Stock chamber volumes
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Cruiser
Joined: Jan 2016
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From: North of Denver
That rule of 'mathematics' is flawed and would assume that all combustion chamber sizes are equal in diameter not just volume, opposed to some being circular/ hemispherical, others being 'bathtub'...it would also assume that the combustion chamber were cylindrical with no taper and that each successive cut produce the same material impact to the remaining volume...
I don't mind the sarcasm...but at least get the 'facts' straight.
The 'rule of thumb' I was requesting was from those that have done this enough to give a ballpark number given a 103" starting point... i.e. if you shave .020" you bump compression by about +.2:1, .040 if you want +.5:1,etc.
Similar to the question...how much do you bump compression switching from a .045" head gasket to a .030? but without having to input every (known) detail into a formula.
I don't mind the sarcasm...but at least get the 'facts' straight.
The 'rule of thumb' I was requesting was from those that have done this enough to give a ballpark number given a 103" starting point... i.e. if you shave .020" you bump compression by about +.2:1, .040 if you want +.5:1,etc.
Similar to the question...how much do you bump compression switching from a .045" head gasket to a .030? but without having to input every (known) detail into a formula.
HDF Community Team
Joined: Jun 2006
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From: san antonio
That rule of 'mathematics' is flawed and would assume that all combustion chamber sizes are equal in diameter not just volume, opposed to some being circular/ hemispherical, others being 'bathtub'...it would also assume that the combustion chamber were cylindrical with no taper and that each successive cut produce the same material impact to the remaining volume...
I don't mind the sarcasm...but at least get the 'facts' straight.
The 'rule of thumb' I was requesting was from those that have done this enough to give a ballpark number given a 103" starting point... i.e. if you shave .020" you bump compression by about +.2:1, .040 if you want +.5:1,etc.
Similar to the question...how much do you bump compression switching from a .045" head gasket to a .030? but without having to input every (known) detail into a formula.
I don't mind the sarcasm...but at least get the 'facts' straight.
The 'rule of thumb' I was requesting was from those that have done this enough to give a ballpark number given a 103" starting point... i.e. if you shave .020" you bump compression by about +.2:1, .040 if you want +.5:1,etc.
Similar to the question...how much do you bump compression switching from a .045" head gasket to a .030? but without having to input every (known) detail into a formula.
I don't know about you but I want to know precisely how much to remove from a head surface to achieve a target chamber volume so that my calculated CCP and corrected CR are as accurate as possible. Cylinder volume, head gasket thickness, chamber volume, piston dome volume, deck height and cam intake close event are all elements of that calculation. I have found that using the information provided in my previous post to be the most accurate when determining how much to remove from a cylinder head to achieve the compression targets. So I thought you might find it useful but if not, please disregard. You are the one with questions, I am one of the guys that has "done this enough" trying to answer........
At the end of the day, you have to trust the head porter to get the chamber volume right and equalized between both heads. I just use the previously provided information to estimate the chamber volume requirement to pass on to the head porter.......
Last edited by djl; Dec 7, 2016 at 04:29 PM.
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Cruiser
Joined: Jan 2016
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From: North of Denver
Fair enough...at this point I'm not trying to determine exactly how much to machine by the thousandths or less... I'm trying to better understand the order of magnitude of the 'moving' parts of the equation.
Example use case...changing cams and the new cams won't run well on stock compression of, say 9.0:1. The recommended compression is >10.5:1, so nothing extreme and still in the 'bolt in' range.
Pistons, cylinders (by way of deck height) and head volume all play a role comprising the combustion chamber... some of them, like moving from a dished piston to a 6cc dome, provide big jumps, whereas setting a zero deck height or shaving a head .020" are more for fine tuning.
You wouldn't want to shave .5" from a head to make up 1.5 points of compression, at that point it would be better to change pistons. So, where do you draw the line from one to the next?
I didn't struggle with high school or college level geometry...but that only gets you so far in understanding what's practical and more importantly reliable, in the way its applied.
Example use case...changing cams and the new cams won't run well on stock compression of, say 9.0:1. The recommended compression is >10.5:1, so nothing extreme and still in the 'bolt in' range.
Pistons, cylinders (by way of deck height) and head volume all play a role comprising the combustion chamber... some of them, like moving from a dished piston to a 6cc dome, provide big jumps, whereas setting a zero deck height or shaving a head .020" are more for fine tuning.
You wouldn't want to shave .5" from a head to make up 1.5 points of compression, at that point it would be better to change pistons. So, where do you draw the line from one to the next?
I didn't struggle with high school or college level geometry...but that only gets you so far in understanding what's practical and more importantly reliable, in the way its applied.
Last edited by GibsonSG; Dec 7, 2016 at 05:00 PM.
HDF Community Team
Joined: Jun 2006
Posts: 12,776
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From: san antonio
When you have done this enough, you will inherently know where to draw the line. Using your example, to bump compression from 9:1 to 10.5:1 in a cylinder with a 3.75" bore will require a domed piston; a fairly large dome. The smaller the cylinder bore, the larger the dome must be to achieve the desired goal which you apparently understand. The timing of the intake close is also a factor and by selecting a cam with a compatible intake close, the size of the dome may be reduced to and achieve the same end results. JMHO but choosing a domed piston is always a better choice than reducing chamber volume from 85cc to say 76cc. When you start cutting .050" and more from an HD head, you introduce issues with intake fitment that must be dealt with as well. Furthermore, those heads become useless on future builds unless used in the same or very similar application. Of course, once you introduce domed pistons, valve to piston clearance requires more attention and so on and so on. I have purchased domed pistons knowing that I might likely have to shave down the dome to achieve the desired results.
We left base gaskets out of the discussion and they can play a role in this issue as well. There are a lot of moving parts and getting the compression right is probably the most important of them all and one that is overlooled by many but truly, it's not rocket science and certainly easier than college geometry...........
We left base gaskets out of the discussion and they can play a role in this issue as well. There are a lot of moving parts and getting the compression right is probably the most important of them all and one that is overlooled by many but truly, it's not rocket science and certainly easier than college geometry...........
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Joined: Dec 2005
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From: poway
The 0.007xxx numbers is based one someone measuring the surface area of the stock HD bathtub combustion chamber probably based on geometric shapes for measured chamber.. Probably an ellipse and square. If you want CCs per 10 thou it will be 1/0.7 or about 1.4ccs per 10 thou..
All other combustion chamber with different surface areas at the flat will have different numbers. Some will be pretty close to this but Hemi chambers without squish will be bore area x height..
All other combustion chamber with different surface areas at the flat will have different numbers. Some will be pretty close to this but Hemi chambers without squish will be bore area x height..
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Andrews published a compression estimate for Evo, and T/C, at one time, when reducing chamber volume.
Bottom line, cc the heads once completed with the valve job, and any chamber modifications, and machine the gasket surface as needed for the given cubic inch, and camshaft.
Scott
Bottom line, cc the heads once completed with the valve job, and any chamber modifications, and machine the gasket surface as needed for the given cubic inch, and camshaft.
Scott
Last edited by Hillsidecycle.com; Dec 9, 2016 at 12:02 PM.
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