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M's Factory
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5,240 Posts
Discussion Starter #1
Standard RB25DET

63 cc combustion chamber
413 cc's per cylinder with 86 mm pistons
9 to 1 compression ratio
23 cc for piston dome
6 cc for a 1 mm head gasket

63 - 23 + 6 = 46
413 / 46 = 9 to 1

This should be workable with other engines and also can assist with people requiring to know what compression ratio to work towards when engine building.
 

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258 Posts
so if i got 2mm head gasket for my Rb25 DET to run 1.5bar boost


413/52=7.9 to 1


is that right or am i thick (dont answer that)
 

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M's Factory
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Discussion Starter #3
:) * cant say a word but nods *
 

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M's Factory
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Discussion Starter #5 (Edited)
Do you also want to talk about how far above sea level this affects your boost pressure under your engine under load ?

:)

Ok just to clarify for people the above is a static compression ratio calculator.
 

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If you like mate, all I wanted to know was how do you work out how much to decrease your compression ratio, to enable you to run higher boost. You can ask the audiance, or phone a friend, but fifty/fifty has gone.
 

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M's Factory
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Discussion Starter #7
OKi doki mate, check this out on my site.

Dynamic Calculator

Unlike the one above which doesnt take into account, Pressure levels of boost, or Elevation in the atmosphere either.
 

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Nice one mush, i will do that for Isaacs engine, so we can work out what size metal head gasket to use for desired boost levels etc. Must say its awfully tempting to put 6 1/2" in the rod size column
 

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M's Factory
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Discussion Starter #9
:) it does work out quite well this is the calculation device I am using at the moment and have put it on my site so if anyone else needs access to it they can.

I have not linked to it yet hence its invisible unless youhave the link above :) let me know how you get on.
 
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