Does anyone know what the volume would be after a 0.040 skim please?
Edited by Minigman, 07 November 2017 - 09:03 AM.
Posted 07 November 2017 - 06:45 AM
Edited by Minigman, 07 November 2017 - 09:03 AM.
Posted 07 November 2017 - 09:18 AM
Best thing to do Is to get a seringe and measure how much fluid you can inject into it with the head upside doen on a flat surface. use parafin as water won't give you an accurate reading due to surface tension..
Posted 07 November 2017 - 09:41 AM
Edited by Minigman, 07 November 2017 - 09:41 AM.
Posted 07 November 2017 - 12:38 PM
Posted 07 November 2017 - 01:47 PM
A bit depends on the valves too as some are flat, some have a small dish, and also how far recessed the valves/seats are, I've just flooked a 12G295 head that needed a skim, going on a 1217 (1098 to 68mm bore) the compression works out a 10.2:1 perfect.
Posted 07 November 2017 - 02:35 PM
Edited by Swift_General, 07 November 2017 - 02:35 PM.
Posted 07 November 2017 - 03:24 PM
Posted 07 November 2017 - 10:02 PM
It is very unlikely a head which has been skimmed/modified can be simply fitted to an engine without accurate measurements, calculations and skimming.
If you don't do this it is a guess as to whether the CR is correct and suitable.
Posted 09 November 2017 - 02:13 PM
Got to be said I can't get my head round what Swift_General has said, or is it just me?
I'm not having a go, I just don't understand it...
The change in chamber volume does depend on the cross sectional area and as said the skim, but also how that CSA changes too, as it does when skimming. As the depth gets shallower the CSA gets smaller too so its the change in the area on a head chamber plus the skim. I can't see how this is worked out to 0.01cc using graph paper or am I missing something?
Cross sectional area has nothing to do with the volume of the chamber unless its a perfect cylinder or am I wrong? And a CSA should not be measured in CC, as CC is a cubic volume measurement not an area, surely an area is measured in cm squared not cubed? and unless its a perfect cylinder the CC cannot be calculated from an area, or am I wrong?
As the chamber in a head is an odd shape CSA has no place in calculating a 3D volume and is uneeded? or am I wrong? I fail to see what use graph paper is or am I missing something? Its a bit confusing.
A piece of perspex with a hole, a syringe and some ATF is all I have ever used, perfectly accurate and simple.
Posted 09 November 2017 - 02:23 PM
Got to be said I can't get my head round what Swift_General has said, or is it just me?
I'm not having a go, I just don't understand it...
The change in chamber volume does depend on the cross sectional area and as said the skim, but also how that CSA changes too, as it does when skimming. As the depth gets shallower the CSA gets smaller too so its the change in the area on a head chamber plus the skim. I can't see how this is worked out to 0.01cc using graph paper or am I missing something?
Cross sectional area has nothing to do with the volume of the chamber unless its a perfect cylinder or am I wrong? And a CSA should not be measured in CC, as CC is a cubic volume measurement not an area, surely an area is measured in cm squared not cubed? and unless its a perfect cylinder the CC cannot be calculated from an area, or am I wrong?
As the chamber in a head is an odd shape CSA has no place in calculating a 3D volume and is uneeded? or am I wrong? I fail to see what use graph paper is or am I missing something? Its a bit confusing.
A piece of perspex with a hole, a syringe and some ATF is all I have ever used, perfectly accurate and simple.
I agree. The CSA changes as the head is skimmed due to the tapered nature of the chamber.
Posted 09 November 2017 - 04:49 PM
I did this for my 998 a year or so back and just checked my notes...
The 295 head I started with had and average chamber volume of 27.70cc, measured using the 'bit of glass and a big syringe' method (repeated many times - it wasn't easy.)
I did the '5x5mm graph paper and count the squares' method to measure the face area to 2875mm2 to work out the skim depth I needed to get to the CR I wanted. (The chamber doesn't taper at the face where it's being skimmed so don't worry about this being inaccurate.)
I got 40 thou skimmed off (so taking off 2.92cc) to give a new average volume of 24.78cc.
My head had new unleaded exhaust valve seats put in before I did all this - I guess that explains the starting volume being different to the expected 28.3cc. I know you're only after a rough figure to decide if it'll work, but It's worth bearing in mind that these heads will likely have been skimmed or otherwise fiddled with before you get hold of them, so if/when you do get the head you obviously need to measure the actual volume before you start fiddling with it yourself.
Hope this helps!
Posted 09 November 2017 - 05:06 PM
First of all it is necessary to establish what you want to achieve. The answer will be a set compression ratio.
The first thing is to establish what you are starting with. Measure the volume of the combustion chambers, the dish (if any) in the top of the piston and the distance from the piston top to the block deck level at TDC.
From this you can establish the existing CR. Assuming this is too low for what is required, the final corrected volume of the combustion chambers can be calculated.
All that is then necessary is to put that exact amount of paraffin (or meths, or ATF, or whatever you decide to use) into a combustion chamber when the head is laid so that the face is upwards and absolutely level. Measure how far from the face the surface of the fluid is and that is the amount to be skimmed to get the CR correct.
It is not difficult and is necessary if the CR is to be as required. Guessing is not an option.
Posted 09 November 2017 - 06:59 PM
...and, remember to include everything in the CR calc. Inculding Head gasket volume, ring land volume, block unswept volume (the unswept bit between the deck and the piston at tdc) all need to be in the unswept side of the calc. These can be either measured or easily googled.
I missed out the block unswept volume in my calc and ended with a CR of 9.5:1 rather than my target of 10:1!
Still goes like stink compared with the standard head though.
Posted 09 November 2017 - 07:11 PM
Posted 09 November 2017 - 07:23 PM
Edited by Minigman, 09 November 2017 - 07:54 PM.
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