This is so much fun....... not
You can eliminate all the confusion by just calculating all elements that comprise the total chamber volume , sum them together and enter them all as one number under chamber volume and zero the rest.
In your case,the deck clearance of .012 that you seem to have assumed is 2.3975cc, rounds to .............................. 2.4cc,
and since the piston is below deck this is a volume that you must ADD to the total.
You have the head volume already listed as 62cc.
The .039 gasket calculates to 8.603, rounds to 8.6cc .
the piston tops have eyebrows that you list as 5cc which add volume,
so adding these up, we get;
2.4 + 62 + 8.6 + 5 =78cc= Total chamber volume; except to be 100%accurate PRH says to add a cc for the clearance volume, so Grand total becomes
79CC.
Now you can plug this into any calculator and it should be dead-on. But you can even do it longhand by calculating your swept volume (which is the volume that you piston sweeps thru, on a one-way trip, down to up), and plugging the two elements into a formula.
In your case at 3.94 bore and 3.315 stroke, your swept comes to 662.3176cc, rounds to
662.3CC .
Here is the formula to figure your compression ratio;
(CV + cv)/cv = Scr; Where
CV is the swept, and
cv is the total chamber volume; So
(662.3 + 79)/79 =Scr = 9.384; rounds to 9.38
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By itself this Dcr number is relatively meaningless, because the actual compression ratio while running, called the Dynamic Compression Ratio (Dcr) is determined by when the Intake valve actually closes; BECAUSE, the engine cannot start building cylinder pressure until the valves are both closed and sealing. It is the Dcr that determines to a large extent, the cranking cylinder pressure that you measure with a compression tester.
In a given engine;
The Dcr is controlled by the moment that the Intake valve closes (measured in crankshaft degrees).
The Scr is a tool we use to manipulate the Dcr when working with a specific Intake-valve Closing Angle (Ica)
The Ica is a tool we use to manipulate the Dcr from a fixed Scr.
The goal of all this is to keep the cranking cylinder pressure as high as possible, without sending the engine into detonation, with whatever octane-rated gas we intend to run, or can get locally..
The less you care about performance, the less important this becomes.
The bigger the cam is,that you want to run, the more important this becomes.
For fuel economy, you can run more pressure than is normally considered safe, if you promise never to try to get full atmospheric pressure into the chambers; or to restrict your Power-Timing below the detonation level.
At WOT operation, and with full-timing, you have to be real careful to stay out of detonation, cuz from the start-line, to the traps is enough time to destroy an engine.
If you are considering alloy heads;here is a tip;
My Chevrolet Orlando is a Direct-Injected VVT 2.5liter Normally aspirated, 6speed marvel of engineering that cranks 220psi on the same gauge that cranks 177/180 on my 367, and 135/140 on a SmoggerTeen. That's a lotta pressure. It switches to the HO part of the cam at 4500rpm and is rated at 167hp @6700rpm........ Ok hang on, that's NET hp. Remind me what your 1980s 318LA which is 5.21 liters, was rated at; was it 180hp, or was it 150? I forget. Allbeit at 4400 IIRC.
The FIRST point I am trying to make is that she runs 220psi and has made 42mpgC, idling down the hiway at ~65mph=2000rpm in Loc-Up.
42C is ~34 mpg USg. That is, in good measure, because of the high operating pressure.
The SECOND point is that you cannot successfully run 220psi in your iron-headed 318LA at full timing/or full-throttle. If you try it; you'll be lucky if all that happens is that the skirts rattle into the oilpan and/or she holes a few pistons.
But you might be able to run 190psi with closed-chamber alloy heads......... And maybe 165 with closed chamber Iron heads.
As for Ica's;
Generally speaking your choices are from 52* to 72*, for a streeter ; going from Lo-performance at 52* to a pretty big street cam at 72*..
The Ica of a typical streeter might fall into the range of 63 degrees +/- 3*, which is about three basic cam sizes. Your local elevation will also figure into the pressure equations.
Happy HotRodding
