft controls when the inlet and exhaust valves open... some cams have a larger overlap when the inlet valve stays open longer than theoreticly needed due to the benefits of air flow inertia (performance cams) and the exhaust valve opens sooner due to high revving engines throughput of gasses...

The amount of air getting in (past the inlet valves) and being retained in the combustion chamber for the compression and ignition stroke determine the reading you get on the compression tester.

Most engines running a 9-10:1 C/R usually experience between 150-200psi after 6 rotations of the engine... this is dependant on engine condition of course, low readings or inconsistent readings between cylinders (i.e more than 20psi variance) can point to engine wear in the block (cylinder wear, piston rings) or could point to the head (a failed head gasket or incorrectly seated/poorly sealing valves or worse.. a crack in the head)

This guide is not looking into dynamic compression in detail and as such if you have varying readings from a compression test or exceptionally low readings i would suggest checking for engine wear be it compression rings, valves, head gasket etc....

STATIC COMPRESSION RATIO CALCULATION.


Firstly the Complex formulae that determines the C/R without knowing a "standard" C/R figure.


b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket).

The above calculation requires very specific information which the majority of us may not have access to, so for the time being i strongly suggest using the second calculation below...
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For this easier calculation i shall take a typical 2.0 zetec engine from the mondeo.

We know it runs a 10:1 ratio so we should get this value at the end.

1998cc (Total cylinder volume) /4 = 499.5cc (per cylinder)

Then to work out the combustion chamber volume based on a standard engine of 10:1 C/R you...

Divide the cylinder capacity 499.5/10 (the C/R) this gives 49.5cc combustion chamber volume.
49.5cc includes the standard head gasket volume.

To work out the ACTUAL combustion chamber volume of a STANDARD 2.0 head you need to subtract the head gasket volume (volume of space in the metal "fire ring")

Standard FORD head gaskets are 2.0mm, others vary so pay attention to this as it effects the C/R considerably...
To get the gasket volume..

Pi x (radius x radius) x height of head gasket.

The bore is 84.8mm diameter, so half (radius) is 42.4mm

This is therefore 3.1415 x (42.4x42.4) x 0.02 = 112.95 mm cubed (or cubic mm)

112.95/10 = 11.295cmm head gasket volume.

49.5 - 11.295 = 38.205mm cubed ACTUAL combustion chamber volume.

So now you have the Cylinder volume and ACTUAL Combustion chamber volume plus the head gasket volume (38.205 + 11/205 - 49.5) you merely divide the cylinder volume 499.5/49.5 to give the compression ratio.

This gives 10... what we knew already.

However say you have a 20 thou skim (half the permissible amount on a zetec) then you use the the amount taken off the head... subtract this volume from the head gasket and it gives you the new compression ratio.

Ok, so lets work this out again...

Cylinder capacity is 499.5
We know that we have taken off 20thou off the head.. this is 20 thousandths of an inch (0.5mm)

Because we are going to take 0.5mm off the thickness of the head gasket...we will leave the ACTUAL combustion chamber volume the same and work out the total combustion chamber volume by reducing the head gasket thickness by the "20 thou" skim (0.5mm in this case)

So... lets work out the NEW head gasket volume.

3.1415 x (42.4x42.4) x 0.015 = 84.715mm cubed /10 (Standard compression ratio) = 8.4715mm cubed

'Actual' combustion chamber volume 38.205 + head gasket volume 8.4715mm cubed = 46.676mm cubed.

Right, we need to "adjust" the total engine capacity as it has now changed.

For the purpose of this calculation the old gasket volume was 11.295cmm, new gasket 8.4715mm, the difference is 2.8235mm cubed per cylinder.
This means the total capacity of the engine is no longer 1998cc but 1986cc.

Now take the total cylinder volume 1986.7cc /4 = 496.67cc divided by 46.676cc and it will give the new STATIC compression ratio.

496.67/46.676 = 10.64:1 compression ratio.

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Now we have the calculation working. Lets work out what will happen with a 1.8 zetec head (with smaller bore and combustion chamber volumes) onto a 2.0 block.

Firstly we need to work out the combustion chamber volume on a 1.8 zetec.
Following the above calculation...


1796cc /4 = 449 per cylinder.

It has a C/R of 10:1 (later ones had 9.8:1 and this is achieved via thickness of head gasket)

449/10.0 = 44.9cc total combustion chamber volume

Now we work out the ACTUAL combustion chamber volume by subtracting the 1.8 head gasket volume from the total combustion chamber volume.

Pi x (radius x radius) x 0.02 = 3.1415 x (40.3 x 40.3) x 0.02 = 102.04mm cubed.

Divide this by 10 (standard C/R) = 102.04/10 = 10.204mm cubed.

44.9 - 10.204 = 34.695mm cubed and this is the ACTUAL combustion chamber volume of a 1.8 head (EXCLUDING head gasket)

Now we take the 1.8 ACTUAL combustion chamber volume, add the 2.0 head gasket volume (as you CAN NOT use a 1.8 HG on a 2.0 block) and this gives the total combustion chamber volume of a 1.8 head with 2,0 head gasket on a 2.0 block.

34.695 + 11.295 = 45.990mm cubed.

Now we get the cylinder volume of a 2.0 block.

1998cc/4 = 499.5 - 49.5 = 450cc

Add the total combustion chamber of the 1,8 head and 2.0 gasket... = 45.990 + 450cc = 495.990 x 4 = an engine capacity of
1983.96 or rounded up 1984 cc so i have reduced the capacity of my engine by approximately 14cc.

right then... now to get the NEW compression ratio.

495.99 / 45.99 = 10.78:1

This means that a 1.8 head on a 2.0 block is effectively more than a 20 thou skim. (0.5mm)

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Kinda related to the guide but on a personal note... i just had my 1.8 head skimmed 8thou (0.2mm)

So lets put this extra 8thou (0.2mm) into the equation by taking off 0.2mm from the head gasket thickness...

1.8 head and 2.0 head gasket but with 0.2mm less thickness...

This is therefore 3.1415 x (42.4x42.4) x 0.015 = 84.7149mm cubed /10 (standard C/R ratio)

Actual combustion chamber volume of 1.8 head is 34.695mm + new head gasket thickness (incorporating the 0.2mm skim) 8.47149 = 43.16649456 mm cubed

2.0 Cylinder capacity 450cc + new TOTAL combustion chamber capacity inc head gasket 43.1664= 493.164cc

493.164/43.1664 = 11.42:1 ratio.

I wondered why it was running hot, lol.

NOTE: All these calculations assume the head gasket is 2.0 when compressed... it may not be, if its compressed to 1.5mm, adjust the calculation accordingly.

Hopefully this has helped clear up STATIC and DYNAMIC Compression, Proves that the 1.8 and 2.0 heads DO have different combustion chamber capacities and allow you to work out your compression if you have had a head skim.

These calculations can be used on any engine, all you need is the bore, and the standard compression ratio and the total displacement to work it out.

Dave