Working Out My Compression Ratio After Head Skim

[Sinbad 16]

I have just bought a used cylinder head to use in my 1.6 gti engine rebuild,i took it for a skim as there was some pitting on the head face.

The engineers said it had been skimmed before and the pitting was quite bad and may need welding then skimming or a big skim and see how it goes.

Anyway they took quite a bit off and the pitting went.

Im wondering what the new CR will be,im having trouble working it out as the calculations involve the head gasket as part of the clearance volume.

I have the cylinder head chamber CC sizes what i measured earlier,what are they as standard?

Will i need a vernier to retard the timing?

Thanks.

[Cameron 16]

If you have the combustion chamber volume then that's the hardest part done!

 

What you need to do next is work out the rest of your volumes:

 

Step 1a: (3.142 * Bore^2) / 4 = Piston Area

Step 1b: Piston Area * Stroke = Swept Volume

 

Step 2: Piston Volume = measured cc's of any piston dish or cut-outs

 

Step 3: Gasket Thickness * Piston Area = Gasket Volume (approximate, but close enough)

 

Then you work out the ratio of the total internal volume to the comressed volume i.e compression ratio.

 

Step 4: Swept Volume + Piston Volume + Combusion Chamber Volume + Gasket Volume = Total Volume

 

Step 5: Piston Volume + Combustion Chamber Volume + Gasket Volume = Compressed Volume

 

Step 6: Total Volume / Compressed Volume = Compression Ratio

 

 

 

 

So for example..

 

Stroke = 73mm, Bore = 83mm (divide by 10 for cm)

 

Step 1a: Piston Area = (3.142 * 8.3^2) / 4

= 54.1cm^2

 

Step 1b: Swept Volume = 54.1 * 7.3

= 395cm^3

 

(On a side note, this is your cylinder displacement - multiply this by number of cylinders gives your engine displacement so 395 * 4 = 1580cc)

 

Step 2: Piston Volume measured to be = 6cm^3

 

Step 3: Gasket Volume = 54.1 * 0.2 (say it's 2mm thick)

= 10.8cm^3

 

Then say you measured your Combustion Chamber to be 28cm^3..

 

Step 4: Total Volume = 395 + 6 + 28 + 10.8

= 439.8cm^3

 

Step 5: Compressed Volume = 6 + 28 + 10.8

= 44.8cm^3

 

Step 6: Compression Ratio = 439.8 / 44.8

= 9.8 : 1

 

Obviously you need to substitute your values for my made up ones!

 

Got it?

 

Good.

Edited March 21, 2012 by Cameron

[Sinbad 16]

Thanks very much for taking the time to type that out.

[Cameron 16]

No problem, hope it helps. It makes sense to me, could be massively confusing for all I know.

[Sinbad 16]

Well i have just worked it out using that and it came to less than it was!

My brain is fizzing lol

The CC of the chambers is 32 all 4 are.

I got to 9:09:1 Fail!! lol

[Cameron 16]

What's the thickness of the gasket and the piston dish / cut-out volume?

[Sinbad 16]

Piston dish is 5cc and gasket thickness is 1.5mm

[Cameron 16]

So your total volume is 395 + 5 + 8.1 + 32 = 440.1

 

And compressed volume is 5 + 8.1 + 32 = 45.1

 

So CR is 440.1 / 45.1 = 9.76 : 1 which sounds about right.

Edited March 21, 2012 by Cameron

[Sinbad 16]

But i thought it was 9.8:1 standard? At least thats what i read they were??

[Cameron 16]

Guess they didn't skim it by much then!

 

Are you measuring a new head gasket, or an old one? As that will make a small difference.

[Sinbad 16]

New gasket

[welshpug 1,657]

1.4mm is the installed height (or 1.6 for o/s)

 

 

doh, that's xu7, o/s xu5/9 is 1.5

Edited March 21, 2012 by welshpug