Ok. I hope this is the last time I have to ask. After speaking extensively w engine builder, it seems like I'm going with a comp cam 268 ah. Part number K11-306-4. The complete kit.
20.5 70s Carribbean mini cruiser. 454, 10.5 comp, 750 vac sec, air gap, 049 heads (stock), MSD 6A ignition and dizzy, aluminum mercruiser center rise exhaust manifolds, jacuzzi wj ( not sure of impeller but tag says "B"). What are your opinions? Is this a good choice? What are the mini cruiser guys running?

I'd be happy w 55-60 max in my cruiser where i would be mostly cruising around 45. Id like to do the cam just once. I will never change to headers. Intake and carb can always be changed later. I just dont want to cut myself short with the cam

Roller is WAY out of budget

What pushrods did you use?

That looks like a pretty nice kit for the cash.

I would recommend going roller even if it's a stretch on the budget. It is one less headache and you can run virtually any automotive motor oil you want, saving you some money down the line. Probably a long payback,  but piece of mind is worth a lot IMHO.

I just about lost it when I bought my mechanical roller and all the parts needed for the valve train,  but I feel good knowing it will be bulletproof.

Not to mention, reusable to an extent.

I have slowly been building my 496 over the last two years. Money has been tight lately.  (Very long story).

GT

Sent from my SCH-I545 using SoCal Jet Boats mobile app

Ok. I hope this is the last time I have to ask. After speaking extensively w engine builder, it seems like I'm going with a comp cam 268 ah. Part number K11-306-4. The complete kit.
20.5 70s Carribbean mini cruiser. 454, 10.5 comp, 750 vac sec, air gap, 049 heads (stock), MSD 6A ignition and dizzy, aluminum mercruiser center rise exhaust manifolds, jacuzzi wj ( not sure of impeller but tag says "B"). What are your opinions? Is this a good choice? What are the mini cruiser guys running?

Pistons are already purchased and installed. Machine shop recommendation. Will the 270 still be a better choice

First off different cam profiles WILL NOT change your static/mechanical compression ratio, only the dynamic compression. Second, this guy just doesn’t seem to trust his engine builder, for some reason and keeps asking questions on here second guessing the one who’s doing the work and or choosing the parts. Who knows maybe neither one of them know what they are doing. Thicker head gaskets are not a good choice; it’s a band aid to try and correct something that is wrong. With a G.M. style wedge shaped combustion chamber you have what’s called a quench area on the head as well as the piston. This is a combination of head gasket and deck clearance volume. To help atomize the fuel by way of turbulence on the compression stroke you really need to minimize this volume as much as possible so the piston’s quench surface is close to the head quench area. Not only will this help burn more of the fuel, it also holds off detonation. The flame front will not travel into this quench area during combustion, but this is where the end gases could ignite to cause the colliding flame fronts (detonation). Ugly things can happen within this area if the volume is large. If you choose someone to help you on a project you should follow their recommendations, if not, find someone else that really knows and pay them, before you have a trail of destruction and are out of a lot of money. I have read many of your posts, and it seems to me that you have already had some real rough times.

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

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