The 291s are listed as a closed chamber rectangular port head.  Some regard this as the best Chevrolet production iron head ever made.

I am estimating your compression ratio to come in at 8.94:1 with the following assumptions:  4.280 bore, 4.0 stroke, 109cc combustion chambers, true flat top pistons at zero deck, and Fel-Pro head (9.700cc).  Any valve relief notches present in the pistons, if the heads are bigger than advertised (mine always are), if the pistons are below the deck at TDC, your calculated compression ratio will drop.  Just for the record, this is purely hypothetical.

Stepping up to a rectangular port head increases your cross-sectional area of your ports in addition to increasing flow.  Not trying to ruffle any feathers here, but I still stand by my previous posts.  I think you will need to strive for a compression ratio of at least 10.5:1 along with an increase in RPM range to take advantage of these heads’ flow potential over your existing 049 castings.

Keeping the port velocity high is critical to how the engine feels (seat of the pants), throttle response, efficiency, and ultimately how much power it will make.  I honestly don’t think that the addition of these heads will give you any noticeable power over what you have now.  However, if decide to build an engine that will utilize this potential, you could end up with good, reliable power.  As was stated by others, you want to build power at an RPM level where you can use it.  Just because you can make 700 horsepower spinning it to 7500 RPM, you will have to cut your impeller way down to get it to RPM that high on the water, but you will lose your launch.

The price you indicated is not a bad price at all when you price out doing the same to your heads.  I would bet that the springs will probably need to be changed to better match your cam selection.

Good luck,

Joe

Trying to decide if I actually need forged pistons or not..

What is the price difference between hypereutectic pistons and forged pistons that you are looking at?  Can that money be better allocated to something that will add horsepower?  If the price difference isn’t significant and it helps you sleep better at night, go for the forged.

To assist in calculating compression ratio:

Compression ratio is defined as the volume above the piston at BDC divided by the volume above the piston at TDC.

I have played around with many of the online compression ratio calculators and I don’t feel totally comfortable using them, especially when they don’t take head gasket or deck height volumes into account.  Does it matter in the final outcome of the engine?  I don’t know, but it does change the numbers, so I factor that in.  Some compression ratio calculators take the top land of the pistons into account.  I think that is much ado about nothing, I just don’t see that slight volume contributing significantly to the combustion process.

For me, it is easier to just convert the bore and stroke dimensions to centimeters so that all of my numbers are in cubic centimeters.  To convert inches to centimeters, multiply inches by 2.54.

Piston Dish or Piston Dome will either be added (dish +) or subtracted (dome -) because a dish adds to the combustion chamber volume and a dome displaces combustion chamber volume.

Volume, Deck Height is a cylinder volume that is created by the distance the piston is below the block deck while at TDC.  Some engines, like the Mopar LA 340 will have a positive deck height where the piston top is actually above the block deck surface.  Most engines have a deck height below the block deck.

^2 = Quantity Squared
π ~ 3.1416
R = Radius = 1/2 Diameter
Area of a Circle = πR^2
Volume of a Cylinder = Area of a Circle * Stroke

The numbers will look something like the following for a .030 over 454:

Bore = 4.280 inches * 2.54 = 10.8712 cm
Stroke = 4.00 inches * 2.54 = 10.16 cm
Volume, Cylinder = Area, Bore * Stroke

Volume, Cylinder + Volume, Deck Height + Volume, Gasket + Volume, Cylinder Head +/- Volume Dish/Dome
---------------------------------------------------------------------------------------------------------------
Volume, Deck Height + Volume, Gasket + Volume, Cylinder Head +/- Volume Dish/Dome

Area, Bore = [(10.8712 cm / 2)^2] * 3.1416
Area, Bore = (5.4356^2) * 3.1416
Area, Bore = 29.54574736 * 3.1416
Area, Bore = 92.8207 sq/cm

Volume, Cylinder = 92.8207 sq/cm * 10.16 cm
Volume, Cylinder = 943.0583cc

Volume, Deck Height = 0 (zero deck)
Volume, Gasket = 9.700cc
Volume, Cylinder Head = 109cc
Volume Dish/Dome = 0cc (true flat top)

943.0583 + 0 + 9.700 + 109 + 0
---------------------------------------  =
0 + 9.700 + 109 + 0

1061.7583
-------------  =  8.9449:1 CR
118.7

Some other features that influence final compression ratio:

Dished vs. flat valves as well as how many valve jobs the heads have had.  Sinking the valves into the seats not only reduces compression ratio, it also reduces port flow as it affects the short side radius of the port.



Any notches in the block for valve clearance or unshrouding.

Thanks for the detailed response! I am not going to go with forged I am believe the hypers will work just fine. The set I am looking at has a 30cc dome. With stock bore/ stroke and has no been zero decked. Do you think that will yield a safe comp ratio with a 119-122cc head?


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It is my humble opinion that the 049 numbered heads are overrated. 99 times out of 100, the parts are so mismatched and mis-machined, a set of truck peanuts will womp their ass.

GT

  Really?

As to the OP... be careful of what piston you are using and what head you decide to go with... an open chamber piston will have issues in a closed chamber head. So if you decide to go closed chamber head, make sure the piston dome shape is for a closed chamber head. The reverse though, is not a problem. Also, keep in mind that you're not gonna be able to get away with quite as much compression if you leave the piston .020+ down in the hole and use a roughly .040 gasket... your quench is getting quite a bit on the wide side. Cam selection may help, or it may make matters worse, so that is also something to consider when dancing around 10:1 with a wide quench area and a decent sized hump on the piston. More times than not... power and tuneability works out better with a smaller dome, zero decked, and heads shaved a bit to arrive at your 10:1... or at least that 's been my experience. Of course, with the exception about the open chamber pistons with closed chamber heads, is all my opinion... so, place as much or as little value on it as you wish.