I realize that too many cooks spoil the broth.  I am not here trying to stir up anything, just crunching some numbers.

I would be seriously concerned about running a thicker head gasket because your Quench Distance is going to open up, making your engine more prone to detonation.

No disrespect to vintage240zracer, but I have never had good results using the online compression ratio calculators.  I like to run the numbers the old fashioned way.

This is pure speculation based on numbers and my experience and NOT based on your actual parts.  The only way to get accurate numbers is to measure the exact parts that will be used in your build.

Looking at your Speed-Pro H581CP30 pistons, they are advertised as having a 30cc dome and a 1.640 compression height.  You really need to know your compression height, deck height, or amount milled from the deck surface to accurately calculate your compression ratio.  Your block started out with a 9.8 deck height, connecting rod length is 6.135, and half of your 4.0” stroke is 2.0.  Using the following numbers:

9.8 – 6.135 – 2.0 – 1.640 = .025

This means, unless your block has been decked, your Speed-Pro H581CP30 pistons will be “in the hole” as much as .025-inch.  This volume adds as much as 5.894cc to the volume of the combustion chamber.  This .025 in the hole combines with the gasket thickness to create the Quench Distance.  The most commonly used Fel-Pro head gaskets have a compressed thickness of .039 and a volume of 9.700cc.  If we use the factory blueprint numbers, the Quench Distance is .064

.039 + .025 = .064

I would make every effort to keep the Quench Distance UNDER .055 and everything I can find on the Big Block Chevy recommends a Quench Distance of .030 to .040.  I think, in an open cooling system boat, you will need a little more clearance.  I like Bostick’s recommendation, “Bore size, clearance, and application will dictate some variants in desired quench... but a general rule on a typical aspirated deal, .040-.045ish.”

Next, the heads are suspect to me.  I am not sure where you came up with 116cc for combustion chamber volume.  Without knowing the history of your heads, I can not be sure just how much material was removed during surfacing.  My experience has been, combustion chambers always are on the high side of the advertised volume, making building compression difficult.  This number depends on many variables such as; how much was surfaced off, how deep were the valve seats cut, shape of the valves, size of the valves, and any chamber work such as unshrouding of the valves.  A few cc’s here and there doesn’t seem like much, but it all adds up, especially if you have an engine that is already compression challenged.

Here is what I come up with for numbers.  I like to convert all dimensions to centimeters, that way all of my units are in cubic centimeters.  To convert inches to centimeters, multiply by the constant 2.54.  Yes, you can convert cubic inches into cubic centimeters by multiplying the ci by 16.387064.  I also prefer to calculate area of a circle by using Pi * R^2

Bore:  4.280-inch  10.8712cm
Stroke:  4.0-inch  10.16cm
Deck Height:  .025-inch  0.0635cm
Gasket Volume:  9.700cc
Piston Dome:  30cc
Combustion Chamber:  116cc

(((((10.8712/2)^2) * 3.1416) * 10.16) + ((((10.8712/2)^2) * 3.1416) * 0.0635) + 9.700 + (-30) + 116) / (((((10.8712/2)^2) * 3.1416) * 0.0635) + 9.700 + (-30) + 116)

(((((5.4356)^2) * 3.1416) * 10.16) + ((((5.4356)^2) * 3.1416) * 0.0635) + 9.700 + (-30) + 116) / ((((5.4356)^2) * 3.1416) * 0.0635) + 9.700 + (-30) + 116)

((((29.5457) * 3.1416) * 10.16) + (((29.5457) * 3.1416) * 0.0635) + 9.700 + (-30) + 116) / (((29.5457) * 3.1416) * 0.0635) + 9.700 + (-30) + 116)

((92.8207 * 10.16) + (92.8207 * 0.0635) + 9.700 + (-30) + 116) / ((92.8207 * 0.0635) + 9.700 + (-30) + 116)

(943.0583 + 5.8941 + 9.700 + (-30) + 116) / (5.8941 + 9.700 + (-30) + 116)

1044.6524 / 101.5941 = 10.2826

Using your numbers, your compression ratio is 10.28:1

Let’s assume that the deck height has been “zero decked” and the top of the pistons are flush with the top of the block.  That bumps the compression ratio to 10.85:1.

If your block is decked so that your Quench Distance is per Bostick’s general recommendation of .040-.045, your compression ratio would range from 10.71:1 to 10.83:1.

All of these numbers assume that the volume of your heads is actually 116cc.  Should you actually have 119cc combustion chambers, you would have 10.01:1 compression instead of 10.28:1.  If your numbers are a little more conservative, and you actually have 113cc combustion chambers, you would have 10.57:1 compression instead of 10.28:1.

These examples should illustrate how the few unknown variables can influence your static compression ratio.

I use a program that I developed in SolidWorks to help me calculate dynamic compression ratio, which is influenced by cam timing, specifically intake valve closing point.  While using the static compression ratio of 10.28:1, comparing the two cams that are mentioned in this thread, the 268AH http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=408&sb=2 and the XM270H http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=404&sb=2

With the 268AH installed 4-degrees advanced (106* Intake Centerline) per CompCams recommendations, your engine would have a dynamic stroke of 3.250 inches and a dynamic compression ratio of 8.54:1

With the 268AH installed 2-degrees advanced (108* Intake Centerline) from CompCams recommendations, your engine would have a dynamic stroke of 3.199 inches and a dynamic compression ratio of 8.42:1

With the 268AH installed straight up (110* Intake Centerline), your engine would have a dynamic stroke of 3.146 inches and a dynamic compression ratio of 8.30:1

With these dynamic compression ratios, you are looking at detonation issues running premium pump gasoline.

With the XM270H installed 4-degrees advanced (108* Intake Centerline), your engine would have a dynamic stroke of 3.172 inches and a dynamic compression ratio of 8.36:1

With the XM270H installed 2-degrees advanced (110* Intake Centerline) per CompCams recommendations, your engine would have a dynamic stroke of 3.119 inches and a dynamic compression ratio of 8.24:1

With the XM270H installed straight up (112* Intake Centerline), your engine would have a dynamic stroke of 3.064 inches and a dynamic compression ratio of 8.11:1

Again, with these numbers, you are right on the edge of detonation and your tune will have to be right on the money.  I would install the XM270H cam straight up at 112-degree intake centerline to favor the top end and reduce the dynamic compression ratio.

I would venture to recommend stepping up another cam size, reducing dynamic compression ratio further, to the XM278H http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=405&sb=0

With the XM278H installed 2-degrees advanced (110* Intake Centerline) per CompCams recommendations, your engine would have a dynamic stroke of 3.008 inches and a dynamic compression ratio of 7.98:1

With the XM278H installed straight up (112* Intake Centerline), your engine would have a dynamic stroke of 2.950 inches and a dynamic compression ratio of 7.85:1

I would install this cam on a 110 degree intake centerline (advanced 2 degrees), per CompCams recommendations, to favor bottom end in the event that you plan on pulling skiers and tubes.  Otherwise, I would install this straight up for top end and for a little more user friendly tune.

If you have read through all of this rambling, I am surprised.  I am actually surprised that I typed all of this.  Anyway, your machine shop should be able to provide you with your deck height and I am willing to CC your heads for you.  Again, until you know the actual numbers, this is all just speculation.  At least it can help you with an educated guess.

Cheers,

Joe

Everyone should have this thread in their "bookmark" list for future reference.  Joe has pointed out some very usable information that should be retained.

Thanks Joe.

As I look over this entire post, once again this thread has become entirely too in depth for this persons application again. No doubt, good info. But too much for the job at hand, I get what GT is saying about going roller, but don’t forget for a second that these cams can fail too, I have seen it many times in factory parts as well as aftermarket. These are not the “God send” of cams, nothing is fool proof. There are more moving parts with these set ups, most of the time it’s a lifter failure or I’ve even seen the hardened surface on the lobes go away. Yes one will spend about another $1,000 or more on a roller valvetrain when building an engine, but there also is no guarantee you will produce more power over a flat tappet grind. There are variables there too, talking of the specific valve timing events and lobe profile, it’s more than just friction reduction. To be certain both cams with almost identical valve timing and lobe profiles need to be run in that engine on a dyno, perhaps the roller will produce a slight bit more power, but you will never feel it in the seat.
Cam and valvetrain choice will always boil down to application, and for some people budget. I am still a big fan of flat tappet cams, both hydraulic and mechanical. I’ve had tremendous success with them and yes the oils and additives are still available for the protection needed. I’m also a fan of roller cams for when the application is also needed. I have been in a dyno cell when an NHRA stock eliminator 396ci engine is being tested with a solid flat tappet cam, this engine made incredible power all the way up into an RPM range where most will think that only a roller will survive, well that’s bullshit. For the owner of this boat, just shove that hydraulic bumpstick in, perform a proper break in and go play, your not building a competition engine for a drag boat.

I do get ya GT, you have obviously been in the industry as long as I and have also seen a lot. But we both know usually (but not all) a direct link to a parts failure can be tied to how that part was maintained. Valvetrain harmonics will always be present with all cams, hydraulic cams are pretty forgiving though, what is hard on components is valve lash with the mechanical versions. I run a mechanical roller in my drag race engine in my dragster, over the years I have had two lifter failures that actually did lobe damage and I keep a watchful eye on all my parts, so you just never know, but that’s racing. I have a solid flat tappet in my BBC engine in my Chevelle that I built in 1992, I have never had a problem with it and it’s not a small cam either…Oh well…it is what it is I guess…It boils down to personal preference, application and budget…All is good....

Thank you.  Aren't those hemispherical or pent-roof style combustion chambers?  What gaskets are you using?  Hope to make it out to watch you race soon.

Cheers,

Joe