Modifying the Head Gasket

[Doug_RS2]

There has been discussions on how to modify the metal laminated head gasket. The idea is that

a) it needs to be thicker because a head has been taken off and thus machined flat - taking 0.002" or 0.003" off - Is that about right?

b) a lower compression ratio means you can run more boost.

The RS2 (metal laminated) Head Gasket is made up of four layers.

The top and bottom layers are 0.008" thick and also have a pressing in them round the cylinders, coolant and oil holes.

There is a shiny metal layer below the top layer - this is 0.004" thick.

Then you get the centre section that is 0.040" thick - now this looks as if it is 2 layers but bonded together.

The 4 seperate layers are not bonded together - they have brass cheapo rivets holding them together. I was also surprised (although I have never seen headgaskets before, so what do I know!) that big holes for the cylinders don't have one of the layers edges rolled so that there aren't multiple edges in the cylinder - Am I making Sense?

Anyway - How do I modify this (I am getting a second headgakset) so that I have lower compression? I was originally going to put 2 headgaskets on but this seemingly eventually blows. I don't want to get into getting a new centre one laser cut!

And can anyone do some sums to tell me what difference in compression ratios I would see with diffrent layers int he headgasket?

[Doug_RS2]

The metal head gasket:

[Doug_RS2]

And a pic showing the layers:

What should I do with my 2 headgaskets?

[Pisobiker]

Doug,

to calculate your real "geometrical" CR you need to find out exact volume of combustion chamber when piston is at TDC (thus including head gasket thickness).
This isn't that easy so I think for a most practical use you could do another way:

Being:

CV=Cyilinder volume in cc (for our 81mm bore x 86.5 mm stroke this is 445.5 cc)
CCV=Combustion chamber volume in cc (including stock headgasket thickness)
CR=Compression Ratio

then if you feel the need to lower your compression ratio lets say of 1 point, it doesn't really matter that much if you actually are going from 9.3:1 to 8.3:1 or from 9.5:1 to 8.5:1.
So lets say we trust Audi and our engine's CR is 9.3:1 then:

CR= (CV+CCV) / CCV

for the original gasket setup this means CCV=53.68 cc

using a thicker head gasket and lets call the relative increase in volume "HGV" (which is the bore surface multiplied for gasket's thickness difference), then
new compression ratio will be:

CR=(CV+CCV+HGV) / (CCV+HGV)


Lets make two examples:

1) New head gasket 0.5mm thicker:

CV=445.5
CCV=53.68
HGV=2.575

=> new CR=8.92 (you lowered your actual CR of about 0.38 points)


2) New head gasket 1.0mm thicker:

CV=445.5
CCV=53.68
HGV=5.15

=> new CR=8.57 (you lowered your actual CR of about 0.73 points)




Marco

[Stick Insect]

I started a new thread instead of replying! here it is in the right section


A local company who build Formula Ford engines use silver paint on their head gaskets when building them.

Do you think that the paint thickness would help lower the compression ratio?

I've got a Mini 8 port head which has 9.7cc combustion chambers which makes the static compression ratio about 13:1 which is a bit high for the road. I have been looking at thicker head gaskets to lower th CR. There is a company who make head gaskets for Triumph Dolomite Sprints which are thicker to correct overskimming. They may be able to make a thicker head gasket for this project.

I have heard that thicker gaskets are available for Rover K' series and the Vauxhall 16V heads.

I think they are around Huddersfield.

I don't know any more info about the supplier I'm afraid.

[Mihnea]

Static compression ratio with a metal head gasket is 9.0:1 instead of 9.3:1 with a paper head gasket...


HTH,


Mihnea

[Pisobiker]

Originally posted by Mihnea
Static compression ratio with a metal head gasket is 9.0:1 instead of 9.3:1 with a paper head gasket...


HTH,


Mihnea

Ah, I see....

then numbers are:

CV=445.5
CCV=55.69

Which means:

1) 0.5mm thicker gasket: CR=8.646

2) 1.0mm thicker gasket: CR=8.323



Marco

[desmo888]

Originally posted by Mihnea
Static compression ratio with a metal head gasket is 9.0:1 instead of 9.3:1 with a paper head gasket...


HTH,


Mihnea

So the metal is thinner? i was going to change from paper to metal on my 3B. Should I rethink that? I have no interest in playing with stacked gaskets (although the unbonded layers make it easy enough).

BTW Doug - thanks for the nice headgasket pictures...

[Pisobiker]

Dave, matel one is thicker.

Lower CR=bigger chamber volume.


Marco

[The Spanish]

Marco and Minhea or anyone else,

For my setup with the RS2 turbo + injectors + Exhaust Manifold should I get the Metal headgasket?
I thought they were the same on the ABY and ADU
I want to have the safest setup for the engine so the Metal Headgasket would be better no?

Please enlight me,

Rafael

[Pisobiker]

Rafael, metal head gasket is recommended for high boost applications.
Mihnea could tell if his 360-370 chip setup will be fine with a paper one.

It seems early ABYs did still have the paper headgasket. Now that you remind it I'll try to figure out if mine is metal or not.
I might be able to see without unbolting anything.


Marco

[Audi Parts Guru]

Guys, the paper gasket is no longer available, it has been replaced by the metal one

[desmo888]

Originally posted by Pisobiker
Dave, matel one is thicker.

Lower CR=bigger chamber volume.


Marco

DOH! See what I get for playing on the internet at work! :wacked:

Somebody get me a pint...

[Doug_RS2]

I'm not getting much confidence in putting a modified head gasket on(too may people saying NO), so I will stick to having just the one head gasket and wait till I builld another engine before I get a custom one.

I was going to have 0.008" + 0.004" +0.04" +0.008" (from 2nd HG) +0.040" (from 2nd HG) +0.008" This way there is a compression layer between the 2 thick steel layers

This would make it 0.048 thicker - about 1.2mm - lowering compression ratio by 1 point (ish) to 8.0:1 (I think)

[Mihnea]

Doug, AFAIK quite a few people do that in Germany and I also figured that what I was trying to translate for you is exactly what you just said. I guess having a gasket in front of you helps...

It's all up to you actually, if you don't go often to 6+k RPMs in 6th gear, then it's not so recommended to drop the CR...