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[project_mistral]

Need some advice really, so any much appreciated.

If you bore the cylinders out to 0.5 oversize, am i right in saying you have buy larger pistons aswell as rings? or can you just use larger rings to take up the slack??

The whole process will cost in the region of £600 is it worth it?? does it gain a large enough lower compression to make it worth while??

Or will the standard pistons and bores be could enough for a reliable 200bhp+

dan

[FezzR]

the only reason they give the option of 0.5 1.0 and 1.5mm overbore is in the event that the cylinder is worn when you come to rebuild it, and the pistion is slack in the cylinder,

thus you over bore by 0.5mm, this takes the cylinder back to a round shape, (wear makes it oval)

there is no performance gain at all (well any there may, is not worth thinking about otherwise you`ll just be disappointed)

[project_mistral]

thanks for that.

the bores in that case seem to be ok on mine. The lip at the top of the bore is minimal, i was able to push the piston out by hand instead of having to use a hammer like i was told! not bad for a79k engine.

cheers

[louma84]

it will lower the compression ratio slightly, but i don't no by how much

[Red16]

it will lower the compression ratio slightly, but i don't no by how much

it wont lower, it will raise the comp ratio

[FezzR]

it will lower the compression ratio slightly, but i don't no by how much

it wont lower, it will raise the comp ratio

indeed as the man says, larger bdc capacity into the same tdc volume,
as long as all other things are equal etc

[louma84]

but you would b increasing cc in each cylinder though so wouldn't that lower it? [ERRR] if ya want to lower the c/r in ur car u can skim the tops of the pistons or use a decomp plate????????

[FezzR]

increasing the capacity isnt the key fact because it can be done in so many ways

as i said before , its about the capacity with the piston at TDC and BDC

wider pistons mean more BDC capacity but the same TDC capacity

decomp plate/ pistons means BDC bigger capacity but with a bigger TDC capacity too

[Red16]

but you would b increasing cc in each cylinder though so wouldn't that lower it? [ERRR] if ya want to lower the c/r in [ERRR] car u can skim the tops of the pistons or use a decomp plate????????

no it wouldnt, but by skimming the pistons you make the swept volume of the cylinder bigger as well as the combustion chamber volume, which is why the c/r will be lower.

if you bore the block and use larger pistons, the swept volume will be larger but the combustion chamber volume will be the same, therefore increasing the c/r.

[Red16]

looks like FezzR beat me to it

[FezzR]

looks like FezzR beat me to it

took me about 10 mins to post it lol
you go and make a cup of tea while half way through

yours is using the correct terminology tho

[Red16]

fezzR, i wouldve made a cuppa but was at me mates flat while posting, and young lads flats are crap, never anything in the bloody cupboards so no coffee got made

heres how you work out compression ratio, get ya thinking caps on


swept volume = (Bore x Bore x 0.785 x Stroke) / 1000


(Swept volume + combustion chamber volume) / combustion chamber volume = compression ratio


but all this is disregarding the volume the head gasket creates