. Depends on engine size, parts prices, what parts you already have, cam size, intended vehicle usage, octane of fuel usually available, etc...

. For general street use, usually start thinking about 9.5:1 for iron heads and 10.5:1 for aluminum heads... that's the Static/mechanical Compression Ratio(SCR)... then take cam spec.s into consideration for Dynamic Compression Ratio(DCR)... low 8's DCR for iron, high 8's for aluminum... (higher for all out racing)


http://www.summitracing.com/popup/calcsandtools/compression-calculator
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You are correct that the combustion chamber cc volume choice is only related to the piston used and the desired compression ratio you want to achieve. Has nothing to do with rpm

About the only particulars is that it is better to use a 64cc chamber and a flat top piston than to use a 72 cc chamber and a domed piston for the same compression ratio, as fuel burn is better when the flame doesn't have to crawl over a dome.

As far as using 64cc and a dish piston or 72cc and a flat top.... with both set-ups having the same compression ratio...I would then choose whichever combination resulted in a lower parts cost, as there will be no real difference in performance.
Flat-top pistons are often the least costly compared to dished or domed pistons, because they are simpler to machine.

 

About the only particulars is that it is better to use a 64cc chamber and a flat top piston

. This is where I start as well... 2-eyebrow flat top piston if possible... stock 'compression height' pistons (usually KB's)... with proper 'engine quench' distance... adjust from there if needed to make the combo work... although the stock open chamber 76cc heads did tend to allow better valve flow than the stock closed chamber 64cc heads... but were usually used in an ultra low compression ratio system by the factory, making for soggy performance...

http://www.crankshaftcoalition.com/wiki/Quench
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Smaller changer is better. The reason is thermal efficiency. Compression isn't the only factor. A small wedge combustion chamber is very efficient for a street motor. The smaller the chamber the less heat is absorbed by the head and more heat then can be converted to mechanical energy. Conversely, a large dome or hemispherical head has a large surface area, good for flow and valve size but very bad for thermal efficacy. This is why hemi's ruse the race track but not the street.

 

Depends on a lot of the subtle features of the chamber. Generally smaller chambers pull the spark plug in toward the center of the chamber. This reduces burn time as the flame front radiates in all directions compared to larger chambers where the spark plug is located to the side causing a longer burn time because the flame front has to travel side to side. This is not always true for aftermarket chambers which often push the spark plug toward the center. This is something you have to look for, never buy a head that you haven't seen the chamber. The older smog heads and some, but not all, stock replacement heads from the aftermarket are large 74-76 cc chambers with the plug located far to the outside of the chamber. Aftermarket, GMPP, and certainly as you get into the late 1980's to mid 1990's production feature chambers that are smaller 58 (aluminum L98), 64, 66 ccs that are approaching the shape but are not exactly like the 1996 L31 Vortec head. You also have to be careful of the mid 80's through mid 90's Swirl Port head this uses the better chambers of that era in both 64 and 76 ccs with the more centered spark plug but fill half the valve pocket with a flow restricting swirl vane. These make great torque and get excellent fuel mileage but cannot make much power as RPMs are restricted to about 4000 max before the port chokes off.

The L31 Vortec was a game changer with improved intake porting and full Ricardo chambers where the volume is small at 64 ccs with the spark plug pushed inward as much as the valve location allows. The intake side of the chamber is shaped with an eye to generating swirl without a vane and mixing the wet flow into the air stream. The squish/quench side of the chamber features a beak that extendeds between the inlet and exhaust valve to redirect the secondary intake flow away from the exhaust valve so fresh mixture is not lost out the exhaust during valve overlap. This reduces emissions and improves cylinder filling resulting in greater power and improved fuel mileage.

This type chanber has been incorporated by all the OEMs and the aftermarket. Some are better laid out than others but all are quite to very good.

For the hot rodder the aftermarket piston companies came up with the D shaped dish, sometimes called the cup, that resides under the valve pocket. The volume of the dish/cup is used along with the volume of the combustion chamber and other clearances to establish the static compression ratio. The factories still can't bring themselves to ditch the round dish. The advantage of the D dish is it allows a larger area of the piston crown to be flat and close closely under the heads squish/quench step as found with a flat top piston. This maximizes the effects of squish that performs a final mix of the fuel and air; then pushes the mixture across the chamber into the valve pocket before the spark plug. This makes it easy for the spark to fire and the high density of the mixture burns quickly reducing the need for large amounts of spark lead which better aligns the period of maximum cylinder pressure with the piston/crankshaft angles most suited to extract the power from that combustion pressure and place it into the crankshaft. Following the initial burn; the quench or heat sink of the farside of the chamber having a lot of surface area and not a lot of volume prevents remaining mixture from self igniting before the flame front gets there. This provides a high amount of detonation resistance (called mechanical octane) which makes the octane of the fuel used appear higher than its rating.

The intake porting of the L31 vortec is unique and took a different direction than classic ot rod porting. First the roof was raised which is classic hot rod porting to raise the roof and widen the port alongside the guide toward the bore wall side of the port where the major flow is. GM pretty much left this alone but widened the floor which leads to the tight bottom turn into the pocket. Widening this would slow the flow velocity in this area which would help keep the flow attached to the floor as it makes the turn into the pocket which should increase this secondary flow onto the back side of the valve. This works extremely well for lifts not over .5 inch which are typical for factory cams. For hot rod max performance with lifts higher than .5 inch the port is opened up in the more conventonal manner. The aftermarket does a lot of mixing of port shapes and locations with the Vortec style chamber, some heads will only accept the raised port and unique bolt patterned intake, others use the older port sizes and locations along with the earlier bolt pattern and still others accept either intake.

So you've got lots of, if not too many choices, I'm sure we'd be happy to comment on your selections if you care to hear.

Bogie

 

Here lies your answer
http://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

 

my selection will be the AFR 195 or 210's but just wondering about the CC size, and if there is anything to gain by going with the 72CC on a 383 stroker,

lots of good info here though

 

The good information here answers your question but needs some peeling apart so lets take the data out and see where it goes. There are two paths that arrive at the same place if you use a modern Ricardo type heart shaped chamber. AFR uses this chamber on both the 64 and 76 cc chambered heads. So the end use is a large dictator where racing using “fuels” will favor the large chamber while racing with gasoline favors a tight chamber and the street may show a preference for the tight chamber but a large chamber can also serve the purpose.

The chamber size selected will drive on the piston selection for a shape and volume that arrives at the needed compression. The compression is dictated by the closing event of the intake valve that generates a Dynamic Compression Ratio (DCR) between 8 and 9 to one. This is less than the Static Compression Ratio (SCR) because it adjusts for cylinder fill lost to the rising piston to the point of valve closure in crankshaft degrees. This amounts to calculating how much stroke is lost to that point which makes the engine appear smaller than it is. The SCR becomes a number you play with to get the needed DCR. So the 10 to 1 your sighting as desired may or not be the best number, you've got to run through a set of equations that use your rod length and intake cam timing of the closed event of the valve. These are on line Keith Black has an easy one to use. Generally you’re shooting for a DCR of 8 to 1 for old style combustion chambers in an iron head; 8.5 for modern chambers in an iron head or old chambers in an aluminum head; 9 to 1 for modem chambers in an aluminum head. Several of us can run these for you but we need the rod center to center length and the cam event timing or cam part number so we can look it up.

While larger chambers show better flow on a flow bench smaller chambers show better power on the dynamometer with gasoline. The drag racers tend toward open chambers especially where "fuels" are involved because of the shear amount of liquid going through the engine. NASCAR distance runners tend toward small chambers where sustained high power is required on gasoline. For the street either is acceptable, my preference is tight chambers for better swirl and wet flow characteristics at moderate RPMs where most street engines operate. Your choice of chamber will dictate the piston in order to tailor the SCR to the available/desired fuel. Entering in this is also any milling done to the block and/or the heads, and the thickness of the head gasket. A very radical long duration cam that will close the intake quite late in the compression stroke could run you into using a domed piston. This is not desirable on a street or distance engine as the dome interferes with the combustion speed and the extra surface area absorbs combustion heat more than a flat top piston. The ideal is a flat top as it maximizes squish/quench; does not obstruct the flame travel; and does not bring excess surface area that conducts heat away from the burn. The D dish piston offers the flat surface of a flat top for optimum squish/quench, does not obstruct the flame front, but adds surface area that absorbs heat; but this is usually the best of possible compromises. An obvious trade in regard to surface area conduction is a small chamber with a D dish piston or a large chamber with a flat top piston. If I were racing the large chamber with a flat top would be my choice, on the street a tight chamber and a D dish would be it.

It is important to optimize the compression with the cam especially on the street and with an automatic as this is where max power and best fuel economy reside. However, if detonation requires a compromise it is better to back off the compression than it is to back off ignition advance. This is because power goes down faster when the optimum advance rate is not used than when the optimum compression ratio is not used. A benefit of modern combustion chambers with their fast burn characteristic is that they are less sensitive to ignition advance versus optimum power output, these leaves space to err to the side of higher compression than can be done with older chamber designs. Again your AFR choices have modern chambers as do most aftermarket heads these days.

Bogie

 

The first thing you have to know is are you building the engine for the street or for the strip. If you are building for the street I would limit my compression ratio to 10.0:1 with aluminum heads. If building for the strip the compression ratio could be up to 14.0:1. Lets talk about a street engine. You know with pump gas you want 10.0:1 c.r. Find a calculator like Keith Blacks. Look at some flat top pistons for the piston head volume. I have seen them from 0-7 cc. Use 64 cc heads, 7 cc P.H.V., .039" gasket thickness, 4.166" gasket bore, 4.040" cylinder bore, Deck clearance zero, 3.480" stroke, 5.7" rod length. The reason I used "0" deck clearance is because 0 + .039" = .039. and the recommendation for the quench is .035"- .045". The static compression ratio using these figures calculated to 10.123. Looking at camshafts I look at the cams power range and the duration. I would want a cam with a power range like 2000-6400 and the duration in the 230's. Most cam manufacturers list the c.r. to use with the camshaft. The heads like Pro-filers list the intake port flow at 308 cfm so they have the potential of making 550-600 hp with the heads ported and 2.02/1.60 valves. For the street I would use an intake like the Edelbrock Performer RPM with a 750 cfm Holley carburetor.