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  #31  
Old 07-01-2018, 07:38 PM
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Jimbo Jimbo is offline
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Quote:
Originally Posted by franky View Post
How do you calculate how much to skim off the head with the varying combustion chamber shapes of heads and if its been skimmed before etc? given the more you take off the more it reduces CC volume with each pass.


Seems like that could be a head scratching thing. Or skim to about what you think then remeasure and go back if its not quite right?
if the head is slightly mishapen then you may find that skimming it will end up with combustion face cc greater at one of than the other so in effect the head is skimmed at an angle and not flat.
to correct it you can machine the combustion chamber to increase cc however its far better to calculate first like pmac said and get the machinist to skim the head accordingly.
ive seen it done where i work where they test for combustion chamber accuracy, if its out the machine has to be calibrated to account for the discrepancy across cylinders.
obviously it wont work if the fire face is banana shaped as the material taken off either end will reduce cc in either ends combustion chamber hence the need to remove material in the combustion chamber in that situation.
to be honest its chasing BHP that neccesitates doing it as in my opinion its a time consuming art form.
you wouldnt bother on a fiat punto or the like.
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  #32  
Old 07-01-2018, 08:07 PM
franky franky is offline
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Quote:
Originally Posted by Jimbo View Post
if the head is slightly mishapen then you may find that skimming it will end up with combustion face cc greater at one of than the other so in effect the head is skimmed at an angle and not flat.
to correct it you can machine the combustion chamber to increase cc however its far better to calculate first like pmac said and get the machinist to skim the head accordingly.
ive seen it done where i work where they test for combustion chamber accuracy, if its out the machine has to be calibrated to account for the discrepancy across cylinders.
obviously it wont work if the fire face is banana shaped as the material taken off either end will reduce cc in either ends combustion chamber hence the need to remove material in the combustion chamber in that situation.
to be honest its chasing BHP that neccesitates doing it as in my opinion its a time consuming art form.
you wouldnt bother on a fiat punto or the like.
I think you've misunderstood my question.

The combustion chamber isn't a circular hole where you can decide to remove 10cc, so given its equal shape you can work out volume and just remove that amount. All the various heads have their own chamber shape, ones that have been re-worked etc. So removing 0.25mm for example doesnt' reduce the CC by the same amount each time, without a 3d computer scan of the chamber you can't know how much to remove to hit a target CC? So do you make the first cut, fairly easy on a stock new head, then measure and do multiple skims?
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  #33  
Old 07-01-2018, 08:11 PM
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jonbills jonbills is offline
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You've got to cc the chambers to start with - say they measure 40. Say you want a chamber size of 37 to get the CR you want. You can use graph paper to measure the area of the chamber - put the graph paper over the chamber and trace round the edge. count the complete squares & treat the partial squares included as 1/2s. each little square is 1 mm2. Maybe you end up with 4000 squares, so 40 cm2. a 1mm slice of that is 4 cm3. so you need to have a 0.75mm skim to get to 37 cm3.

And yes, the calculation assumes that the the walls of the chamber are vertical, which they aren't of course. because they slope in, the volume you'll remove is a little below the estimate. so you will have to measure again and skim again possibly.
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  #34  
Old 07-01-2018, 11:46 PM
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Jimbo Jimbo is offline
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Quote:
Originally Posted by franky View Post
So do you make the first cut, fairly easy on a stock new head, then measure and do multiple skims?
pretty much, but to be honest the people who do skimming professionally know from experience what to take off from the cc readouts and a couple of cc tests after a test skim and like jonbills said you can use the graph paper to predict roughly what to take off.
for example the heads we skim the company must have skimmed 5 of the same heads at different heights and measured the ccs as we have graphs with plotted cc against material removed so we know what to expect at different heights, albeit what i do is standard production not modifying to get higher compression ratios but the theory is the same.
the other problem of course is the combustion faces i have to deal with are machined so most are pretty equal opposed to those that are left rough cast which plays havoc with readings
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