Aftermarket pistons and compression ratio

[sandman1111]

Hey

I have been thinking here, and haven´t found any information on my "problem". I got an ACE engine, with 10.8 compression, but people have been measuring, and have said, that it is actually 10.5. So my concern is, what kind of compression I end up If I buy Wössner 8.0:1 pistons, is it 8.0:1 or is it 7.7:1 ? If you look at the Wiseco 9.0:1 pistons, it says -11cc, that would actually make 8.7:1?
Hope someone can enlighten me!

Wiseco -11cc (supposed to be 9.0:1) And calculating with engine/wiseco specs., I get 8.56:1 -compression ratio.

So what is right?

I have done some calculations with given specs on JE, Wiseco, Wössner, and come up with 0.3-0.4 lower compression.

Thanks

[diesel des]

I can confirm that the compression ratio on the wossners in a 20vt are lower than advertised.

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[Error404]

I calc'd my Wossners to be close to 8.5:1, box spec is 8.0:1

Post up the figures you are using for your calculation

[gnutz2]

Years ago I calculated the head was about 45cc and the gasket is about 8.2cc, you can work out the rest from there.

http://www.s2forum.com/forum/showthread.php?t=25254&highlight=Calculating+compression+ratio

[diesel des]

I calc'd my Wossners to be close to 8.5:1, box spec is 8.0:1

Post up the figures you are using for your calculation

The pistons i used were for a 9a turbo. But they definitely were about 0.25 less than the advertised figure.

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[sandman1111]

Okey I´ll post some figures.

Lets take Wiseco -11 for calculating.

Wiseco specs:

First number in inch, second mm

Bore Size: 3.287 / 83.5
Oversizes: .040
Stroke: 3.654 / 92.8
Rod Length: 5.669 / 144
Comp Height: 1.172 / 29.77
Head cc's: 46, should be about right, some claim 45.5 is the correct cc.
Dome Dish: -11cc
Gasket Thickness: 0.065 / 1.650 correct
Deck Clearance: 0.007+ / 0.180+
Block Height: 8.661 / 220
Comp Ratio: 9.0

Only spec I don´t know is head gasket bore diameter - I use 84

I´m using this calculator - http://www.csgnetwork.com/compcalc.html

And it tells me it is not 9.0, it is 8.57:1

So assuming, my engines deck clearance and head cc are about the same, I should end up with the calculated compression ratio. Ofc there is always a chance, that the calculator is "broken".

[gnutz2]

Your calculations are correct but what head gasket are you using?

Stock mls gasket is 82mm diameter but with only .18mm clearance between piston and deck the pistons would touch with gasket.

[sandman1111]

Well this is calculated for future project, so I don´t have a correct head gasket yet . I thought the OEM is 83mm, I got a few, I should measure. And +0.180, means that it is below deck.

Edit: Maybe Wiseco +0.180 means above? After recalculating, I get 8.8:1 compression ratio, with Wiseco -11cc.

[gnutz2]

Every gasket I've checked is 82mm.

I know + means below deck but it leaves very little clearance for expansion of piston and bearing clearance, also remember that the pistons will rock at tdc.

[Error404]

Baz I calc'd mine with proper measuring gear and a burette etc and got pretty much the same CCs as you give or take 1%

I posted an excel CR calculator up on the forum:

http://www.s2forum.com/forum/showthread.php?t=66589&highlight=CR+calculator

[sandman1111]

I measured my Victor Reinz head gasket, and it is 83.5mm, so its fine for 83.5 pistons. Part nr. 61-29305-00

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[Error404]

I would go 0.5mm less to allow for expansion & some carbon buildup etc.

[sandman1111]

I have to bore my block anyway, so I´m sure the guys can bore my gasket also

[gnutz2]

Sorry for the confusion, I'm talking about an i5 mls gasket.

I've bored mls gaskets beforehand and it didn't seal properly when on boost, my problem was the centre layer which is folded back to create a seal when the head is clamped, once the gasket was bored this would drop out or move. I suppose it could have worked had I been very careful handling the gasket after the machining but I think 83.5mm will work fine for you.