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Discussion Starter #1
Using Summits Compression Calculator on their website I got to thinking if maybe I'm not capturing everything I need to calculate compression correctly.

They are asking for the:
Bore
Stroke
Cylinder Head Volume
Effective Dome
Deck Clearance
Compressed Gasket Thickness
Number of Cylinders

Cylinder Head Volume is the one I'm concerned with because I have never added in the compressed gasket volume itself along with the cylinder head volume and I don't think they are capturing that stat correctly with this calculator.

I could have all kinds of different bore sizes of gasket and that would change the cylinder head volume.
Looking at it from a 4.03 bore the gasket I was looking at was 4.1 in size and had a compressed volume of 10.1cc. Wouldn't I have to add this into the cylinder head combustion chamber volume to get an accurate compression ratio?
 

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You also need piston relieves or dome volume cc and head cc.
 

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Using Summits Compression Calculator on their website I got to thinking if maybe I'm not capturing everything I need to calculate compression correctly.

They are asking for the:
Bore
Stroke
Cylinder Head Volume
Effective Dome
Deck Clearance
Compressed Gasket Thickness
Number of Cylinders

Cylinder Head Volume is the one I'm concerned with because I have never added in the compressed gasket volume itself along with the cylinder head volume and I don't think they are capturing that stat correctly with this calculator.

I could have all kinds of different bore sizes of gasket and that would change the cylinder head volume.
Looking at it from a 4.03 bore the gasket I was looking at was 4.1 in size and had a compressed volume of 10.1cc. Wouldn't I have to add this into the cylinder head combustion chamber volume to get an accurate compression ratio?
most don't know the gasket cc's , just makes it simple .

you know you head gasket cc , add to chamber cc , enter 0 for gasket
 

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Nobody mentioned when the cam closed the intake and exhaust valves for the compression stroke, I'm sure that somewhere in there is the credit for Volume of air being compressed.

Or did I miss where that was a calculation above. It is after all the single most important piece of the pie, because if that valve don't close you pretty much got jack schitt, don't ya?
 

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Nobody mentioned when the cam closed the intake and exhaust valves for the compression stroke, I'm sure that somewhere in there is the credit for Volume of air being compressed.

Or did I miss where that was a calculation above. It is after all the single most important piece of the pie, because if that valve don't close you pretty much got jack schitt, don't ya?
what cam ? don't even need one to get the ratio
the cam has nothing to do with static compression ratio .
 

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Ok, the ratio then is the amount of squish when the piston is at tdc? that's it?

Does static compression have to do with the cam timing then?
 

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Ok, the ratio then is the amount of squish when the piston is at tdc? that's it?

Does static compression have to do with the cam timing then?
Compression ratio is the ratio of volume from its largest capacity to its smallest capacity

Lets say with the piston at the bottom of its stroke the volume is 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be described as a 10:1 compression ratio.
 

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Compression ratio is the ratio of volume from its largest capacity to its smallest capacity

Lets say with the piston at the bottom of its stroke the volume is 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be described as a 10:1 compression ratio.
OK I see what you're saying. Static compression is the areas of total volume the piston can pus from BTC to TDC, and that means Dynamic is the actual ratio of the A/F mixture that actually get's compressed?
 

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OK I see what you're saying. Static compression is the areas of total volume the piston can pus from BTC to TDC, and that means Dynamic is the actual ratio of the A/F mixture that actually get's compressed?
NO>you are thinking about cylinder pressure

Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC).

Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.
The difference between the two can be substantial. For example, with a cam that closes the intake valve at 70º ABDC, the piston has risen 0.9053" from BDC in a stock rod 350 at the intake closing point. This decreases the sweep volume of the cylinder considerably, reducing the stroke length by almost an inch. Thereby reducing the compression ratio. This is the only difference between calculating the SCR and the DCR. All other values used in calculating the CR are the same. Note that the DCR is always lower than the SCR.


Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors.
 

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Discussion Starter #13
Thanks for the replies.
I found an on line calculator that more closely scrutinizes head gasket and asks the question "Enter Head Gasket Bore Diameter" which on the particular gasket I was figuring was 4.1 inches. It doesn't make a lot of difference but it is a few 1/10th off on the Summit site Thanks again.
 

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You're all over-complicating it.

There are 2 types of compression ratios that we shall be concerned with. Static and Dynamic.

Static compression ratio is the ratio of the total volume of the cylinder at BDC to the total volume of the cylinder at TDC. That is it.

Dynamic compression ratio takes into consideration the cam timing, rod length, stroke with emphasis on the intake valve closing point in crankshaft degrees (crank degrees relating to where the piston is at in the cylinder).



Copied from Lunati's web page ( Helpful Piston Calculations - Lunati Power )


<table cellpadding="2" cellspacing="0"> <tbody><tr> <td colspan="3"> Compression ratio = (swept volume + total chamber volume) / total chamber volume </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td colspan="3"> It is important that we understand two terms and their relationship to compression ratio: Swept Volume and Total Chamber Volume. Swept Volume is the area the piston travels through bottom dead center to top dead center. Total Chamber Volume is all the area above the piston at top dead center. This would include the area above the piston in the cylinder block, the area of the compressed head gasket, the combustion chamber, the valve pocket, and the dome of the piston. The compression ratio is the relationship of the swept volume to the total chamber volume. </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td colspan="3"> To start, we need to know the Swept Volume of one cylinder. The size of one cylinder figured in cubic centimeters. </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td colspan="3"> Swept volume (cc) = cylinder bore diameter (inches) X cylinder bore diameter (inches) X stroke (inches) X 12.8704 </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td> </td> <td> Example: </td> <td> Cylinder head cc = 72.18 cc </td> </tr> <tr> <td> </td> <td> </td> <td> Piston = flat top with two valve pockets that measure a total of 4 cc </td> </tr> <tr> <td> </td> <td> </td> <td> Head gasket = 4.000" round and 0.038" thick when compressed </td> </tr> <tr> <td> </td> <td> </td> <td> Deck clearance = The piston at top dead center is 0.010" below the surface of the deck </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td> </td> <td> </td> <td> Gasket cc = bore X bore X compressed thickness X 12.8704 </td> </tr> <tr> <td> </td> <td> </td> <td> Gasket cc = 4.000 X 4.000 X 0.038 X 12.8704 </td> </tr> <tr> <td> </td> <td> </td> <td> Gasket cc = 7.83 cc </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td> </td> <td> </td> <td> Deck clearance volume = bore X bore X deck clearance X 12.8704 </td> </tr> <tr> <td> </td> <td> </td> <td> Deck clearance volume = 4.000 X 4.000 X 0.010 12.8704 </td> </tr> <tr> <td> </td> <td> </td> <td> Deck clearance volume = 2.059 cc </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td> </td> <td> </td> <td> Total chamber volume = 72.18 + 7.83 + 4 + 2.059 </td> </tr> <tr> <td> </td> <td> </td> <td> Total chamber volume = 86.07 cc </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td colspan="3"> Now we are finally ready to calculate the compression ratio! </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td> </td> <td> Example: </td> <td> Swept volume = 716.62 cc </td> </tr> <tr> <td> </td> <td> </td> <td> Total chamber volume = 86.07 cc </td> </tr> <tr> <td colspan="3"> </td> </tr> <tr> <td> </td> <td> </td> <td> Compression ratio = (swept volume + total chamber volume) / total chamber volume </td> </tr> <tr> <td> </td> <td> </td> <td> Compression ratio = (716.16 + 86.07) / 86.07 </td> </tr> <tr> <td> </td> <td> </td> <td> Compression ratio = 9.33:1 </td></tr></tbody></table>


Now: dynamic compression ratio. You need to use a calculator for that unless you're really good at math (and I am not). Try here:
Wallace Racing: Dynamic Compression Ratio Calculator

That one is pretty accurate. If you have all your figures correct and a gauge that is accurate, that calculator is pretty much dead nuts. Mine said it would crank 298 psi. And after assembly, it cranks 295 on the gauge. Yes it required a heck of a starter motor! But the cam is not right for this engine.
 
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