whats the best way to raise compression

[dale325i]

how do you go about raising the compression ratio, the guy has put my 2.7 together and done a compression ratio test and it came out at 8.4, ive been told that the best compression ratio would be 10.1 so how do they get it up to the right compression, is it done by skimming the head or block or some other way. cheers

[E30Adam]

There are various ways of doing it. Usually you need to change the pistons to get a higher compression but make sure that the tops are correct for the cylinder head. Another way is to deck the block slightly but this will throw your timing out and you'll need a vernier pulley to adjust it correctly.

What pisons has he used in your 2.7 build?

[dale325i]

Hi Adam, ive got 325 pistons with 320i rods and eta crank with a 320i block bored out, i already have a vernier pulley, so decking the block might be the way to go. cheers

[Jimbob]

What head is on there?
Does this mean all the work on the 2.5 is now wasted?

[Ant]

what pistons Dale ? high comp units from a pre facelift modded to suit (skirts machined) should have achieved a comp ratio of 9.75:1 + 8% due to the capacity increase with stock valve to piston clearances.

post facelift units should have given a tad under 10:1 using the same rough method to calculate the CR.

stock CR + capacity increase = compression ratio increase IMHO, that assumes the block was decked to give the same squish as the original 2.5 block.

[dale325i]

What head is on there?
Does this mean all the work on the 2.5 is now wasted?

Hi Jimbob, the head is from a late 325 casting number 885

[dale325i]

what pistons Dale ? high comp units from a pre facelift modded to suit (skirts machined) should have achieved a comp ratio of 9.75:1 + 8% due to the capacity increase with stock valve to piston clearances.

post facelift units should have given a tad under 10:1 using the same rough method to calculate the CR.

Hi Ant, i think the pistons are from a late 325