i have an 1275 a + engine and i will be fitting 10>1 compression ratio pistons, will skimming the deck of the block achieve anything.If yes how much can be removed?
Block Skimming
Started by
chrisproject
, Jul 01 2010 07:30 PM
6 replies to this topic
#2
Dave33
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Mini Mad
Posted 01 July 2010 - 10:56 PM
Ideally you want the pistons tops flush with the top of the block,so a dry build will be necessary to determine how much needs removing.
the compression ratio you desire,will be achieved by,the combustion chamber volume cc,any dish in pistion cc,gasket thickness will also play a part in this,and the area above the piston rings in the bore.
Have a look in david vizards book,or keith calvers website for a more detailed description,its a bit more in depth than i can fit in here.
But with regards to block skimming,dont leave the piston further down the bore to achieve a lower compression ratio as it will cause detonation probs.
So get it skimmed so pistons are flush,then sort you c/r ratio out.
hth
dave
the compression ratio you desire,will be achieved by,the combustion chamber volume cc,any dish in pistion cc,gasket thickness will also play a part in this,and the area above the piston rings in the bore.
Have a look in david vizards book,or keith calvers website for a more detailed description,its a bit more in depth than i can fit in here.
But with regards to block skimming,dont leave the piston further down the bore to achieve a lower compression ratio as it will cause detonation probs.
So get it skimmed so pistons are flush,then sort you c/r ratio out.
hth
dave
#3
Pauly
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Up Into Fourth
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Posted 02 July 2010 - 07:11 PM
Do a dry build with your new pistons and take measurments, as the crown heights are different for different pistons.
#4
Dave33
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Mini Mad
Posted 03 July 2010 - 12:09 AM
Out of curiosity,why are you aiming for 10.1?
The cam spec/duration will have a large bearing on what static c/r is required,longer duration will require a higher c/r than a short duration type.
The reason is the the cylinder wont start compressing til the inlets closed,so the later this happens the lower the 'dynamic compression' will be,hence the need for a higher static c/r needed on a long duration cam.
so if you just up the compression on a standard spec motor you wont gain anything,as higher compression will take more effort to spin over,and will make it more susceptible to detonation/pre-ignition probs.
hth
Dave
The cam spec/duration will have a large bearing on what static c/r is required,longer duration will require a higher c/r than a short duration type.
The reason is the the cylinder wont start compressing til the inlets closed,so the later this happens the lower the 'dynamic compression' will be,hence the need for a higher static c/r needed on a long duration cam.
so if you just up the compression on a standard spec motor you wont gain anything,as higher compression will take more effort to spin over,and will make it more susceptible to detonation/pre-ignition probs.
hth
Dave
#5
miniobsessed
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Posted 03 July 2010 - 09:57 AM
so if you just up the compression on a standard spec motor you wont gain anything,as higher compression will take more effort to spin over,and will make it more susceptible to detonation/pre-ignition probs.
hth
Dave
Have to disagree here Dave...
Who said the factory CR was/is correct? Actually calculating the correct CR and building your motor to that you will obviously achieve the most out it.
But completely agree you should not go skimming the block for the sake of it especially as if you go too far the pistons would be sticking out the top. And do not skim the block until you have done a dry build to determine the exact amount that needs to come off
#6
Dave33
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Mini Mad
Posted 03 July 2010 - 10:29 AM
yep i see your point,the likelihood of standard c/r being spot on is not high,i didnt take this into account,was merely stating that IF it is what it should be then just upping the c/r is a waste of time.so if you just up the compression on a standard spec motor you wont gain anything,as higher compression will take more effort to spin over,and will make it more susceptible to detonation/pre-ignition probs.
hth
Dave
Have to disagree here Dave...
Who said the factory CR was/is correct? Actually calculating the correct CR and building your motor to that you will obviously achieve the most out it.
But completely agree you should not go skimming the block for the sake of it especially as if you go too far the pistons would be sticking out the top. And do not skim the block until you have done a dry build to determine the exact amount that needs to come off
It was rather late though......
Dave
#7
miniobsessed
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Posted 04 July 2010 - 01:43 PM
yep i see your point,the likelihood of standard c/r being spot on is not high,i didnt take this into account,was merely stating that IF it is what it should be then just upping the c/r is a waste of time.so if you just up the compression on a standard spec motor you wont gain anything,as higher compression will take more effort to spin over,and will make it more susceptible to detonation/pre-ignition probs.
hth
Dave
Have to disagree here Dave...
Who said the factory CR was/is correct? Actually calculating the correct CR and building your motor to that you will obviously achieve the most out it.
But completely agree you should not go skimming the block for the sake of it especially as if you go too far the pistons would be sticking out the top. And do not skim the block until you have done a dry build to determine the exact amount that needs to come off
It was rather late though......
Dave
No worries mate, I wasn't having a go
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