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Home > 998cc > 998 volume and CR

andy120

4 Posts
Member #: 2270
Junior Member

15th Feb, 2008 at 01:53:02am

Guys
I have a 1286 head with 24.5cc combustion chamber ans dished pistons, my measurements put the CR around the 10:1. Without using decompression sets how do you get the CR down towards the 8:1 mark? Or have i missed something?

Many thanks
Andy
NZ

BENROSS

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9812 Posts
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Resident Cylinder Head Modifier

Mitsi Evo 7, 911, Cossie. & all the chavs ...... won no problem

15th Feb, 2008 at 02:00:20am

you can open the chambers up some what
making sure you dont go out of the gasket area

wil_h

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Betwix Harrogate and York

15th Feb, 2008 at 08:16:39am

Well my calculations show a CR of 8.5.

That's with 998 cc, 24.5 cc chambers and 6cc dish (plus 2.5 cc in the gasket)

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

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the design shows a distinct lack of imagination,
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I'm mainly concerned about swirl in the runners caused by the tangential entry.


Rob H

4314 Posts
Member #: 700
Formerly British Open Classic

The West Country

15th Feb, 2008 at 12:44:23pm

I get almost spot on 10:1 with flat top pistons.

EDIT: That included 0.75 for ring lan

Options are:

Dish pistons (either machine the pisons or replace them, depends what they are to start with)
Open up the chambers in the head (or change the head depending on what the head is to start with)
Bodge it (twin head gaskets / decomp plate)

Edited by Rob H on 15th Feb, 2008.

Isambard Kingdom Brunel said:
Nothing is impossible if you are an Engineer

andy120

4 Posts
Member #: 2270
Junior Member

15th Feb, 2008 at 07:34:09pm

Heres my math.

bore dish comb c Gask
249.5 6 24.5 2.50 282.5 cc
24.5 2.5 27 cc


actual CR 10.46

something screwey here
Andy

da
forgot to add piston dish to the combustion chamber size.

Edited by andy120 on 15th Feb, 2008.


Paul S

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8604 Posts
Member #: 573
Formerly Axel

Podland

15th Feb, 2008 at 07:39:30pm

No,

Total chamber volume is 33cc.

Total chamber plus swept volume is 282.5cc

The CR is 282.5/33 = 8.56

You are not accounting for the 6cc in the pistons.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."
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