7 replies to this topic

[stretch tech]

hi all, i rebuilt my engine a few months back with small modifications and never bothered to check the compression ratio. now, after looking deeper into engine projects on this forum, i realised i need to check it

i down loaded an app called mini head on my phone and that gave me a figure of 9.2 ish, is this right ? is this bad ? sorry for the newbish questions but i need some expert (or better informed any way ) opinions/advice

my engine spec is as follows (well the bits that matter) :

998 bored +40
skimmed block and standard unleaded metro head
flat top pistons
new gaskets (of course)

cheers

Rhys

[ACDodd]

You need to know the piston deck height and the head gasket compressed volume to work out the CR.

AC

[stretch tech]

You need to know the piston deck height and the head gasket compressed volume to work out the CR.

AC

what is the piston deck height ? it has a standard size head gasket if that helps

[mini-luke]

The height the piston is below the deck (top of block).

[stretch tech]

The height the piston is below the deck (top of block).

ok thanks for clearing that up, it should be standard height then bar a light skim...how much would this deck height be ?

[Cooperman]

As 'AC' says, you need to know some basic dimensions which can really only be measured with the head off.
First of all you need to know what the volume is between the top face of the pistons and the cylinder block top level. You work this out by measuring how far down the block the top of each piston is. With a 998 for each 0.010" (i.e. each 10 thou) the piston sits from the block surface at TDC there is about 0.86 cc of volume. There is no actual set amount as it depends on the manufacturing tolerances of pistons, crank, rods and block and, of course, the type (as in part number, of piston fitted).
If it's a copper-faced 998 head gasket the gasket volume is 2.7 cc.
The head chamber volume needs to be measured with a burette or a 10 cc shrynge and some paraffin.
Only then can the C.R. be calculated.

Edited by Cooperman, 30 July 2012 - 08:07 PM.

[stretch tech]

ok then, thanks for that cooper man, that really helps i'll have to try and dig out how much the block and head was skimmed by as they were both quite badly worn i've tried the calculation again with some of the info above and an educated guess on deck size etc and got just above 9.5:1 which by looking around seems a bit better

[Cooperman]

Do a static compression test and if the compressions per cylinder are over around 150 psi just drive it and don't worry too much about it. If it's a road car, rather than a competition car, you probably won't really notice much difference. Next time you strip the head off come on here and we'll tell you what to measure and help with the calculations.