# Compression ratio help: VR6 content



## GinsterMan98 (May 19, 2008)

There was a recent thread that raised the question of milling piston dishes to achieve a lower compression ratio. My first question is has anyone here done this to OEM 81mm bore VR6 12v pistons?
If so, than I need some help with the math to figure out the amount of material needed to be removed.
From what I understand this is how you calculate the compression ratio of your engine. 

You need to know the following info for your engine.
1. Bore= 81.0mm or 3.189 in, To the second power= 10.170 inches
2. Stroke= 90mm or 3.543 in
3. Piston dish volume= 25cc or 1.525 cu in, but I am unsure of this value
4. Head gasket volume= unknown
5. Head chamber volume= 0 I would think due to the flat cylinder head.
6. Piston deck height= unknown for the VR6

The next step is the math.
1. Cylinder sweep volume= .7853982 X Bore2 X Stroke in inches, I got 28.300
2. Clearence volume= .7853982 X Bore2 X Piston deck height
3. Piston Dish volume= cc to cu in= 25 X .0610237 witch equals 1.525
4. Chamber volume= 0

Add 1-4 together = unknown
Then divided by the sum of 5-8

5. Clearence volume
6. Piston volume= 1.525
7. Gasket volume= unknown
8. Chamber volume= 0

If someone could help me fill in the blanks I would be very greatful


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## GinsterMan98 (May 19, 2008)

To expand alittle, I need to know the bore diameter of a stock VR6 headgasket. I also need to know the compressed thickness of the gasket to account for its volume. I also am confused about the piston deck height as the VR6 piston is not at a 90 deg angle to the deck surface. I also need to verify the stock pistons dish volume, I have read on another site its 25cc. I am not in the states right now or I would have check one of my spare blocks, or had a machine shop check it. I know someone has this info on here. Thanks to all that chime in.


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## GinsterMan98 (May 19, 2008)

Bump


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## GinsterMan98 (May 19, 2008)

Look, In am just an aircraft mechanic, not an engineer. I do not see the diffrence if I shave a few tenthousands off a piston to run more boost vs a headspacer or forged pistons. I know people have run high boost on a stock cr VR6, so lowering the Cr will balance out and meaterial I take off wouldn't it? I am in Iraq right now and do not have an engine to look at right now so someone tell me I am wrong or help me out please. I already got in contact with a machine shop that said they can either make the dish wider or deepen it or both.


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## rjones1214 (Apr 24, 2009)

From the VR pistons I saw in my thread, I would say widen the dish of the piston. 
How much did the machine shop say they would charge?


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## GinsterMan98 (May 19, 2008)

75 or so, but I know them real well. They have been my machineshop for some time now and he always likes new projects from import guys. He mainly does 350's and stuff like that.


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## GinsterMan98 (May 19, 2008)

bump


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## Rheinland Technik (Apr 2, 2010)

You do not have to machine the dish in the VR6 pistons any deeper. You should rather machine them larger in diameter to reduce your compression. If you are using a Mk3 head gasket, you will have to remove 7cc's of material to achieve 9:1 compression.


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## EL DRIFTO (Jul 11, 2004)

Rheinland Technik said:


> You should rather machine them larger in diameter to reduce your compression.


 i like this answer, i was considering all vw did to get the CC right in the slanted vr & how to keep that right


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## GinsterMan98 (May 19, 2008)

Would that be 1.17cc's per cylinder or 7 cc's per cylinder. Thanks for the reply, but how did you get these figures if you don't mind me asking?


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## GinsterMan98 (May 19, 2008)

> i like this answer, i was considering all vw did to get the CC right in the slanted vr & how to keep that right


 Thats what I was thinking, but I have no experience with this kind of work. I ws only suggesting possibilities.


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## GinsterMan98 (May 19, 2008)

Bump for more info.


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## Rheinland Technik (Apr 2, 2010)

That would be 7 cc's per cylinder.


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## GinsterMan98 (May 19, 2008)

Trade secret? how did you get this value? I have never tried to calculate CR before and would like to know how its done pretaining to a VR6.


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## GinsterMan98 (May 19, 2008)

Nevermind, After asking a few people in the know, its a bad idea.


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## rjones1214 (Apr 24, 2009)

what reasons made this a bad idea?


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## GinsterMan98 (May 19, 2008)

The reason its not such a good idea is it will reduce the mass of the piston. This mass helps cool the piston and the cylinder temps. This cooling helps prevent knock. Headspacer will not reduce the ability of the pistons cooling potential. Cost is also about the same when done. Not going this route.


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